3.2. Basics - containers and beans
In Spring, those objects that form the backbone of your application and that
are managed by the Spring IoC
container are referred to as
beans. A bean is simply an object that is instantiated,
assembled and otherwise managed by a Spring IoC container; other than that, there is
nothing special about a bean (it is in all other respects one of probably many objects
in your application). These beans, and the
dependencies between
them, are reflected in the
configuration metadata used by a
container.
The
org.springframework.beans.factory.BeanFactory
is the actual representation of the Spring IoC
container that is
responsible for containing and otherwise managing the aforementioned beans.
The
BeanFactory interface is the central IoC container
interface in Spring. Its responsibilities include instantiating or sourcing application
objects, configuring such objects, and assembling the dependencies between these objects.
There are a number of implementations of the
BeanFactory
interface that come supplied straight out-of-the-box with Spring. The most commonly used
BeanFactory implementation is the
XmlBeanFactory class. This implementation allows you to express the
objects that compose your application, and the doubtless rich interdependencies between such
objects, in terms of XML. The
XmlBeanFactory takes this
XML
configuration metadata and uses it to create a
fully configured system or application.
3.2.1.1. Configuration metadata
As can be seen in the above image, the Spring IoC container consumes some form of
configuration metadata; this configuration metadata is nothing
more than how you (as an application developer) inform the Spring container as to
how to “
instantiate, configure, and assemble [the objects in your
application]”. This configuration metadata is typically supplied in a
simple and intuitive XML format. When using XML-based configuration metadata, you
write
bean definitions for those beans that you want the
Spring IoC container to manage, and then let the container do it's stuff.
![[Note]](https://lh3.googleusercontent.com/blogger_img_proxy/AEn0k_u-FNJSkjUh9zPYeNjhrizQcabEiEisIHQdgBgxpBrs3mv46sef_voMN8SHPZYwnHi08BENtFuEeJLxZFhKI0__TocHU4DuQrUdbltZrCSKrUQ4nx1WcleVuK9TFI30Yc96leIiw1NJObI=s0-d) | Note |
|---|
| XML-based metadata is by far the most commonly used form of configuration
metadata. It is not however the only form of
configuration metadata that is allowed. The Spring IoC container itself
is totally decoupled from the format in which this
configuration metadata is actually written.
At the time of writing, you can supply this configuration metadata
using either XML, the Java properties format, or programmatically (using
Spring's public API). The XML-based configuration metadata format really is
simple though, and so the remainder of this chapter will use the XML format
to convey key concepts and features of the Spring IoC container. |
Please be advised that in the vast majority of application scenarios,
explicit user code is not required to instantiate one or more instances
of a Spring IoC container. For example, in a web application scenario, a simple
eight (or so) lines of absolutely boilerplate J2EE web descriptor XML in the
web.xml file of the application will typically suffice
(see
Section 3.8.4, “Convenient ApplicationContext instantiation for web applications”).
Spring configuration consists of at least one bean definition that the
container must manage, but typically there will be more than one bean definition.
When using XML-based configuration metadata, these beans are configured as
<bean/> elements inside a top-level
<beans/>
element.
These bean definitions correspond to the actual objects that make up your
application. Typically you will have bean definitions for your service layer
objects, your data access objects (DAOs), presentation objects such as Struts
Action instances, infrastructure objects such as
Hibernate
SessionFactory instances, JMS
Queue references, etc. (the possibilities are of
course endless, and are limited only by the scope and complexity of your application).
(Typically one does not configure fine-grained domain objects in the container.)
Find below an example of the basic structure of XML-based configuration metadata.
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd">
<bean id="..." class="...">
<!-- collaborators and configuration for this bean go here -->
</bean>
<bean id="..." class="...">
<!-- collaborators and configuration for this bean go here -->
</bean>
<!-- more bean definitions go here... -->
</beans>
3.2.2. Instantiating a container
Instantiating a Spring IoC container is easy; find below some examples of how
to do just that:
Resource resource = new FileSystemResource("beans.xml");
BeanFactory factory = new XmlBeanFactory(resource);
... or...
ClassPathResource resource = new ClassPathResource("beans.xml");
BeanFactory factory = new XmlBeanFactory(resource);
... or...
ApplicationContext context = new ClassPathXmlApplicationContext(
new String[] {"applicationContext.xml", "applicationContext-part2.xml"});
// of course, an ApplicationContext is just a BeanFactory
BeanFactory factory = context;
3.2.2.1. Composing XML-based configuration metadata
It can often be useful to split up container definitions into multiple
XML files. One way to then load an application context which is configured
from all these XML fragments is to use the application context constructor
which takes multiple
Resource locations. With
a bean factory, a bean definition reader can be used multiple times to read
definitions from each file in turn.
Generally, the Spring team prefers the above approach, since it
keeps container configuration files unaware of the fact that they are
being combined with others. An alternate approach is to use one or more
occurrences of the
<import/> element to load bean definitions
from another file (or files). Let's look at a sample:
<beans>
<import resource="services.xml"/>
<import resource="resources/messageSource.xml"/>
<import resource="/resources/themeSource.xml"/>
<bean id="bean1" class="..."/>
<bean id="bean2" class="..."/>
</beans>
In this example, external bean definitions are being loaded from 3
files,
services.xml,
messageSource.xml, and
themeSource.xml. All location paths are considered
relative to the definition file doing the importing, so
services.xml in this case must be in the same directory
or classpath location as the file doing the importing, while
messageSource.xml and
themeSource.xml must be in a
resources location below the location of the importing
file. As you can see, a leading slash is actually ignored, but given that
these are considered relative paths, it is probably better form not to use
the slash at all.
The contents of the files being imported must be fully valid XML
bean definition files according to the Schema or DTD, including the
top level
<beans/> element.
As mentioned previously, a Spring IoC container manages one or more
beans. These beans are created using the instructions
defined in the configuration metadata that has been supplied to the container
(typically in the form of XML
<bean/> definitions).
Within the container itself, these bean definitions are represented as
BeanDefinition objects, which contain (among other
information) the following metadata:
- a package-qualified class name: this is
normally the actual implementation class of the bean being defined.
However, if the bean is to be instantiated by invoking a
static factory method
instead of using a normal constructor, this will actually be the
class name of the factory class.
- bean behavioral configuration elements, which state how the
bean should behave in the container (prototype or singleton,
autowiring mode, initialization and destruction callbacks, and so forth).
- constructor arguments and property values to set in the
newly created bean. An example would be the number of connections
to use in a bean that manages a connection pool (either specified
as a property or as a constructor argument), or the pool size
limit.
- other beans which are needed for the bean to do its work, that is
collaborators (also called dependencies).
The concepts listed above directly translate to a set of properties
that each bean definition consists of. Some of these properties are listed
below, along with a link to further documentation about each of them.
Table 3.1. The bean definition
| Feature | Explained in... |
|---|
| class | Section 3.2.3.2, “Instantiating beans” |
| name | Section 3.2.3.1, “Naming beans” |
| scope | Section 3.4, “Bean scopes” |
| constructor arguments | Section 3.3.1, “Injecting dependencies” |
| properties | Section 3.3.1, “Injecting dependencies” |
| autowiring mode | Section 3.3.6, “Autowiring collaborators” |
| dependency checking mode | Section 3.3.7, “Checking for dependencies” |
| lazy-initialization mode | Section 3.3.5, “Lazily-instantiated beans” |
| initialization method | Section 3.5.1, “Lifecycle interfaces” |
| destruction method | Section 3.5.1, “Lifecycle interfaces” |
Besides bean definitions which contain information on how to
create a specific bean, certain
BeanFactory
implementations also permit the registration of existing objects that have
been created outside the factory (by user code). The
DefaultListableBeanFactory class supports this
through the
registerSingleton(..) method. Typical applications
solely work with beans defined through metadata bean definitions, though.
Every bean has one or more ids (also called identifiers, or names;
these terms refer to the same thing). These ids must be unique within
the container the bean is hosted in. A bean will almost always have only one
id, but if a bean has more than one id, the extra ones can essentially be
considered aliases.
When using XML-based configuration metadata, you use the
'id'
or
'name' attributes to specify the bean identifier(s). The
'id' attribute allows you to specify exactly one id, and as
it is a real XML element ID attribute, the XML parser is able to do
some extra validation when other elements reference the id; as such, it is the
preferred way to specify a bean id. However, the XML specification does limit
the characters which are legal in XML IDs. This is usually not a constraint, but
if you have a need to use one of these special XML characters, or want to introduce
other aliases to the bean, you may also or instead specify one or more bean ids,
separated by a comma (
,), semicolon (
;), or
whitespace in the
'name' attribute.
Please note that you are not required to supply a name for a bean. If
no name is supplied explicitly, the container will generate a (unique) name
for that bean. The motivations for not supplying a name for a bean will be
discussed later (one use case is
inner beans).
3.2.3.1.1. Aliasing beans
In a bean definition itself, you may supply more than one name for
the bean, by using a combination of up to one name specified via the
id attribute, and any number of other names via the
name attribute. All these names can be considered
equivalent aliases to the same bean, and are useful for some situations,
such as allowing each component used in an application to refer to a
common dependency using a bean name that is specific to that component
itself.
Having to specify all aliases when the bean is actually defined is not
always adequate however. It is sometimes desirable to introduce an alias
for a bean which is defined elsewhere. In XML-based configuration metadata
this may be accomplished via the use of the standalone
<alias/> element. For example:
<alias name="fromName" alias="toName"/>
In this case, a bean in the same container which is named
'fromName', may also after the use of this alias
definition, be referred to as
'toName'.
As a concrete example, consider the case where component A defines a
DataSource bean called componentA-dataSource, in its XML fragment.
Component B would however like to refer to the DataSource as
componentB-dataSource in its XML fragment. And the main application,
MyApp, defines its own XML fragment and assembles the final application
context from all three fragments, and would like to refer to the
DataSource as myApp-dataSource. This scenario can be easily handled by
adding to the MyApp XML fragment the following standalone aliases:
<alias name="componentA-dataSource" alias="componentB-dataSource"/>
<alias name="componentA-dataSource" alias="myApp-dataSource" />
Now each component and the main app can refer to the dataSource via
a name that is unique and guaranteed not to clash with any other
definition (effectively there is a namespace), yet they refer to the same
bean.
3.2.3.2. Instantiating beans
A bean definition can be seen as a recipe for creating one or more
actual objects. The container looks at the recipe for a named bean when
asked, and uses the configuration metadata encapsulated by that bean
definition to create (or acquire) an actual object.
If you are using XML-based configuration metadata, you can specify
the type (or class) of object that is to be instantiated using the
'class' attribute of the
<bean/>
element. This
'class' attribute (which internally
eventually boils down to being a
Class property on a
BeanDefinition instance) is normally
mandatory (see
Section 3.2.3.2.3, “Instantiation using an instance factory method” and
Section 3.6, “Bean definition inheritance” for the two exceptions)
and is used for one of two purposes. The class property specifies the
class of the bean to be constructed in the much more common case where the
container itself directly creates the bean by calling its constructor
reflectively (somewhat equivalent to Java code using the
'new' operator). In the less common case where the
container invokes a
static,
factory
method on a class to create the bean, the class property specifies the actual
class containing the
static factory method that is to
be invoked to create the object (the type of the object returned from the
invocation of the
static factory method may be the same
class or another class entirely, it doesn't matter).
3.2.3.2.1. Instantiation using a constructor
When creating a bean using the constructor approach, all normal
classes are usable by and compatible with Spring. That is, the
class being created does not need to implement any specific interfaces
or be coded in a specific fashion. Just specifying the bean class
should be enough. However, depending on what type of IoC you are going
to use for that specific bean, you may need a default (empty)
constructor.
Additionally, the Spring IoC container isn't limited to just managing
true JavaBeans, it is also able to manage virtually
any
class you want it to manage. Most people using Spring prefer to
have actual JavaBeans (having just a default (no-argument) constructor
and appropriate setters and getters modeled after the properties)
in the container, but it is also possible to have more exotic
non-bean-style classes in your container. If, for example, you
need to use a legacy connection pool that absolutely does not adhere
to the JavaBean specification, Spring can manage it as well.
When using XML-based configuration metadata you can specify your bean
class like so:
<bean id="exampleBean" class="examples.ExampleBean"/>
<bean name="anotherExample" class="examples.ExampleBeanTwo"/>
The mechanism for supplying arguments to the constructor (if required),
or setting properties of the object instance after it has been constructed,
will be described shortly.
3.2.3.2.2. Instantiation using a static factory method
When defining a bean which is to be created using a static
factory method, along with the
class attribute
which specifies the class containing the
static factory method,
another attribute named
factory-method is needed to
specify the name of the factory method itself. Spring expects to be
able to call this method (with an optional list of arguments as
described later) and get back a live object, which from that point on
is treated as if it had been created normally via a constructor. One
use for such a bean definition is to call
static
factories in legacy code.
The following example shows a bean definition which specifies
that the bean is to be created by calling a factory-method. Note that
the definition does not specify the type (class) of the returned
object, only the class containing the factory method. In this example,
the
createInstance() method must be a
static method.
<bean id="exampleBean"
class="examples.ExampleBean2"
factory-method="createInstance"/>
The mechanism for supplying (optional) arguments to the factory
method, or setting properties of the object instance after it has been
returned from the factory, will be described shortly.
3.2.3.2.3. Instantiation using an instance factory method
In a fashion similar to instantiation via a
static factory method,
instantiation using an instance factory method is where the factory method of an
existing bean from the container is invoked to create the new bean.
To use this mechanism, the
'class' attribute
must be left empty, and the
'factory-bean' attribute
must specify the name of a bean in the current (or parent/ancestor) container
that contains the factory method. The factory method itself must still be set
via the
'factory-method' attribute (as seen in the example
below).
<!-- the factory bean, which contains a method called createInstance() -->
<bean id="myFactoryBean" class="...">
...
</bean>
<!-- the bean to be created via the factory bean -->
<bean id="exampleBean"
factory-bean="myFactoryBean"
factory-method="createInstance"/>
Although the mechanisms for setting bean properties are still to
be discussed, one implication of this approach is that the factory
bean itself can be managed and configured via DI.
3.2.4. Using the container
A
BeanFactory is essentially nothing more
than the interface for an advanced factory capable of maintaining a registry
of different beans and their dependencies. The
BeanFactory
enables you to read bean definitions and access them using the bean factory.
When using just the
BeanFactory you would create
one and read in some bean definitions in the XML format as follows:
InputStream is = new FileInputStream("beans.xml");
BeanFactory factory = new XmlBeanFactory(is);
Basically that's all there is to it. Using
getBean(String)
you can retrieve instances of your beans; the client-side view of the
BeanFactory is surprisingly simple. The
BeanFactory interface has only six methods for
client code to call:
- boolean containsBean(String): returns true
if the BeanFactory contains a bean
definition or bean instance that matches the given name
- Object getBean(String): returns an instance
of the bean registered under the given name. Depending on how the
bean was configured by the BeanFactory configuration, either a
singleton and thus shared instance or a newly created bean will be
returned. A BeansException will be thrown when
either the bean could not be found (in which case it'll be a
NoSuchBeanDefinitionException), or an exception
occurred while instantiating and preparing the bean
- Object getBean(String, Class): returns a
bean, registered under the given name. The bean returned will be
cast to the given Class. If the bean could not be cast,
corresponding exceptions will be thrown
(BeanNotOfRequiredTypeException). Furthermore,
all rules of the getBean(String) method apply (see above)
- Class getType(String name): returns the
Class of the bean with the given name. If no bean
corresponding to the given name could be found, a
NoSuchBeanDefinitionException will be thrown
- boolean isSingleton(String): determines
whether or not the bean definition or bean instance registered under
the given name is a singleton (bean scopes such as singleton are explained
later). If no bean corresponding
to the given name could be found, a
NoSuchBeanDefinitionException will be thrown
- String[] getAliases(String): Return the
aliases for the given bean name, if any were defined in the bean
definition
Your typical enterprise application is not made up of a single object (or
bean in the Spring parlance). Even the simplest of applications will no doubt
have at least a handful of objects that work together to present what the
end-user sees as a coherent application. This next section explains how you go
from defining a number of bean definitions that stand-alone, each to themselves,
to a fully realized application where objects work (or collaborate) together
to achieve some goal (usually an application that does what the end-user wants).
3.3.1. Injecting dependencies
The basic principle behind
Dependency Injection (DI)
is that objects define their dependencies (that is to say the other
objects they work with) only through constructor arguments, arguments
to a factory method, or properties which are set on the object instance
after it has been constructed or returned from a factory method. Then, it
is the job of the container to actually
inject those
dependencies when it creates the bean. This is fundamentally the inverse,
hence the name
Inversion of Control (IoC), of the bean
itself being in control of instantiating or locating its dependencies on its
own using direct construction of classes, or something like the
Service Locator pattern.
It becomes evident upon usage that code gets much cleaner when the DI
principle is applied, and reaching a higher grade of decoupling is much
easier when beans do not look up their dependencies, but are provided
with them (and additionally do not even know where the dependencies are
located and of what actual class they are).
As touched on in the previous paragraph, DI exists in two major variants,
namely
Setter Injection, and
Constructor Injection.
3.3.1.1. Setter Injection
Setter-based DI is realized by calling setter methods
on your beans after invoking a no-argument constructor or no-argument
static factory method to instantiate your bean.
Find below an example of a class that can only be dependency injected
using pure setter injection. Note that there is nothing
special about this class... it is plain old Java.
public class SimpleMovieLister {
// the SimpleMovieLister has a dependency on the MovieFinder
private MovieFinder movieFinder;
// a setter method so that the Spring container can 'inject' a MovieFinder
public void setMovieFinder(MovieFinder movieFinder) {
this.movieFinder = movieFinder;
}
// business logic that actually 'uses' the injected MovieFinder is omitted...
}
3.3.1.2. Constructor Injection
Constructor-based DI
is realized by invoking a constructor with a number of arguments,
each representing a collaborator. Additionally,
calling a
static factory method with specific
arguments to construct the bean, can be considered almost equivalent,
and the rest of this text will consider arguments to a constructor and
arguments to a
static factory method similarly.
Find below an example of a class that could only be dependency injected
using constructor injection. Again, note that there is nothing
special about this class.
public class SimpleMovieLister {
// the SimpleMovieLister has a dependency on the MovieFinder
private MovieFinder movieFinder;
// a constructor so that the Spring container can 'inject' a MovieFinder
public SimpleMovieLister(MovieFinder movieFinder) {
this.movieFinder = movieFinder;
}
// business logic that actually 'uses' the injected MovieFinder is omitted...
}
The
BeanFactory supports both of these
variants for injecting dependencies into beans it manages. (It in fact
also supports injecting setter-based dependencies after some
dependencies have already been supplied via the constructor approach.)
The configuration for the dependencies comes in the form of a
BeanDefinition, which is used together with
PropertyEditor instances to know how to
convert properties from one format to another. However, most users of
Spring will not be dealing with these classes directly
(that is programmatically), but rather with an XML definition file which
will be converted internally into instances of these classes, and used
to load an entire Spring IoC container instance.
Bean dependency resolution generally happens as follows:
-
The BeanFactory is created
and initialized with a configuration which describes all
the beans. (Most Spring users use a
BeanFactory or
ApplicationContext implementation
that supports XML format configuration files.)
-
Each bean has dependencies expressed in the form of
properties, constructor arguments, or arguments to the
static-factory method when that is used instead of a normal
constructor. These dependencies will be provided to the bean,
when the bean is actually created.
-
Each property or constructor argument is either an actual
definition of the value to set, or a reference to another bean in
the container.
-
Each property or constructor argument which is a value must
be able to be converted from whatever format it was specified
in, to the actual type of that property or constructor argument.
By default Spring can convert a value supplied in string format
to all built-in types, such as int,
long, String,
boolean, etc.
The Spring container validates the configuration of each bean as the container
is created, including the validation that properties which are bean references
are actually referring to valid beans. However, the bean properties themselves
are not set until the bean
is actually created. For those
beans that are singleton-scoped and set to be pre-instantiated (such as
singleton beans in an
ApplicationContext),
creation happens at the time that the container is created, but otherwise this is
only when the bean is requested. When a bean actually has to be created, this will
potentially cause a graph of other beans to be created, as its dependencies and its
dependencies' dependencies (and so on) are created and assigned.
You can generally trust Spring to do the right thing. It will detect
mis-configuration issues, such as references to non-existent beans and
circular dependencies, at container load-time. It will actually set
properties and resolve dependencies as late as possible, which is when
the bean is actually created. This means that
a Spring container which has loaded correctly can later generate an
exception when you request a bean if there is a problem creating that
bean or one of its dependencies. This could happen if the bean throws
an exception as a result of a missing or invalid property, for example.
This potentially delayed visibility of some configuration issues is why
ApplicationContext implementations by default
pre-instantiate singleton beans. At the cost of some upfront time
and memory to create these beans before they are actually needed,
you find out about configuration issues when the
ApplicationContext is created, not later.
If you wish, you can still override this default behavior and set any of these
singleton beans to lazy-initialize (that is not be pre-instantiated).
Finally, if it is not immediately apparent, it is worth mentioning that when
one or more collaborating beans are being injected into a dependent bean, each
collaborating bean is
totally configured prior to being
passed (via one of the DI flavors) to the dependent bean.
This means that if bean A has a dependency on bean B, the Spring IoC container
will
totally configure bean B prior to invoking the setter method
on bean A; you can read '
totally configure'
to mean that the bean will be instantiated (if not a pre-instantiated singleton),
all of its dependencies will be set, and the relevant lifecycle methods (such as
a
configured init method
or the
IntializingBean callback method)
will all be invoked.
First, an example of using XML-based configuration metadata for setter-based DI.
Find below a small part of a Spring XML configuration file specifying some bean
definitions.
<bean id="exampleBean" class="examples.ExampleBean">
<!-- setter injection using the nested <ref/> element -->
<property name="beanOne"><ref bean="anotherExampleBean"/></property>
<!-- setter injection using the neater 'ref' attribute -->
<property name="beanTwo" ref="yetAnotherBean"/>
<property name="integerProperty" value="1"/>
</bean>
<bean id="anotherExampleBean" class="examples.AnotherBean"/>
<bean id="yetAnotherBean" class="examples.YetAnotherBean"/>
public class ExampleBean {
private AnotherBean beanOne;
private YetAnotherBean beanTwo;
private int i;
public void setBeanOne(AnotherBean beanOne) {
this.beanOne = beanOne;
}
public void setBeanTwo(YetAnotherBean beanTwo) {
this.beanTwo = beanTwo;
}
public void setIntegerProperty(int i) {
this.i = i;
}
}
As you can see, setters have been declared to match against the properties
specified in the XML file.
Now, an example of using constructor-based DI. Find below a snippet from an XML
configuration that specifies constructor arguments, and the corresponding Java class.
<bean id="exampleBean" class="examples.ExampleBean">
<!-- constructor injection using the nested <ref/> element -->
<constructor-arg><ref bean="anotherExampleBean"/></constructor-arg>
<!-- constructor injection using the neater 'ref' attribute -->
<constructor-arg ref="yetAnotherBean"/>
<constructor-arg type="int" value="1"/>
</bean>
<bean id="anotherExampleBean" class="examples.AnotherBean"/>
<bean id="yetAnotherBean" class="examples.YetAnotherBean"/>
public class ExampleBean {
private AnotherBean beanOne;
private YetAnotherBean beanTwo;
private int i;
public ExampleBean(
AnotherBean anotherBean, YetAnotherBean yetAnotherBean, int i) {
this.beanOne = anotherBean;
this.beanTwo = yetAnotherBean;
this.i = i;
}
}
As you can see, the constructor arguments specified in the
bean definition will be used to pass in as arguments to the constructor
of the
ExampleBean.
Now consider a variant of this where instead of using a
constructor, Spring is told to call a
static factory
method to return an instance of the object:
<bean id="exampleBean" class="examples.ExampleBean"
factory-method="createInstance">
<constructor-arg ref="anotherExampleBean"/>
<constructor-arg ref="yetAnotherBean"/>
<constructor-arg value="1"/>
</bean>
<bean id="anotherExampleBean" class="examples.AnotherBean"/>
<bean id="yetAnotherBean" class="examples.YetAnotherBean"/>
public class ExampleBean {
// a private constructor
private ExampleBean(...) {
...
}
// a static factory method; the arguments to this method can be
// considered the dependencies of the bean that is returned,
// regardless of how those arguments are actually used.
public static ExampleBean createInstance (
AnotherBean anotherBean, YetAnotherBean yetAnotherBean, int i) {
ExampleBean eb = new ExampleBean (...);
// some other operations...
return eb;
}
}
Note that arguments to the
static factory method
are supplied via
constructor-arg elements, exactly
the same as if a constructor had actually been used. Also, it is
important to realize that the type of the class being returned by
the factory method does not have to be of the same type as the class
which contains the
static factory method, although
in this example it is. An instance (non-static) factory method would
be used in an essentially identical fashion (aside from the use of the
factory-bean attribute instead of the
class attribute), so details will not be discussed here.
3.3.2. Constructor Argument Resolution
Constructor argument resolution matching occurs using the
argument's type. If there is no potential for ambiguity in
the constructor arguments of a bean definition, then the order
in which the constructor arguments are defined in a bean definition
is the order in which those arguments will be supplied to the
appropriate constructor when it is being instantiated. Consider the
following class:
package x.y;
public class Foo {
public Foo(Bar bar, Baz baz) {
// ...
}
}
There is no potential for ambiguity here (assuming of course that
Bar
and
Baz classes are not related in an inheritance hierarchy).
Thus the following configuration will work just fine, and you do not need to
specify the constructor argument indexes and / or types explicitly.
<beans>
<bean name="foo" class="x.y.Foo">
<constructor-arg>
<bean class="x.y.Bar"/>
</constructor-arg>
<constructor-arg>
<bean class="x.y.Baz"/>
</constructor-arg>
</bean>
</beans>
When another bean is referenced, the type is known, and
matching can occur (as was the case with the preceding example).
When a simple type is used, such as
<value>true<value>, Spring cannot
determine the type of the value, and so cannot match by type without
help. Consider the following class:
package examples;
public class ExampleBean {
// No. of years to the calculate the Ultimate Answer
private int years;
// The Answer to Life, the Universe, and Everything
private String ultimateAnswer;
public ExampleBean(int years, String ultimateAnswer) {
this.years = years;
this.ultimateAnswer = ultimateAnswer;
}
}
3.3.2.1. Constructor Argument Type Matching
The above scenario
can use type matching
with simple types by explicitly specifying the type of the constructor
argument using the
'type' attribute. For example:
<bean id="exampleBean" class="examples.ExampleBean">
<constructor-arg type="int" value="7500000"/>
<constructor-arg type="java.lang.String" value="42"/>
</bean>
3.3.2.2. Constructor Argument Index
Constructor arguments can have their index specified explicitly by use of
the
index attribute. For example:
<bean id="exampleBean" class="examples.ExampleBean">
<constructor-arg index="0" value="7500000"/>
<constructor-arg index="1" value="42"/>
</bean>
As well as solving the ambiguity problem of multiple simple values, specifying
an index also solves the problem of ambiguity where a constructor may have two
arguments of the same type. Note that the
index is 0 based.
3.3.3. Bean properties and constructor arguments detailed
As mentioned in the previous section, bean properties and
constructor arguments can be defined as either references to other
managed beans (collaborators), or values defined inline. Spring's XML-based
configuration metadata supports a number of sub-element types
within its
<property/> and
<constructor-arg/> elements for just this purpose.
3.3.3.1. Straight values (primitives, Strings, etc.)
The
<value/> element specifies a property or
constructor argument as a human-readable string representation.
As mentioned previously,
JavaBeans
PropertyEditors are used to convert these
string values from a
String to the actual type of the
property or argument.
<bean id="myDataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close">
<!-- results in a setDriverClassName(String) call -->
<property name="driverClassName">
<value>com.mysql.jdbc.Driver</value>
</property>
<property name="url">
<value>jdbc:mysql://localhost:3306/mydb</value>
</property>
<property name="username">
<value>root</value>
</property>
<property name="password">
<value>masterkaoli</value>
</property>
</bean>
The
<property/> and
<constructor-arg/>
elements also support the use of the
'value' attribute, which can lead
to much more succinct configuration. When using the
'value' attribute,
the above bean definition reads like so:
<bean id="myDataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close">
<!-- results in a setDriverClassName(String) call -->
<property name="driverClassName" value="com.mysql.jdbc.Driver"/>
<property name="url" value="jdbc:mysql://localhost:3306/mydb"/>
<property name="username" value="root"/>
<property name="password" value="masterkaoli"/>
</bean>
The Spring team generally prefer the attribute style over the use of nested
<value/> elements. If you are reading this reference manual
straight through from top to bottom (wow!) then we are getting slightly ahead of ourselves here,
but you can also configure a
java.util.Properties instance like so:
<bean id="mappings" class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
<!-- typed as a java.util.Properties -->
<property name="properties">
<value>
jdbc.driver.className=com.mysql.jdbc.Driver
jdbc.url=jdbc:mysql://localhost:3306/mydb
</value>
</property>
</bean>
Can you see what is happening? The Spring container is converting the text inside
the
<value/> element into a
java.util.Properties
instance using the JavaBeans
PropertEditor mechanism.
This is a nice shortcut, and is one of a few places where the Spring team do favor the
use of the nested
<value/> element over the
'value'
attribute style.
3.3.3.1.1. The idref element
The
idref element is simply an error-proof way to
pass the
id of another bean in the container (to
a
<constructor-arg/> or
<property/>
element).
<bean id="theTargetBean" class="..."/>
<bean id="theClientBean" class="...">
<property name="targetName">
<idref bean="theTargetBean" />
</property>
</bean>
The above bean definition snippet is
exactly equivalent
(at runtime) to the following snippet:
<bean id="theTargetBean" class="..."/>
<bean id="client" class="...">
<property name="targetName">
<value>theTargetBean</value>
</property>
</bean>
The main reason the first form is preferable to
the second is that using the
idref tag allows the
container to validate
at deployment time that the
referenced, named bean actually exists. In the second variation,
no validation is performed on the value that is passed to the
'targetName' property of the
'client'
bean. Any typo will only be discovered (with most likely fatal results)
when the
'client' bean is actually instantiated.
If the
'client' bean is a
prototype bean, this typo
(and the resulting exception) may only be discovered long after the
container is actually deployed.
Additionally, if the bean being referred to is in the same XML unit, and
the bean name is the bean
id, the
'local'
attribute may be used, which allows the XML parser itself to validate the bean
id even earlier, at XML document parse time.
<property name="targetName">
<!-- a bean with an id of 'theTargetBean' must exist,
otherwise an XML exception will be thrown -->
<idref local="theTargetBean"/>
</property>
By way of an example, one common place (at least in pre-Spring 2.0
configuration) where the <idref/> element brings value is in the
configuration of
AOP interceptors in a
ProxyFactoryBean bean definition. If you use
<idref/> elements when specifying the interceptor names, there is
no chance of inadvertently misspelling an interceptor id.
3.3.3.2. References to other beans (collaborators)
The
ref element is the final element allowed
inside a
<constructor-arg/> or
<property/> definition element. It is used to
set the value of the specified property to be a reference to another
bean managed by the container (a collaborator). As
mentioned in a previous section, the referred-to bean is considered to
be a dependency of the bean who's property is being set, and will be
initialized on demand as needed (if it is a singleton bean it may have
already been initialized by the container) before the property is set.
All references are ultimately just a reference to another object, but
there are 3 variations on how the id/name of the other object may be
specified, which determines how scoping and validation is handled.
Specifying the target bean by using the
bean
attribute of the
<ref/> tag is the most general form,
and will allow creating a reference to any bean in the same
container (whether or not in the same XML file), or parent container.
The value of the
'bean' attribute may be the same as either the
'id' attribute of the target bean, or one of the
values in the
'name' attribute of the target bean.
<ref bean="someBean"/>
Specifying the target bean by using the
local
attribute leverages the ability of the XML parser to validate XML id
references within the same file. The value of the
local attribute must be the same as the
id attribute of the target bean. The XML parser
will issue an error if no matching element is found in the same file.
As such, using the local variant is the best choice (in order to know
about errors as early as possible) if the target bean is in the same
XML file.
<ref local="someBean"/>
Specifying the target bean by using the
'parent'
attribute allows a reference to be created to a bean which is in a
parent container of the current container. The value of the
'parent' attribute may be the same as either the
'id' attribute of the target bean, or one of the
values in the
'name' attribute of the target bean,
and the target bean must be in a parent container to the current one.
The main use of this bean reference variant is when you have a hierarchy
of containers and you want to wrap an existing bean in a parent container
with some sort of proxy which will have the same name as the parent bean.
<!-- in the parent context -->
<bean id="accountService" class="com.foo.SimpleAccountService">
<!-- insert dependencies as required as here -->
</bean>
<!-- in the child (descendant) context -->
<bean id="accountService" <-- notice that the name of this bean is the same as the name of the 'parent' bean
class="org.springframework.aop.framework.ProxyFactoryBean">
<property name="target">
<ref parent="accountService"/> <-- notice how we refer to the parent bean
</property>
<!-- insert other configuration and dependencies as required as here -->
</bean>
A
<bean/> element inside the
<property/> or
<constructor-arg/>
elements is used to define a so-called
inner bean. An
inner bean definition does not need to have any id or name defined, and it is
best not to even specify any id or name value because the id or name value
simply will be ignored by the container.
<bean id="outer" class="...">
<!-- instead of using a reference to a target bean, simply define the target bean inline -->
<property name="target">
<bean class="com.mycompany.Person"> <!-- this is the inner bean -->
<property name="name" value="Fiona Apple"/>
<property name="age" value="25"/>
</bean>
</property>
</bean>
Note that in the specific case of inner beans, the
'scope'
flag and any
'id' or
'name' attribute are
effectively ignored. Inner beans are
always anonymous and they
are
always scoped as
prototypes. Please also note
that it is
not possible to inject inner beans into
collaborating beans other than the enclosing bean.
The
<list/>,
<set/>,
<map/>, and
<props/> elements
allow properties and arguments of the Java
Collection
type
List,
Set,
Map, and
Properties,
respectively, to be defined and set.
<bean id="moreComplexObject" class="example.ComplexObject">
<!-- results in a setAdminEmails(java.util.Properties) call -->
<property name="adminEmails">
<props>
<prop key="administrator">administrator@somecompany.org</prop>
<prop key="support">support@somecompany.org</prop>
<prop key="development">development@somecompany.org</prop>
</props>
</property>
<!-- results in a setSomeList(java.util.List) call -->
<property name="someList">
<list>
<value>a list element followed by a reference</value>
<ref bean="myDataSource" />
</list>
</property>
<!-- results in a setSomeMap(java.util.Map) call -->
<property name="someMap">
<map>
<entry>
<key>
<value>yup an entry</value>
</key>
<value>just some string</value>
</entry>
<entry>
<key>
<value>yup a ref</value>
</key>
<ref bean="myDataSource" />
</entry>
</map>
</property>
<!-- results in a setSomeSet(java.util.Set) call -->
<property name="someSet">
<set>
<value>just some string</value>
<ref bean="myDataSource" />
</set>
</property>
</bean>
Note that the value of a map key or value, or a set value,
can also again be any of the following elements:
bean | ref | idref | list | set | map | props | value | null
3.3.3.4.1. Collection merging
As of Spring 2.0, the container also supports the
merging
of collections. This allows an application developer to define a parent-style
<list/>,
<map/>,
<set/> or
<props/> element, and
have child-style
<list/>,
<map/>,
<set/> or
<props/> elements
inherit and override values from the parent collection; that is to say the child collection's
values will be the result obtained from the merging of the elements of the parent
and child collections, with the child's collection elements overriding values
specified in the parent collection.
Please note that this section on merging makes use of the
parent-child bean mechanism. This concept has not yet been introduced, so
readers unfamiliar with the concept of parent and child bean definitions
may wish to read the relevant section
before continuing.
Find below an example of the collection merging feature:
<beans>
<bean id="parent" abstract="true" class="example.ComplexObject">
<property name="adminEmails">
<props>
<prop key="administrator">administrator@somecompany.com</prop>
<prop key="support">support@somecompany.com</prop>
</props>
</property>
</bean>
<bean id="child" parent="parent">
<property name="adminEmails">
<!-- the merge is specified on the *child* collection definition -->
<props merge="true">
<prop key="sales">sales@somecompany.com</prop>
<prop key="support">support@somecompany.co.uk</prop>
</props>
</property>
</bean>
<beans>
Notice the use of the
merge=true attribute on the
<props/> element of the
adminEmails property of the
child
bean definition. When the
child bean is actually resolved
and instantiated by the container, the resulting instance will have an
adminEmails Properties collection
that contains the result of the merging of the child's
adminEmails collection with the parent's
adminEmails collection.
administrator=administrator@somecompany.com
sales=sales@somecompany.com
support=support@somecompany.co.uk
Notice how the child
Properties collection's
value set will have inherited all the property elements from the parent
<props/>. Notice also how the child's value for
the
support value overrides the value in the parent collection.
This merging behavior applies similarly to the
<list/>,
<map/>, and
<set/> collection
types. In the specific case of the
<list/> element, the
semantics associated with the
List collection type, that is the
notion of an
ordered collection of values, is maintained;
the parent's values will precede all of the child list's values. In the
case of the
Map,
Set,
and
Properties collection types, there is no notion
of ordering and hence no ordering semantics are in effect for the collection types that
underlie the associated
Map,
Set and
Properties
implementation types used internally by the container.
Finally, some minor notes about the merging support are in order; you
cannot merge different collection types (e.g. a
Map and
a
List), and if you do attempt to do so an appropriate
Exception will be thrown; and in case it is not
immediately obvious, the
'merge' attribute must be specified
on the lower level, inherited, child definition; specifying the
'merge' attribute on a parent collection definition is
redundant and will not result in the desired merging; and (lastly), please
note that this merging feature is only available in Spring 2.0 (and later
versions).
3.3.3.4.2. Strongly-typed collection (Java 5+ only)
If you are using Java 5 or Java 6, you will be aware that it is possible to
have strongly typed collections (using generic types). That is, it is possible to
declare a
Collection type such that it can only
contain
String elements (for example).
If you are using Spring to dependency inject a strongly-typed
Collection into a bean, you can take advantage
of Spring's type-conversion support such that the elements of your strongly-typed
Collection instances will be converted to the
appropriate type prior to being added to the
Collection.
public class Foo {
private Map<String, Float> accounts;
public void setAccounts(Map<String, Float> accounts) {
this.accounts = accounts;
}
}
<beans>
<bean id="foo" class="x.y.Foo">
<property name="accounts">
<map>
<entry key="one" value="9.99"/>
<entry key="two" value="2.75"/>
<entry key="six" value="3.99"/>
</map>
</property>
</bean>
</beans>
When the
'accounts' property of the
'foo'
bean is being prepared for injection, the generics information about the element
type of the strongly-typed
Map<String, Float> is
actually available via reflection, and so Spring's type conversion infrastructure
will actually recognize the various value elements as being of type
Float and so the string values
'9.99', '2.75',
and
'3.99' will be converted into an actual
Float
type.
The
<null/> element is used to handle
null values. Spring treats empty arguments for
properties and the like as empty
Strings. The following
XML-based configuration metadata snippet results in the email property
being set to the empty
String value ("")
<bean class="ExampleBean">
<property name="email"><value/></property>
</bean>
This is equivalent to the following Java code:
exampleBean.setEmail(""). The special
<null> element may be used to indicate a
null value. For example:
<bean class="ExampleBean">
<property name="email"><null/></property>
</bean>
The above configuration is equivalent to the following Java code:
exampleBean.setEmail(null).
3.3.3.6. Shortcuts and other convenience options for XML-based configuration metadata
The configuration metadata shown so far is a tad verbose. That is why there
are several options available for you to limit the amount of XML you have to write to configure your
components. The first is a shortcut to define values and references to other beans
as part of a
<property/> definition. The second is slightly different format
of specifying properties alltogether.
3.3.3.6.1. XML-based configuration metadata shortcuts
The
<property/>,
<constructor-arg/>, and
<entry/> elements all support a
'value'
attribute which may be used instead of embedding a full
<value/>
element. Therefore, the following:
<property name="myProperty">
<value>hello</value>
</property>
<constructor-arg>
<value>hello</value>
</constructor-arg>
<entry key="myKey">
<value>hello</value>
</entry>
are equivalent to:
<property name="myProperty" value="hello"/>
<constructor-arg value="hello"/>
<entry key="myKey" value="hello"/>
The
<property/> and
<constructor-arg/> elements support a similar
shortcut
'ref' attribute which may be used instead
of a full nested
<ref/> element. Therefore,
the following:
<property name="myProperty">
<ref bean="myBean">
</property>
<constructor-arg>
<ref bean="myBean">
</constructor-arg>
... are equivalent to:
<property name="myProperty" ref="myBean"/>
<constructor-arg ref="myBean"/>
Note however that the shortcut form is equivalent to a
<ref bean="xxx"> element; there is no
shortcut for
<ref local="xxx">. To enforce a
strict local reference, you must use the long form.
Finally, the entry element allows a shortcut form to specify the
key and/or value of the map, in the form of the
'key'
/
'key-ref' and
'value' /
'value-ref' attributes. Therefore, the following:
<entry>
<key>
<ref bean="myKeyBean" />
</key>
<ref bean="myValueBean" />
</entry>
is equivalent to:
<entry key-ref="myKeyBean" value-ref="myValueBean"/>
Again, the shortcut form is equivalent to a
<ref bean="xxx"> element; there is no shortcut for
<ref local="xxx">.
3.3.3.6.2. The p-namespace and how to use it to configure properties
The second option you have to limit the amount of XML you have to write to configure your components
is to use the special "p-namespace". Spring 2.0 and later features support for extensible configuration
formats
using namespaces. Those namespaces are
all based on an XML Schema definition. In fact, the
beans configuration format that you've been
reading about is defined in an XML Schema document.
One special namespace is not defined in an XSD file, and only exists in the core of Spring itself. The so-called
p-namespace doesn't need a schema definition and is an alternative way of configuring your properties differently
than the way you have seen so far. Instead of using
nested property
elements, using the p-namespace you can use attributes as part of the
bean element that
describe your property values. The values of the attributes will be taken as the values for your properties.
The following two XML snippets boil down to the same thing in the end: the first is using the format
you're familiar with (the
property elements) whereas the second example is using the p-namespace
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:p="http://www.springframework.org/schema/p"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-2.0.xsd">
<bean name="classic" class="com.mycompany.ExampleBean">
<property name="email" value="foo@bar.com/>
</bean>
<bean name="p-namespace"
class="com.mycompany.ExampleBean"
p:email="foo@bar.com"/>
</beans>
As you can see, we are including an attribute from the p-namespace called email in the bean definition. This is telling
Spring that it should include a property declaration. As previously mentioned, the p-namespace doesn't have a schema
definition, so the name of the attribute can be set to whatever name your property has.
This next example includes two more bean definitions that both have a reference to another bean:
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:p="http://www.springframework.org/schema/p"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-2.0.xsd">
<bean name="john-classic" class="com.mycompany.Person">
<property name="name" value="John Doe"/>
<property name="spouse" ref="jane"/>
</bean>
<bean name="john-modern"
class="com.mycompany.Person"
p:name="John Doe"
p:spouse-ref="jane"/>
<bean name="jane" class="com.mycompany.Person">
<property name="name" value="Jane Doe"/>
</bean>
</beans>
As you can see, this example doesn't only include a property value using the p-namespace, but also uses a special
format to declare property references. Whereas the first bean definition uses
<property name="spouse" ref="jane"/>
to create a reference from bean john to bean jane, the second bean definition uses
p:spouse-ref="jane" as an attribute to do the exact same thing. In this case
spouse
is the property name whereas the
-ref part tells Spring this is not a value but a bean reference.
| Note |
|---|
| Note that we recommend you to choose carefully which approach you are going to use in your project.
You should also communicate this to your team members so you won't end up with XML documents using all
three approaches at the same time. This will prevent people from not understanding
the application because of different ways of configuring it, and will add to the
consistency of your codebase. Also note that this functionality is only available as of Spring 2.0.
|
3.3.3.7. Compound property names
Compound or nested property names are perfectly legal
when setting bean properties, as long as all components of the path
except the final property name are not null. For example, in this bean
definition:
<bean id="foo" class="foo.Bar">
<property name="fred.bob.sammy" value="123" />
</bean>
The
foo bean has a
fred property
which has a
bob property, which has a
sammy property, and that final
sammy
property is being set to the value
123. In order
for this to work, the
fred property of
foo,
and the
bob property of
fred must both
be non-null after the bean is constructed, or a
NullPointerException will be thrown.
For most situations, the fact that a bean is a dependency of another is
expressed by the fact that one bean is set as a property of another.
This is typically accomplished with the
<ref/> element
in XML-based configuration metadata. For the relatively infrequent situations
where dependencies between beans are less direct (for example, when a static
initializer in a class needs to be triggered, such as database driver registration),
the
'depends-on' attribute may be used to explicitly
force one or more beans to be initialized before the bean using this
element is initialized. Find below an example of using the
'depends-on' attribute to express a dependency on a single bean.
<bean id="beanOne" class="ExampleBean" depends-on="manager"/>
<bean id="manager" class="ManagerBean" />
If you need to express a dependency on multiple beans, you can supply a
list of bean names as the value of the
'depends-on'
attribute, with commas, whitespace and semi-colons all valid delimiters, like so:
<bean id="beanOne" class="ExampleBean" depends-on="manager,accountDao">
<property name="manager" ref="manager" />
</bean>
<bean id="manager" class="ManagerBean" />
<bean id="accountDao" class="x.y.jdbc.JdbcAccountDao" />
| Note |
|---|
| The 'depends-on' attribute and property is used not only to
specify an initialization time dependency, but also to specify
the corresponding destroy time dependency (in the case of
singleton beans only). Dependant beans that are defined in the
'depends-on' attribute will be destroyed
first prior to the relevant bean itself being destroyed. This thus
allows you to control shutdown order too. |
3.3.5. Lazily-instantiated beans
The default behavior for
ApplicationContext
implementations is to eagerly pre-instantiate all
singleton beans
at startup. Pre-instantiation means that an
ApplicationContext
will eagerly create and configure all of its
singleton
beans as part of its initialization process. Generally this is
a good thing,
because it means that any errors in the configuration or in the surrounding environment
will be discovered immediately (as opposed to possibly hours or even days down the line).
However, there are times when this behavior is
not what is wanted.
If you do not want a singleton bean to be pre-instantiated when using
an
ApplicationContext, you can selectively
control this by marking a bean definition as lazy-initialized. A lazily-initialized
bean indicates to the IoC container whether or not a bean instance should be
created at startup or when it is first requested.
When configuring beans via XML, this lazy loading is controlled by the
'lazy-init' attribute on the
<bean/>
element; for example:
<bean id="lazy" class="com.foo.ExpensiveToCreateBean" lazy-init="true"/>
<bean name="not.lazy" class="com.foo.AnotherBean"/>
When the above configuration is consumed by an
ApplicationContext, the bean named
'lazy' will
not be eagerly pre-instantiated
when the
ApplicationContext is starting up, whereas
the
'not.lazy' bean will be eagerly pre-instantiated.
One thing to understand about lazy-initialization is that even though a bean
definition may be marked up as being lazy-initialized, if the lazy-initialized bean
is the dependency of a singleton bean that is not lazy-initialized, when the
ApplicationContext is eagerly pre-instantiating the
singleton, it will have to satisfy all of the singletons dependencies,
one of which will be the lazy-initialized bean! So don't be confused if the IoC
container creates one of the beans that you have explicitly configured as
lazy-initialized at startup; all that means is that the lazy-initialized bean
is being injected into a non-lazy-initialized singleton bean elsewhere.
It is also possible to control lazy-initialization at the container level by using
the
'default-lazy-init' attribute on the
<beans/>
element; for example:
<beans default-lazy-init="true">
<!-- no beans will be pre-instantiated... -->
</beans>
3.3.6. Autowiring collaborators
The Spring container is able to
autowire
relationships between collaborating beans. This means that it is possible to
automatically let Spring resolve collaborators (other beans) for your
bean by inspecting the contents of the
BeanFactory.
The autowiring functionality has five modes. Autowiring is specified
per bean and can thus be enabled for some beans,
while other beans will not be autowired. Using autowiring, it is possible
to reduce or eliminate the need to specify properties or constructor
arguments, thus saving a significant amount of typing.
[2]
When using XML-based configuration metadata, the autowire mode for a bean definition
is specified by using the
autowire attribute of the
<bean/> element. The following values are allowed:
Table 3.2. Autowiring modes
| Mode | Explanation |
|---|
| no | No autowiring at all. Bean references must be defined
via a ref element. This is the default, and
changing this is discouraged for larger deployments, since
explicitly specifying collaborators gives greater control and
clarity. To some extent, it is a form of documentation about
the structure of a system. |
| byName | Autowiring by property name. This option will inspect
the container and look for a bean named exactly the same as
the property which needs to be autowired. For example, if you
have a bean definition which is set to autowire by name, and
it contains a master property (that is,
it has a setMaster(..) method), Spring
will look for a bean definition named master,
and use it to set the property. |
| byType | Allows a property to be autowired if there is exactly
one bean of the property type in the container. If there is
more than one, a fatal exception is thrown, and this indicates
that you may not use byType autowiring
for that bean. If there are no matching beans, nothing
happens; the property is not set. If this is not desirable,
setting the dependency-check="objects"
attribute value specifies that an error should be thrown in
this case. |
| constructor | This is analogous to byType, but
applies to constructor arguments. If there isn't exactly one
bean of the constructor argument type in the container, a
fatal error is raised. |
| autodetect | Chooses constructor or
byType through introspection of the bean
class. If a default constructor is found, the
byType mode will be applied. |
Note that explicit dependencies in
property and
constructor-arg settings
always
override autowiring. Please also note that it is not currently possible to autowire
so-called
simple properties such as primitives,
Strings, and
Classes (and arrays of such
simple properties).(This is by-design and should be considered a
feature.)
Autowire behavior can be combined with dependency checking, which will
be performed after all autowiring has been completed.
It is important to understand the various advantages and disadvantages of autowiring.
Some advantages of autowiring include:
- Autowiring can significantly reduce the volume of configuration
required. However, mechanisms such as the use of a bean template
(discussed elsewhere in this chapter)
are also valuable in this regard.
- Autowiring can cause configuration to keep itself up to date
as your objects evolve. For example, if you need to add an additional
dependency to a class, that dependency can be satisfied
automatically without the need to modify configuration. Thus there
may be a strong case for autowiring during development, without
ruling out the option of switching to explicit wiring when the code
base becomes more stable.
Some disadvantages of autowiring:
- Autowiring is more magical than explicit wiring. Although,
as noted in the above table, Spring is careful to avoid guessing
in case of ambiguity which might have unexpected results, the
relationships between your Spring-managed objects are no longer
documented explicitly.
- Wiring information may not be available to tools that may
generate documentation from a Spring container.
- Autowiring by type will only work when there is a single bean
definition of the type specified by the setter method or constructor
argument. You need to use explicit wiring if there is any potential
ambiguity.
There is no wrong or right answer in all cases. A degree of
consistency across a project is best though; for example, if autowiring is
not used in general, it might be confusing to developers to use it just
to wire one or two bean definitions.
3.3.6.1. Excluding a bean from being available for autowiring
You can also (on a per-bean basis) totally exclude a bean from being an
autowire candidate. When configuring beans using Spring's XML format, the
'autowire-candidate' attribute of the
<bean/> element can be set to
'false'; this has the effect of making the container
totally exclude that specific bean definition from being available
to the autowiring infrastructure.
This can be useful when you have a bean that you absolutely never ever
want to have injected into other beans via autowiring. It does not
mean that the excluded bean cannot itself be configured using autowiring...
it can, it is rather that it itself will not be considered as a candidate
for autowiring other beans.
3.3.7. Checking for dependencies
The Spring IoC container also has the ability to check for the
existence of unresolved dependencies of a bean deployed into the container.
These are JavaBeans properties of the bean, which do not have actual values
set for them in the bean definition, or alternately provided
automatically by the autowiring feature.
This feature is sometimes useful when you want to ensure that all
properties (or all properties of a certain type) are set on a bean. Of
course, in many cases a bean class will have default values for many
properties, or some properties do not apply to all usage scenarios, so
this feature is of limited use. Dependency checking can also be enabled
and disabled per bean, just as with the autowiring functionality. The
default is to
not check dependencies. Dependency
checking can be handled in several different modes. When using
XML-based configuration metadata, this is specified via the
'dependency-check' attribute in a bean definition,
which may have the following values.
Table 3.3. Dependency checking modes
| Mode | Explanation |
|---|
| none | No dependency checking. Properties of the bean which
have no value specified for them are simply not set. |
| simple | Dependency checking is performed for primitive types
and collections (everything except collaborators). |
| object | Dependency checking is performed for collaborators only. |
| all | Dependency checking is done for collaborators, primitive types
and collections. |
If you are using Java 5 and thus have access to source-level annotations,
you may find the section entitled
Section 25.3.1, “@Required”
to be of interest.
For most application scenarios, the majority of the beans in the
container will be
singletons.
When a singleton bean needs to collaborate with another singleton bean, or a
non-singleton bean needs to collaborate with another non-singleton bean, the
typical and common approach of handling this dependency by defining one bean
to be a property of the other is quite adequate. There is a problem when the bean
lifecycles are different. Consider a singleton bean A which needs to use
a non-singleton (prototype) bean B, perhaps on each method invocation on
A. The container will only create the singleton bean A once, and thus
only get the opportunity to set the properties once. There is no
opportunity for the container to provide bean A with a new instance of
bean B every time one is needed.
One solution to this issue is to forgo some inversion of
control. Bean A can be
made aware of the container
by implementing the
BeanFactoryAware interface, and
use programmatic means to
ask the container via a
getBean("B") call for
(a typically new) bean B instance every time it needs it. Find below an
admittedly somewhat contrived example of this approach:
// a class that uses a stateful Command-style class to perform some processing
package fiona.apple;
// lots of Spring-API imports
import org.springframework.beans.BeansException;
import org.springframework.beans.factory.BeanFactory;
import org.springframework.beans.factory.BeanFactoryAware;
public class CommandManager implements BeanFactoryAware {
private BeanFactory beanFactory;
public Object process(Map commandState) {
// grab a new instance of the appropriate Command
Command command = createCommand();
// set the state on the (hopefully brand new) Command instance
command.setState(commandState);
return command.execute();
}
// the Command returned here could be an implementation that executes asynchronously, or whatever
protected Command createCommand() {
return (Command) this.beanFactory.getBean("command"); // notice the Spring API dependency
}
public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
this.beanFactory = beanFactory;
}
}
The above example is generally not a desirable solution since the
business code is then aware of and coupled to the Spring Framework. Method Injection,
a somewhat advanced feature of the Spring IoC container, allows this use case to be
handled in a clean fashion.
3.3.8.1. Lookup method injection
Lookup method injection refers to the ability of the container
to override methods on
container managed beans, to
return the result of looking up another named bean in the container.
The lookup will typically be of a prototype bean as in the scenario
described above. The Spring Framework implements this method injection
by dynamically generating a subclass overriding the method, using bytecode
generation via the CGLIB library.
So if you look at the code from previous code snippet (the
CommandManager class), the Spring container
is going to dynamically override the implementation of the
createCommand() method. Your
CommandManager class is not going to have any
Spring dependencies, as can be seen in this reworked example below:
package fiona.apple;
// no more Spring imports!
public abstract class CommandManager {
public Object process(Object commandState) {
// grab a new instance of the appropriate Command interface
Command command = createCommand();
// set the state on the (hopefully brand new) Command instance
command.setState(commandState);
return command.execute();
}
// okay... but where is the implementation of this method?
protected abstract Command createCommand();
}
In the client class containing the method to be injected (the
CommandManager in this case), the method that
is to be 'injected' must have a signature of the following form:
<public|protected> [abstract] <return-type> theMethodName(no-arguments);
If the method is
abstract, the dynamically-generated
subclass will implement the method. Otherwise, the dynamically-generated subclass
will override the concrete method defined in the original class. Let's look at an
example:
<!-- a stateful bean deployed as a prototype (non-singleton) -->
<bean id="command" class="fiona.apple.AsyncCommand" scope="prototype">
<!-- inject dependencies here as required -->
</bean>
<!-- commandProcessor uses statefulCommandHelper -->
<bean id="commandManager" class="fiona.apple.CommandManager">
<lookup-method name="createCommand" bean="command"/>
</bean>
The bean identified as
commandManager will call its
own method
createCommand() whenever it needs
a new instance of the
command bean. It
is important to note that the person deploying the beans must be
careful to deploy the
command bean as a
prototype (if that is actually what is needed). If it is deployed
as a
singleton, the same instance of
the
command bean will be returned each time!
Please be aware that in order for this dynamic subclassing to work,
you will need to have the CGLIB jar(s) on your classpath. Additionally, the
class that the Spring container is going to subclass cannot be
final, and the method that is being overridden cannot be
final either. Also, testing a class that has an
abstract method can be somewhat odd in that you will have
to subclass the class yourself and supply a stub implementation of the
abstract method. Finally, objects that have been the target
of method injection cannot be serialized.
![[Tip]](https://lh3.googleusercontent.com/blogger_img_proxy/AEn0k_sHu5WZzgq6JeeNwvqD-5GJBQkMa5HsfcSlD7Ch-zSdkmGrGy787qYti6vTOxTTciyt6FYfK1IHfPdmBMp3-k1khbOeTHmYl8Zx1QCjy0b2V3kmwEdeB9mEU5-PhWDOned51AIhEd2OHFo=s0-d) | Tip |
|---|
| The interested reader may also find the ServiceLocatorFactoryBean
(in the org.springframework.beans.factory.config package) to be
of use; the approach is similar to that of the
ObjectFactoryCreatingFactoryBean, but it allows you
to specify your own lookup interface as opposed to having to use a Spring-specific
lookup interface such as the ObjectFactory.
Consult the (copious) Javadocs for the ServiceLocatorFactoryBean
for a full treatment of this alternative approach (that does
reduce the coupling to Spring). |
3.3.8.2. Arbitrary method replacement
A less commonly useful form of method injection than Lookup
Method Injection is the ability to replace arbitrary methods in a
managed bean with another method implementation. Users may safely skip
the rest of this section (which describes this somewhat advanced
feature), until this functionality is actually needed.
When using XML-based configuration metadata, the
replaced-method
element may be used to replace an existing method implementation with
another, for a deployed bean. Consider the following class, with a
method computeValue, which we want to override:
public class MyValueCalculator {
public String computeValue(String input) {
// some real code...
}
// some other methods...
}
A class implementing the
org.springframework.beans.factory.support.MethodReplacer
interface provides the new method definition.
/** meant to be used to override the existing computeValue(String)
implementation in MyValueCalculator
*/
public class ReplacementComputeValue implements MethodReplacer {
public Object reimplement(Object o, Method m, Object[] args) throws Throwable {
// get the input value, work with it, and return a computed result
String input = (String) args[0];
...
return ...;
}
}
The bean definition to deploy the original class and specify the
method override would look like this:
<bean id="myValueCalculator class="x.y.z.MyValueCalculator">
<!-- arbitrary method replacement -->
<replaced-method name="computeValue" replacer="replacementComputeValue">
<arg-type>String</arg-type>
</replaced-method>
</bean>
<bean id="replacementComputeValue" class="a.b.c.ReplacementComputeValue"/>
One or more contained
<arg-type/> elements
within the
<replaced-method/> element may be used to
indicate the method signature of the method being overridden. Note
that the signature for the arguments is actually only needed in the
case that the method is actually overloaded and there are multiple
variants within the class. For convenience, the type string for an
argument may be a substring of the fully qualified type name. For
example, all the following would match
java.lang.String.
java.lang.String
String
Str
Since the number of arguments is often enough to distinguish between
each possible choice, this shortcut can save a lot of typing, by allowing
you to type just the shortest string that will match an argument type.
When you create a bean definition what you are actually creating is a
recipe for creating actual instances of the class
defined by that bean definition. The idea that a bean definition is a recipe
is important, because it means that, just like a class, you can potentially
have many object instances created from a single recipe.
You can control not only the various dependencies and
configuration values that are to be plugged into an object that is created
from a particular bean definition, but also the
scope
of the objects created from a particular bean definition. This approach is very
powerful and gives you the flexibility to
choose the scope
of the objects you create through configuration instead of having to 'bake in'
the scope of an object at the Java class level. Beans can be defined to be
deployed in one of a number of scopes: out of the box, the Spring Framework
supports exactly five scopes (of which three are available only if you are using
a web-aware
ApplicationContext).
The scopes supported out of the box are listed below:
Table 3.4. Bean scopes
| Scope | Description |
|---|
| singleton | Scopes a single bean definition to a single object instance per Spring IoC container. |
| prototype | Scopes a single bean definition to any number of object instances. |
| request | Scopes a single bean definition to the lifecycle of
a single HTTP request; that is each and every HTTP request will
have its own instance of a bean created off the back of a
single bean definition. Only valid in the context of a
web-aware Spring ApplicationContext. |
| session | Scopes a single bean definition to the lifecycle of
a HTTP Session. Only valid in
the context of a web-aware Spring
ApplicationContext. |
| global session | Scopes a single bean definition to the lifecycle of a
global HTTP Session. Typically
only valid when used in a portlet context. Only valid in the
context of a web-aware Spring
ApplicationContext. |
3.4.1. The singleton scope
When a bean is a singleton, only one
shared
instance of the bean will be managed, and all requests for beans with
an id or ids matching that bean definition will result in that one
specific bean instance being returned by the Spring container.
To put it another way, when you define a bean definition and it
is scoped as a singleton, then the Spring IoC container will create
exactly one instance of the object defined by
that bean definition. This single instance will be
stored in a cache of such singleton beans, and
all subsequent
requests and references for that named bean will result in
the cached object being returned.
Please be aware that Spring's concept of a singleton bean is quite
different from the Singleton pattern as defined in the seminal Gang of
Four (GoF) patterns book. The GoF Singleton hardcodes the scope of
an object such that one
and only one instance of a
particular class will ever be created
per
ClassLoader. The scope of the Spring singleton
is best described as
per container and per bean.
This means that if you define one bean for a particular class in a single
Spring container, then the Spring container will create one
and only one instance of the class defined by that
bean definition.
The singleton scope is the default scope in Spring.
To define a bean as a singleton in XML, you would write configuration like so:
<bean id="accountService" class="com.foo.DefaultAccountService"/>
<!-- the following is equivalent, though redundant (singleton scope is the default); using spring-beans-2.0.dtd -->
<bean id="accountService" class="com.foo.DefaultAccountService" scope="singleton"/>
<!-- the following is equivalent and preserved for backward compatibility in spring-beans.dtd -->
<bean id="accountService" class="com.foo.DefaultAccountService" singleton="true"/>
3.4.2. The prototype scope
The non-singleton, prototype scope of bean deployment results in
the
creation of a new bean instance every time a
request for that specific bean is made (that is, it is injected into another
bean or it is requested via a programmatic
getBean()
method call on the container). As a rule of thumb, you should
use the prototype scope for all beans that are stateful, while the
singleton scope should be used for stateless beans.
The following diagram illustrates the Spring prototype scope.
Please note that a DAO would not typically be configured as a
prototype, since a typical DAO would not hold any conversational state;
it was just easier for this author to reuse the core of the singleton
diagram.
To define a bean as a prototype in XML, you would write configuration like so:
<!-- using spring-beans-2.0.dtd -->
<bean id="accountService" class="com.foo.DefaultAccountService" scope="prototype"/>
<!-- the following is equivalent and preserved for backward compatibility in spring-beans.dtd -->
<bean id="accountService" class="com.foo.DefaultAccountService" singleton="false"/>
There is one quite important thing to be aware of when deploying a bean
in the prototype scope, in that the lifecycle of the bean changes slightly.
Spring does not manage the complete lifecycle of a prototype bean: the
container instantiates, configures, decorates and otherwise assembles a
prototype object, hands it to the client and then has no further knowledge
of that prototype instance. This means that while
initialization
lifecycle callback methods will be called on all objects regardless of scope,
in the case of prototypes, any configured
destruction
lifecycle callbacks will
not be called. It is the
responsibility of the client code to clean up
prototype scoped objects and release any expensive resources that the prototype
bean(s) are holding onto. (One possible way to get the Spring container
to release resources used by prototype-scoped beans is through the use
of a custom
bean post-processor
which would hold a reference to the beans that need to be cleaned up.)
In some respects, you can think of the Spring containers role when talking
about a prototype-scoped bean as somewhat of a replacement for the Java
'new' operator. All lifecycle aspects past that point
have to be handled by the client. (The lifecycle of a bean in the Spring
container is further described in the section entitled
Section 3.5.1, “Lifecycle interfaces”.)
3.4.3. Singleton beans with prototype-bean dependencies
When using singleton-scoped beans that have dependencies on
beans that are scoped as prototypes, please be aware that
dependencies are resolved at instantiation time.
This means that if you dependency inject a prototype-scoped
bean into a singleton-scoped bean, a brand new prototype bean
will be instantiated and then dependency injected into the singleton
bean...
but that is all. That exact same
prototype instance will be the sole instance that is ever supplied
to the singleton-scoped bean, which is fine if that is what you want.
However, sometimes what you actually want is for the
singleton-scoped bean to be able to acquire a brand new instance of
the prototype-scoped bean again and again and again at runtime.
In that case it is no use just dependency injecting a prototype-scoped
bean into your singleton bean, because as explained above, that
only happens
once when the Spring container
is instantiating the singleton bean and resolving and injecting
its dependencies. If you are in the scenario where you need to get
a brand new instance of a (prototype) bean again and again and again
at runtime, you are referred to the section entitled
Section 3.3.8, “Method Injection”
| Backwards compatibility note: specifying the lifecycle scope in XML |
|---|
If you are referencing the 'spring-beans.dtd' DTD in
a bean definition file(s), and you are being explicit about the lifecycle scope
of your beans you must use the "singleton" attribute to
express the lifecycle scope (remembering that the
singleton lifecycle scope is
the default). If you are referencing the 'spring-beans-2.0.dtd'
DTD or the Spring 2.0 XSD schema, then you will need to use the "scope"
attribute (because the "singleton" attribute was removed from the
definition of the new DTD and XSD files in favour of the "scope"
attribute).
To be totally clear about this, this means that if you use the
"singleton" attribute in an XML bean definition then you
must be referencing the 'spring-beans.dtd'
DTD in that file. If you are using the "scope"
attribute then you must be referencing either the
'spring-beans-2.0.dtd' DTD or the
'spring-beans-2.0.xsd' XSD in that file. |
The other scopes, namely
request,
session,
and
global session are for use only in web-based applications
(and can be used irrespective of which particular web application framework you are
using, if indeed any). In the interest of keeping related concepts together in
one place in the reference documentation, these scopes are described here.
| Note |
|---|
| The scopes that are described in the following paragraphs are
only available if you are using a web-aware
Spring ApplicationContext implementation
(such as XmlWebApplicationContext).
If you try using these next scopes with regular Spring IoC containers such as
the XmlBeanFactory or
ClassPathXmlApplicationContext, you will
get an IllegalStateException complaining about
an unknown bean scope. |
3.4.4.1. Initial web configuration
In order to support the scoping of beans at the
request,
session, and
global session levels
(web-scoped beans), some minor initial configuration is required
before you can set about defining your bean definitions. Please note that
this extra setup is
not required if you just want to
use the 'standard' scopes (namely singleton and prototype).
Now as things stand, there are a couple of ways to effect this
initial setup depending on your particular Servlet environment...
If you are accessing scoped beans within Spring Web MVC, i.e.
within a request that is processed by the Spring
DispatcherServlet,
or
DispatcherPortlet, then no special setup is necessary:
DispatcherServlet and
DispatcherPortlet
already expose all relevant state.
When using a Servlet 2.4+ web container, with requests processed
outside of Spring's DispatcherServlet (e.g. when using JSF or Struts),
you need to add the following
javax.servlet.ServletRequestListener
to the declarations in your web application's
'web.xml' file.
<web-app>
...
<listener>
<listener-class>org.springframework.web.context.request.RequestContextListener</listener-class>
</listener>
...
</web-app>
If you are using an older web container (Servlet 2.3), you will need to use
the provided
javax.servlet.Filter implementation.
Find below a snippet of XML configuration that has to be included
in the
'web.xml' file of your web application if you
want to have access to web-scoped beans in requests outside of Spring's
DispatcherServlet on a Servlet 2.3 container. (The filter mapping depends on
the surrounding web application configuration and so you will have to change
it as appropriate.)
<web-app>
..
<filter>
<filter-name>requestContextFilter</filter-name>
<filter-class>org.springframework.web.filter.RequestContextFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>requestContextFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
...
</web-app>
That's it.
DispatcherServlet,
RequestContextListener and
RequestContextFilter all do exactly the
same thing, namely bind the HTTP request object to the
Thread that is servicing that request. This makes
beans that are request- and session-scoped available further down the
call chain.
3.4.4.2. The request scope
Consider the following bean definition:
<bean id="loginAction" class="com.foo.LoginAction" scope="request"/>
With the above bean definition in place, the Spring container will create
a brand new instance of the
LoginAction bean
using the
'loginAction' bean definition for each and
every HTTP request. That is, the
'loginAction' bean will be
effectively scoped at the HTTP request level. You can change or dirty
the internal state of the instance that is created as much as you want,
safe in the knowledge that other requests that are also using instances created
off the back of the same
'loginAction' bean definition
will not be seeing these changes in state since they are particular to an individual
request. When the request is finished processing, the bean that is scoped
to the request will be discarded.
3.4.4.3. The session scope
Consider the following bean definition:
<bean id="userPreferences" class="com.foo.UserPreferences" scope="session"/>
With the above bean definition in place, the Spring container will create
a brand new instance of the
UserPreferences bean
using the
'userPreferences' bean definition for the
lifetime of a single HTTP
Session.
In other words, the
'userPreferences' bean will be
effectively scoped at the HTTP
Session level.
Just like
request-scoped beans, you can change the internal
state of the instance that is created as much as you want, safe in the
knowledge that other HTTP
Session instances
that are also using instances created off the back of the same
'userPreferences' bean definition
will not be seeing these changes in state since they are particular to an individual
HTTP
Session. When the HTTP
Session is eventually discarded, the bean
that is scoped to that particular HTTP
Session
will also be discarded.
3.4.4.4. The global session scope
Consider the following bean definition:
<bean id="userPreferences" class="com.foo.UserPreferences" scope="globalSession"/>
The
global session scope is similar to the
standard HTTP
Session scope
(
described immediately above),
and really only makes sense in the context of portlet-based web applications.
The portlet specification defines the notion of a global
Session that is shared amongst all
of the various portlets that make up a single portlet web application.
Beans defined at the
global session scope are scoped
(or bound) to the lifetime of the global portlet
Session.
Please note that if you are writing a standard Servlet-based
web application and you define one or more beans as having
global session scope, the standard HTTP
Session scope will be used, and no
error will be raised.
3.4.4.5. Scoped beans as dependencies
Being able to define a bean scoped to a HTTP request or
Session (or indeed
a custom scope of
your own devising) is all very well, but one of the
main value-adds of the Spring IoC container is that it manages
not only the instantiation of your objects (beans), but also
the wiring up of collaborators (or dependencies). If you want to inject
a (for example) HTTP request scoped bean into another bean, you will need
to inject an AOP proxy in place of the scoped bean. That is, you need to
inject a proxy object that exposes the same public interface as the scoped
object, but that is smart enough to be able to retrieve the real, target
object from the relevant scope (for example a HTTP request) and delegate
method calls onto the real object.
| Note |
|---|
| You do not need to use the
<aop:scoped-proxy/> in conjunction with
beans that are scoped as singletons or
prototypes. It is an error to try to create a
scoped proxy for a singleton bean (and the resulting
BeanCreationException will certainly set
you straight in this regard). |
Let's look at the configuration that is required to effect this;
the configuration is not hugely complex (it takes just one line), but it
is important to understand the “
why” as well as the
“
how” behind it.
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:aop="http://www.springframework.org/schema/aop"
xsi:schemaLocation="
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd
http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.0.xsd">
<!-- a HTTP Session-scoped bean exposed as a proxy -->
<bean id="userPreferences" class="com.foo.UserPreferences" scope="session">
<!-- this next element effects the proxying of the surrounding bean -->
<aop:scoped-proxy/>
</bean>
<!-- a singleton-scoped bean injected with a proxy to the above bean -->
<bean id="userService" class="com.foo.SimpleUserService">
<!-- a reference to the proxied 'userPreferences' bean -->
<property name="userPreferences" ref="userPreferences"/>
</bean>
</beans>
To create such a proxy, you need only to insert a child
<aop:scoped-proxy/> element into a scoped bean definition
(you may also need the CGLIB library on your classpath so that the container can
effect class-based proxying; you will also need to be using
Appendix A, XML Schema-based configuration).
So, just why do you need this
<aop:scoped-proxy/> element in the
definition of beans scoped at the
request,
session,
globalSession and
'
insert your custom scope here' level? The reason is best explained
by picking apart the following bean definition (please note that the following
'userPreferences' bean definition as it stands is
incomplete):
<bean id="userPreferences" class="com.foo.UserPreferences" scope="session"/>
<bean id="userManager" class="com.foo.UserManager">
<property name="userPreferences" ref="userPreferences"/>
</bean>
From the above configuration it is evident that the singleton bean
'userManager' is being injected with a reference to the HTTP
Session-scoped bean
'userPreferences'.
The salient point here is that the
'userManager' bean is a
singleton... it will be instantiated
exactly once per container,
and its dependencies (in this case only one, the
'userPreferences'
bean) will also only be injected (once!). This means that the
'userManager' will (conceptually) only ever operate on the exact
same
'userPreferences' object, that is the one that it was originally
injected with. This is
not what you want when you inject a HTTP
Session-scoped bean as a dependency into a
collaborating object (typically). Rather, what we
do want is a
single
'userManager' object, and then, for the lifetime of a HTTP
Session, we want to see and use a
'userPreferences' object that is specific to said HTTP
Session.
Rather what you need then is to inject some sort of object that exposes the
exact same public interface as the
UserPreferences class (ideally
an object that
is a UserPreferences instance)
and that is smart enough to be able to go off and fetch the
real UserPreferences object from
whatever underlying scoping mechanism we have chosen (HTTP request,
Session, etc.). We can then safely inject this proxy
object into the
'userManager' bean, which will be blissfully unaware
that the
UserPreferences reference that it is holding onto is a
proxy. In the case of this example, when a
UserManager
instance invokes a method on the dependency-injected
UserPreferences
object, it is really invoking a method on the proxy... the proxy will then go off and
fetch the real
UserPreferences object from (in this case) the HTTP
Session, and delegate the method invocation onto the
retrieved real
UserPreferences object.
That is why you need the following, correct and complete, configuration
when injecting
request-,
session-, and
globalSession-scoped beans into collaborating objects:
<bean id="userPreferences" class="com.foo.UserPreferences" scope="session">
<aop:scoped-proxy/>
</bean>
<bean id="userManager" class="com.foo.UserManager">
<property name="userPreferences" ref="userPreferences"/>
</bean>
3.4.4.5.1. Choosing the type of proxy created
By default, when the Spring container is creating a proxy for a bean
that is marked up with the
<aop:scoped-proxy/>
element,
a CGLIB-based class proxy will be created.
This means that you need to have the CGLIB library on the classpath
of your application.
Note: CGLIB proxies will only intercept public method
calls! Do not call non-public methods on such a proxy;
they will not be delegated to the scoped target object.
You can choose to have the Spring container create 'standard'
JDK interface-based proxies for such scoped beans by specifying
'
false' for the value of the '
proxy-target-class'
attribute of the
<aop:scoped-proxy/> element.
Using JDK interface-based proxies does mean that you don't need any additional
libraries on your application's classpath to effect such proxying, but it
does mean that the class of the scoped bean must implement at least one
interface, and
all of the collaborators into which
the scoped bean is injected must be referencing the bean via one of its
interfaces.
<!-- DefaultUserPreferences implements the UserPreferences interface -->
<bean id="userPreferences" class="com.foo.DefaultUserPreferences" scope="session">
<aop:scoped-proxy proxy-target-class="false"/>
</bean>
<bean id="userManager" class="com.foo.UserManager">
<property name="userPreferences" ref="userPreferences"/>
</bean>
The section entitled
Section 6.6, “Proxying mechanisms”
may also be of some interest with regard to understanding the nuances
of choosing whether class-based or interface-based proxying is right
for you.
As of Spring 2.0, the bean scoping mechanism in Spring is extensible. This means
that you are not limited to just the bean scopes that Spring provides out of the box;
you can define your own scopes, or even redefine the existing scopes (although that last
one would probably be considered bad practice - please note that you
cannot override the built-in
singleton and
prototype scopes).
3.4.5.1. Creating your own custom scope
Scopes are defined by the
org.springframework.beans.factory.config.Scope
interface. This is the interface that you will need to implement in order
to integrate your own custom scope(s) into the Spring container, and is described
in detail below. You may wish to look at the
Scope
implementations that are supplied with the Spring Framework itself for an idea of
how to go about implementing your own. The
Scope JavaDoc
explains the main class to implement when you need your own scope in more detail too.
The
Scope interface has four methods dealing with getting objects
from the scope, removing them from the scope and allowing them to be 'destroyed' if needed.
The first method should return the object from the underlying scope. The session scope
implementation for example will return the session-scoped bean (and if it does not exist, return a
new instance of the bean, after having bound it to the session for future reference).
Object get(String name, ObjectFactory objectFactory)
The second method should remove the object from the underlying scope. The session scope implementation
for example, removes the session-scoped bean from the underlying session. The object should be returned
(you are allowed to return null if the object with the specified name wasn't found)
Object remove(String name)
The third method is used to register callbacks the scope should execute when it is destroyed or
when the specified object in the scope is destroyed. Please refer to the JavaDoc or a Spring scope
implementation for more information on destruction callbacks.
void registerDestructionCallback(String name, Runnable destructionCallback)
The last method deals with obtaining the conversation identifier for the underlying scope. This identifier is
different for each scope. For a session for example, this can be the session identifier.
String getConversationId()
SPR-2600 - TODO
3.4.5.2. Using a custom scope
After you have written and tested one or more custom
Scope
implementations, you then need to make the Spring container aware of your new scope(s).
The central method to register a new
Scope with the
Spring container is declared on the
ConfigurableBeanFactory
interface (implemented by most of the concrete
BeanFactory
implementations that ship with Spring); this central method is displayed below:
void registerScope(String scopeName, Scope scope);
The first argument to the
registerScope(..) method is the
unique name associated with a scope; examples of such names in the Spring container
itself are
'singleton' and
'prototype'.
The second argument to the
registerScope(..) method is an
actual instance of the custom
Scope implementation
that you wish to register and use.
Let's assume that you have written your own custom
Scope implementation, and you have registered it
like so:
// note: the ThreadScope class does not ship with the Spring Framework
Scope customScope = new ThreadScope();
beanFactory.registerScope("thread", scope);
You can then create bean definitions that adhere to the scoping rules of your
custom
Scope like so:
<bean id="..." class="..." scope="thread"/>
If you have your own custom
Scope implementation(s),
you are not just limited to only programmatic registration of the custom scope(s).
You can also do the
Scope registration declaratively,
using the
CustomScopeConfigurer class.
The declarative registration of custom
Scope
implementations using the
CustomScopeConfigurer class is
shown below:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:aop="http://www.springframework.org/schema/aop"
xsi:schemaLocation="
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd
http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.0.xsd">
<bean class="org.springframework.beans.factory.config.CustomScopeConfigurer">
<property name="scopes">
<map>
<entry key="thread">
<bean class="com.foo.ThreadScope"/>
</entry>
</map>
</property>
</bean>
<bean id="bar" class="x.y.Bar" scope="thread">
<property name="name" value="Rick"/>
<aop:scoped-proxy/>
</bean>
<bean id="foo" class="x.y.Foo">
<property name="bar" ref="bar"/>
</bean>
</beans>
3.6. Bean definition inheritance
A bean definition potentially contains a large amount of
configuration information, including container specific information (for example
initialization method, static factory method name, and so forth) and constructor
arguments and property values. A child bean definition is a bean
definition that inherits configuration data from a parent definition. It
is then able to override some values, or add others, as needed. Using
parent and child bean definitions can potentially save a lot of typing.
Effectively, this is a form of templating.
When working with a
BeanFactory programmatically, child bean
definitions are represented by the
ChildBeanDefinition
class. Most users will never work with them on this level, instead
configuring bean definitions declaratively in something like the
XmlBeanFactory. When using XML-based configuration metadata a child bean
definition is indicated simply by using the
'parent'
attribute, specifying the parent bean as the value of this attribute.
<bean id="inheritedTestBean" abstract="true"
class="org.springframework.beans.TestBean">
<property name="name" value="parent"/>
<property name="age" value="1"/>
</bean>
<bean id="inheritsWithDifferentClass"
class="org.springframework.beans.DerivedTestBean"
parent="inheritedTestBean" init-method="initialize">
<property name="name" value="override"/>
<!-- the age property value of 1 will be inherited from parent -->
</bean>
A child bean definition will use the bean class from the parent
definition if none is specified, but can also override it. In the latter
case, the child bean class must be compatible with the parent, that is it
must accept the parent's property values.
A child bean definition will inherit constructor argument values,
property values and method overrides from the parent, with the option to
add new values. If any init-method, destroy-method and/or
static
factory method settings are specified, they will override the corresponding parent
settings.
The remaining settings will
always be taken
from the child definition:
depends on,
autowire mode,
dependency check,
singleton,
scope,
lazy init.
Note that in the example above, we have explicitly marked the parent
bean definition as abstract by using the
abstract
attribute. In the case that the parent definition does not specify a
class, and so explicitly marking the parent bean definition as
abstract is required:
<bean id="inheritedTestBeanWithoutClass" abstract="true">
<property name="name" value="parent"/>
<property name="age" value="1"/>
</bean>
<bean id="inheritsWithClass" class="org.springframework.beans.DerivedTestBean"
parent="inheritedTestBeanWithoutClass" init-method="initialize">
<property name="name" value="override"/>
<!-- age will inherit the value of 1 from the parent bean definition-->
</bean>
The parent bean cannot get instantiated on its own since it is
incomplete, and it is also explicitly marked as
abstract.
When a definition is defined to be
abstract like this,
it is usable only as a pure template bean definition that will serve as a
parent definition for child definitions. Trying to use such an
abstract parent bean on its own (by referring to it as
a ref property of another bean, or doing an explicit
getBean() call with the parent bean id), will
result in an error. Similarly, the container's internal
preInstantiateSingletons() method will completely
ignore bean definitions which are defined as abstract.
| Note |
|---|
| ApplicationContexts (but not
BeanFactories) will by default pre-instantiate
all singletons. Therefore it is important (at least for singleton beans)
that if you have a (parent) bean definition which you intend to use only
as a template, and this definition specifies a class, you must make sure
to set the 'abstract' attribute to 'true',
otherwise the application context will actually (attempt to) pre-instantiate
the abstract bean. |
3.7. Container extension points
The IoC component of the Spring Framework has been designed for extension.
There is typically no need for an application developer to subclass any of the various
BeanFactory or
ApplicationContext
implementation classes. The Spring IoC container can be infinitely extended by
plugging in implementations of special integration interfaces. The next few sections are
devoted to detailing all of these various integration interfaces.
3.7.1. Customizing beans using BeanPostProcessors
The first extension point that we will look at is the
BeanPostProcessor interface. This interface defines
a number of
callback methods that you as an application
developer can implement in order to provide your own (or override the containers default)
instantiation logic, dependency-resolution logic, and so forth. If you want to do
some custom logic after the Spring container has finished instantiating, configuring
and otherwise initializing a bean, you can plug in one or more
BeanPostProcessor implementations.
You can configure multiple
BeanPostProcessors if you wish.
You can control the order in which these
BeanPostProcessors
execute by setting the
'order' property (you can only set this property
if the
BeanPostProcessor
implements the
Ordered interface; if you write your own
BeanPostProcessor you should consider implementing the
Ordered interface too); consult the Javadocs for the
BeanPostProcessor and
Ordered
interfaces for more details.
| Note |
|---|
BeanPostProcessors operate on bean (or object)
instances; that is to say, the Spring IoC container will
have instantiated a bean instance for you, and then
BeanPostProcessors get a chance to do their stuff.
If you want to change the actual bean definition (that is the recipe that
defines the bean), then you rather need to use a
BeanFactoryPostProcessor (described below in the
section entitled Section 3.7.2, “Customizing configuration metadata with BeanFactoryPostProcessors”.
Also, BeanPostProcessors are scoped per-container.
This is only relevant if you are using container hierarchies. If you define a
BeanPostProcessor in one container, it will
only do its stuff on the beans in that container. Beans that
are defined in another container will not be post-processed by
BeanPostProcessors in another container, even if both containers
are part of the same hierarchy. |
The
org.springframework.beans.factory.config.BeanPostProcessor
interface consists of exactly two callback methods. When such a class is
registered as a post-processor with the container (see below for how this registration
is effected), for each bean instance that is created by the container, the post-processor
will get a callback from the container both
before any container
initialization methods (such as
afterPropertiesSet and any declared
init method) are called, and also afterwards. The post-processor is free to do what it
wishes with the bean instance, including ignoring the callback completely. A bean
post-processor will typically check for marker interfaces, or do something such as wrap
a bean with a proxy; some of the Spring AOP infrastructure classes are implemented as bean
post-processors and they do this proxy-wrapping logic.
It is important to know that a
BeanFactory treats bean
post-processors slightly differently than an
ApplicationContext.
An
ApplicationContext will
automatically detect
any beans which are defined in the configuration metadata which is supplied to it that
implement the
BeanPostProcessor interface, and register them
as post-processors, to be then called appropriately by the container on bean creation. Nothing
else needs to be done other than deploying the post-processors in a similar fashion to any
other bean. On the other hand, when using a
BeanFactory
implementation, bean post-processors explicitly have to be registered, with code like this:
ConfigurableBeanFactory factory = new XmlBeanFactory(...);
// now register any needed BeanPostProcessor instances
MyBeanPostProcessor postProcessor = new MyBeanPostProcessor();
factory.addBeanPostProcessor(postProcessor);
// now start using the factory
This explicit registration step is not convenient, and this is one of the
reasons why the various
ApplicationContext
implementations are preferred above plain
BeanFactory
implementations in the vast majority of Spring-backed applications, especially
when using
BeanPostProcessors.
| BeanPostProcessors and AOP auto-proxying |
|---|
Classes that implement the BeanPostProcessor
interface are special, and so they are treated differently
by the container. All BeanPostProcessors
and their directly referenced beans will be instantiated
on startup, as part of the special startup phase of the
ApplicationContext, then
all those BeanPostProcessors will be registered
in a sorted fashion - and applied to all further beans. Since AOP
auto-proxying is implemented as a BeanPostProcessor
itself, no BeanPostProcessors or directly
referenced beans are eligible for auto-proxying (and thus will not have
aspects 'woven' into them.
For any such bean, you should see an info log message:
“Bean 'foo' is not eligible for getting processed by all BeanPostProcessors (for example: not eligible for auto-proxying)”. |
Find below some examples of how to write, register, and use
BeanPostProcessors in the context of an
ApplicationContext.
3.7.1.1. Example: Hello World, BeanPostProcessor-style
This first example is hardly compelling, but serves to illustrate basic
usage. All we are going to do is code a custom
BeanPostProcessor
implementation that simply invokes the
toString()
method of each bean as it is created by the container and prints the resulting
string to the system console. Yes, it is not hugely useful, but serves to get
the basic concepts across before we move into the second example which
is actually useful.
Find below the custom
BeanPostProcessor
implementation class definition:
package scripting;
import org.springframework.beans.factory.config.BeanPostProcessor;
import org.springframework.beans.BeansException;
public class InstantiationTracingBeanPostProcessor implements BeanPostProcessor {
// simply return the instantiated bean as-is
public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
return bean; // we could potentially return any object reference here...
}
public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
System.out.println("Bean '" + beanName + "' created : " + bean.toString());
return bean;
}
}
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:lang="http://www.springframework.org/schema/lang"
xsi:schemaLocation="
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd
http://www.springframework.org/schema/lang http://www.springframework.org/schema/lang/spring-lang-2.0.xsd">
<lang:groovy id="messenger"
script-source="classpath:org/springframework/scripting/groovy/Messenger.groovy">
<lang:property name="message" value="Fiona Apple Is Just So Dreamy."/>
</lang:groovy>
<!--
when the above bean ('messenger') is instantiated, this custom
BeanPostProcessor implementation will output the fact to the system console
-->
<bean class="scripting.InstantiationTracingBeanPostProcessor"/>
</beans>
Notice how the
InstantiationTracingBeanPostProcessor is
simply defined; it doesn't even have a name, and because it is a bean it can be
dependency injected just like any other bean. (The above configuration also just so
happens to define a bean that is backed by a Groovy script. The Spring 2.0 dynamic
language support is detailed in the chapter entitled
Chapter 24, Dynamic language support.)
Find below a small driver script to exercise the above code and
configuration;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import org.springframework.scripting.Messenger;
public final class Boot {
public static void main(final String[] args) throws Exception {
ApplicationContext ctx = new ClassPathXmlApplicationContext("scripting/beans.xml");
Messenger messenger = (Messenger) ctx.getBean("messenger");
System.out.println(messenger);
}
}
The output of executing the above program will be (something like) this:
Bean 'messenger' created : org.springframework.scripting.groovy.GroovyMessenger@272961
org.springframework.scripting.groovy.GroovyMessenger@272961
3.7.1.2. Example: The RequiredAnnotationBeanPostProcessor
Using marker interfaces or annotations in conjunction with a custom
BeanPostProcessor implementation is a common
means of extending the Spring IoC container. This next example is a bit of
a cop-out, in that you are directed to the section entitled
Section 25.3.1, “@Required” which demonstrates the usage of a
custom
BeanPostProcessor implementation that
ships with the Spring distribution which ensures that JavaBean properties on
beans that are marked with an (arbitrary) annotation are actually (configured to
be) dependency-injected with a value.
3.7.2. Customizing configuration metadata with BeanFactoryPostProcessors
The next extension point that we will look at is the
org.springframework.beans.factory.config.BeanFactoryPostProcessor.
The semantics of this interface are similar to the
BeanPostProcessor,
with one major difference:
BeanFactoryPostProcessors operate on the
bean configuration metadata; that is, the Spring IoC container will
allow
BeanFactoryPostProcessors to read the configuration metadata
and potentially change it
before the container has actually
instantiated any other beans.
You can configure multiple
BeanFactoryPostProcessors if you wish.
You can control the order in which these
BeanFactoryPostProcessors
execute by setting the
'order' property (you can only set this property
if the
BeanFactoryPostProcessor
implements the
Ordered interface; if you write your own
BeanFactoryPostProcessor you should consider
implementing the
Ordered interface too); consult
the Javadocs for the
BeanFactoryPostProcessor
and
Ordered interfaces for more details.
| Note |
|---|
If you want to change the actual bean instances
(the objects that are created from the configuration metadata), then
you rather need to use a BeanPostProcessor
(described above in the section entitled
Section 3.7.1, “Customizing beans using BeanPostProcessors”.
Also, BeanFactoryPostProcessors are scoped
per-container. This is only relevant if you are
using container hierarchies. If you define a
BeanFactoryPostProcessor in one container,
it will only do its stuff on the bean definitions in
that container. Bean definitions in another container will not be
post-processed by BeanFactoryPostProcessors in another
container, even if both containers are part of the same hierarchy. |
A bean factory post-processor is executed manually (in the case of a
BeanFactory) or automatically (in the case of an
ApplicationContext) to apply changes of
some sort to the configuration metadata that defines a container. Spring
includes a number of pre-existing bean factory post-processors, such as
PropertyResourceConfigurer and
PropertyPlaceholderConfigurer, both described below,
and
BeanNameAutoProxyCreator, which is very useful for wrapping
other beans transactionally or with any other kind of proxy, as described
later in this manual. The
BeanFactoryPostProcessor
can be used to add custom property editors.
In a
BeanFactory, the process of applying a
BeanFactoryPostProcessor is manual, and will be
similar to this:
XmlBeanFactory factory = new XmlBeanFactory(new FileSystemResource("beans.xml"));
// bring in some property values from a Properties file
PropertyPlaceholderConfigurer cfg = new PropertyPlaceholderConfigurer();
cfg.setLocation(new FileSystemResource("jdbc.properties"));
// now actually do the replacement
cfg.postProcessBeanFactory(factory);
This explicit registration step is not convenient, and this is one of the
reasons why the various
ApplicationContext
implementations are preferred above plain
BeanFactory
implementations in the vast majority of Spring-backed applications, especially
when using
BeanFactoryPostProcessors.
An
ApplicationContext will detect any beans which
are deployed into it which implement the
BeanFactoryPostProcessor
interface, and automatically use them as bean factory post-processors, at the appropriate
time. Nothing else needs to be done other than deploying these post-processor in a similar
fashion to any other bean.
| Note |
|---|
| Just as in the case of BeanPostProcessors, you typically
don't want to have BeanFactoryPostProcessors
marked as being lazily-initialized. If they are marked as such, then the Spring
container will never instantiate them, and thus they won't get a chance
to apply their custom logic. If you are using the
'default-lazy-init' attribute on the declaration of your
<beans/> element, be sure to mark your various
BeanFactoryPostProcessor bean definitions with
'lazy-init="false"'.
|
3.7.2.1. Example: the PropertyPlaceholderConfigurer
The
PropertyPlaceholderConfigurer
is used to externalize property values from a
BeanFactory definition, into another
separate file in the standard Java
Properties format.
This is useful to allow the person deploying an application to customize
enviroment-specific properties (for example database URLs, usernames and passwords), without the
complexity or risk of modifying the main XML definition file or files for the
container.
Consider the following XML-based configuration metadata fragment, where a
DataSource with placeholder values is defined. We
will configure some properties from an external
Properties
file, and at runtime, we will apply a
PropertyPlaceholderConfigurer to the metadata which will
replace some properties of the datasource:
<bean class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
<property name="locations">
<value>classpath:com/foo/jdbc.properties</value>
</property>
</bean>
<bean id="dataSource" destroy-method="close"
class="org.apache.commons.dbcp.BasicDataSource">
<property name="driverClassName" value="${jdbc.driverClassName}"/>
<property name="url" value="${jdbc.url}"/>
<property name="username" value="${jdbc.username}"/>
<property name="password" value="${jdbc.password}"/>
</bean>
The actual values come from another file in the standard Java
Properties format:
jdbc.driverClassName=org.hsqldb.jdbcDriver
jdbc.url=jdbc:hsqldb:hsql://production:9002
jdbc.username=sa
jdbc.password=root
The
PropertyPlaceholderConfigurer doesn't only
look for properties in the
Properties file you
specify, but also checks against the Java
System
properties if it cannot find a property you are trying to use. This
behavior can be customized by setting the
systemPropertiesMode
property of the configurer. It has three values, one to tell the configurer
to always override, one to let it
never override and
one to let it override only if the property cannot be found in the
properties file specified. Please consult the Javadoc for the
PropertyPlaceholderConfigurer for more information.
| Class name substitution |
|---|
The PropertyPlaceholderConfigurer
can be used to substitute class names, which is sometimes useful
when you have to pick a particular implementation class at runtime.
For example:
<bean class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
<property name="locations">
<value>classpath:com/foo/strategy.properties</value>
</property>
<property name="properties">
<value>custom.strategy.class=com.foo.DefaultStrategy</value>
</property>
</bean>
<bean id="serviceStrategy" class="${custom.strategy.class}"/>
If the class is unable to be resolved at runtime to a valid class,
resolution of the bean will fail once it is about to be created
(which is during the preInstantiateSingletons()
phase of an ApplicationContext for a
non-lazy-init bean.) |
3.7.2.2. Example: the PropertyOverrideConfigurer
The
PropertyOverrideConfigurer, another bean
factory post-processor, is similar to the
PropertyPlaceholderConfigurer, but in contrast to the
latter, the original definitions can have default values or no values at
all for bean properties. If an overriding
Properties file
does not have an entry for a certain bean property, the default context definition
is used.
Note that the bean factory definition is
not
aware of being overridden, so it is not immediately obvious when looking
at the XML definition file that the override configurer is being used.
In case that there are multiple
PropertyOverrideConfigurer
instances that define different values for the same bean property, the last one
will win (due to the overriding mechanism).
Properties file configuration lines are expected to be in the format:
beanName.property=value
An example properties file might look like this:
dataSource.driverClassName=com.mysql.jdbc.Driver
dataSource.url=jdbc:mysql:mydb
This example file would be usable against a container definition
which contains a bean called
dataSource,
which has
driver and
url
properties.
Note that compound property names are also supported, as long as
every component of the path except the final property being overridden is
already non-null (presumably initialized by the constructors). In this
example...
foo.fred.bob.sammy=123
... the
sammy property of the
bob property
of the
fred property of the
foo
bean is being set to the scalar value
123.
3.7.3. Customizing instantiation logic using FactoryBeans
The
org.springframework.beans.factory.FactoryBean
interface is to be implemented by objects that
are themselves
factories.
The
FactoryBean interface is a point of pluggability
into the Spring IoC containers instantiation logic. If you have some complex
initialization code that is better expressed in Java as opposed to a (potentially)
verbose amount of XML, you can create your own
FactoryBean,
write the complex initialization inside that class, and then plug your custom
FactoryBean into the container.
The
FactoryBean interface provides three methods:
- Object getObject(): has to return an
instance of the object this factory creates. The instance can
possibly be shared (depending on whether this factory returns
singletons or prototypes).
- boolean isSingleton(): has to return
true if this FactoryBean
returns singletons, false otherwise
- Class getObjectType(): has to return
either the object type returned by the
getObject() method or null
if the type isn't known in advance
The
FactoryBean concept and interface
is used in a number of places within the Spring Framework; at the time of writing
there are over 50 implementations of the
FactoryBean
interface that ship with Spring itself.
Finally, there is sometimes a need to ask a container for an actual
FactoryBean instance itself, not the bean it produces.
This may be achieved by prepending the bean id with
'&'
(sans quotes) when calling the
getBean method of
the
BeanFactory (including
ApplicationContext). So for a given
FactoryBean with an id of
myBean,
invoking
getBean("myBean") on the container will return the
product of the
FactoryBean, but invoking
getBean("&myBean") will return the
FactoryBean instance itself.
3.8. The ApplicationContext
While the
beans package provides basic
functionality for managing and manipulating beans, often in a programmatic
way, the
context package adds
ApplicationContext,
which enhances
BeanFactory functionality in a more
framework-oriented style. Many users will use
ApplicationContext in a completely declarative fashion,
not even having to create it manually, but instead relying on support classes such as
ContextLoader to automatically start an
ApplicationContext as part of the
normal startup process of a J2EE web-app. Of course, it is still possible
to programmatically create an ApplicationContext.
The basis for the context package is the
ApplicationContext interface, located in the
org.springframework.context package. Deriving from the
BeanFactory interface, it provides all the functionality of
BeanFactory. To allow working in a more framework-oriented
fashion, using layering and hierarchical contexts, the context package also provides the
following functionality:
- MessageSource, providing access to
messages in i18n-style
- Access to resources, such as URLs and files
- Event propagation to beans implementing
the ApplicationListener interface
- Loading of multiple (hierarchical)
contexts, allowing each to be focused on one particular
layer, for example the web layer of an application
As the
ApplicationContext includes all functionality of the
BeanFactory, it is generally recommended that it be used over the
BeanFactory, except for a few limited situations such as perhaps in an
Applet, where memory consumption might be critical, and a few extra
kilobytes might make a difference. The following sections describe
functionality that
ApplicationContext adds to basic
BeanFactory
capabilities.
3.8.1. Internationalization using MessageSources
The
ApplicationContext interface extends an interface called
MessageSource, and therefore provides messaging (i18n
or internationalization) functionality. Together with the
HierarchicalMessageSource, capable of resolving
hierarchical messages, these are the basic interfaces Spring provides to
do message resolution. Let's quickly review the methods defined there:
- String getMessage(String code, Object[] args,
String default, Locale loc): the basic method used to
retrieve a message from the MessageSource. When no message is
found for the specified locale, the default message is used. Any
arguments passed in are used as replacement values, using the
MessageFormat functionality provided by the
standard library.
- String getMessage(String code, Object[] args,
Locale loc): essentially the same as the previous
method, but with one difference: no default message can be
specified; if the message cannot be found, a
NoSuchMessageException is thrown.
- String getMessage(MessageSourceResolvable
resolvable, Locale locale): all properties used in the
methods above are also wrapped in a class named
MessageSourceResolvable, which you can use via
this method.
When an
ApplicationContext gets loaded, it automatically searches
for a
MessageSource bean defined in the context. The bean has to have
the name
'messageSource'. If such a bean is found, all calls to the methods
described above will be delegated to the message source that was found. If no message source was
found, the
ApplicationContext attempts to see if it has a parent
containing a bean with the same name. If so, it uses that bean as the
MessageSource. If it can't find any source for messages, an empty
StaticMessageSource will be instantiated in order to be able to accept calls
to the methods defined above.
Spring currently provides two
MessageSource
implementations. These are the
ResourceBundleMessageSource and the
StaticMessageSource. Both implement
HierarchicalMessageSource in order to do nested messaging.
The
StaticMessageSource is hardly ever used but provides programmatic
ways to add messages to the source. The
ResourceBundleMessageSource is
more interesting and is the one we will provide an example for:
<beans>
<bean id="messageSource"
class="org.springframework.context.support.ResourceBundleMessageSource">
<property name="basenames">
<list>
<value>format</value>
<value>exceptions</value>
<value>windows</value>
</list>
</property>
</bean>
</beans>
This assumes you have three resource bundles defined on your
classpath called
format,
exceptions and
windows. Using the
JDK standard way of resolving messages through ResourceBundles, any
request to resolve a message will be handled. For the purposes of the example,
lets assume the contents of two of the above resource bundle files are...
# in 'format.properties'
message=Alligators rock!
# in 'exceptions.properties'
argument.required=The '{0}' argument is required.
Some (admittedly trivial) driver code to exercise the
MessageSource functionality can be found below. Remember
that all
ApplicationContext implementations are also
MessageSource implementations and so can be cast to the
MessageSource interface.
public static void main(String[] args) {
MessageSource resources = new ClassPathXmlApplicationContext("beans.xml");
String message = resources.getMessage("message", null, "Default", null);
System.out.println(message);
}
The resulting output from the above program will be...
Alligators rock!
So to summarize, the
MessageSource is defined in a file
called
'beans.xml' (this file exists at the root of your classpath).
The
'messageSource' bean definition refers to a number
of resource bundles via it's
basenames property; the three files
that are passed in the list to the
basenames property exist as files
at the root of your classpath (and are called
format.properties,
exceptions.properties,
and
windows.properties respectively).
Lets look at another example, and this time we will look at passing arguments
to the message lookup; these arguments will be converted into strings and inserted
into placeholders in the lookup message. This is perhaps best explained with an
example:
<beans>
<!-- this MessageSource is being used in a web application -->
<bean id="messageSource" class="org.springframework.context.support.ResourceBundleMessageSource">
<property name="baseName" value="WEB-INF/test-messages"/>
</bean>
<!-- let's inject the above MessageSource into this POJO -->
<bean id="example" class="com.foo.Example">
<property name="messages" ref="messageSource"/>
</bean>
</beans>
public class Example {
private MessageSource messages;
public void setMessages(MessageSource messages) {
this.messages = messages;
}
public void execute() {
String message = this.messages.getMessage("argument.required",
new Object [] {"userDao"}, "Required", null);
System.out.println(message);
}
}
The resulting output from the invocation of the
execute()
method will be...
The 'userDao' argument is required.
With regard to internationalization (i18n), Spring's various
MessageResource implementations follow the same locale resolution
and fallback rules as the standard JDK
ResourceBundle. In short, and continuing with
the example
'messageSource' defined previously, if you want to resolve
messages against the British (en-GB) locale, you would create files called
format_en_GB.properties,
exceptions_en_GB.properties,
and
windows_en_GB.properties respectively.
Locale resolution is typically going to be managed by the surrounding environment
of the application. For the purpose of this example though, we'll just manually
specify the locale that we want to resolve our (British) messages against.
# in 'exceptions_en_GB.properties'
argument.required=Ebagum lad, the '{0}' argument is required, I say, required.
public static void main(final String[] args) {
MessageSource resources = new ClassPathXmlApplicationContext("beans.xml");
String message = resources.getMessage("argument.required",
new Object [] {"userDao"}, "Required", Locale.UK);
System.out.println(message);
}
The resulting output from the running of the above program will be...
Ebagum lad, the 'userDao' argument is required, I say, required.
The
MessageSourceAware interface can also be used to acquire
a reference to any
MessageSource that has been defined. Any bean
that is defined in an
ApplicationContext that implements the
MessageSourceAware interface will be injected with the
application context's
MessageSource when it (the bean) is being
created and configured.
Event handling in the
ApplicationContext is provided
through the
ApplicationEvent class and
ApplicationListener interface. If a bean which
implements the
ApplicationListener interface is
deployed into the context, every time an
ApplicationEvent
gets published to the
ApplicationContext, that
bean will be notified. Essentially, this is the standard
Observer
design pattern. Spring provides three standard events:
Table 3.5. Built-in Events
| Event | Explanation |
|---|
| ContextRefreshedEvent | Published when the
ApplicationContext is
initialized or refreshed. Initialized here means that all
beans are loaded, singletons are pre-instantiated and the
ApplicationContext is ready
for use. |
| ContextClosedEvent | Published when the ApplicationContext is closed,
using the close() method on the
ApplicationContext. Closed here means
that singleton beans (only!) are destroyed. |
| RequestHandledEvent | A web-specific event telling all beans that a HTTP
request has been serviced (this will be published
after the request has been finished).
Note that this event is only applicable for web applications
using Spring's DispatcherServlet. |
Implementing custom events can be done as well. Simply call the
publishEvent() method on the
ApplicationContext,
specifying a parameter which is an instance of your custom event class
implementing
ApplicationEvent. Event listeners receive events
synchronously. This means the
publishEvent() method blocks until all
listeners have finished processing the event (it is possible to supply
an alternate event publishing strategy via a
ApplicationEventMulticaster implementation).
Furthermore, when a listener receives an event it operates inside the transaction context of
the publisher, if a transaction context is available.
Let's look at an example. First, the
ApplicationContext:
<bean id="emailer" class="example.EmailBean">
<property name="blackList">
<list>
<value>black@list.org</value>
<value>white@list.org</value>
<value>john@doe.org</value>
</list>
</property>
</bean>
<bean id="blackListListener" class="example.BlackListNotifier">
<property name="notificationAddress" value="spam@list.org"/>
</bean>
Now, let's look at the actual classes:
public class EmailBean implements ApplicationContextAware {
private List blackList;
private ApplicationContext ctx;
public void setBlackList(List blackList) {
this.blackList = blackList;
}
public void setApplicationContext(ApplicationContext ctx) {
this.ctx = ctx;
}
public void sendEmail(String address, String text) {
if (blackList.contains(address)) {
BlackListEvent evt = new BlackListEvent(address, text);
ctx.publishEvent(evt);
return;
}
// send email...
}
}
public class BlackListNotifier implements ApplicationListener {
private String notificationAddress;
public void setNotificationAddress(String notificationAddress) {
this.notificationAddress = notificationAddress;
}
public void onApplicationEvent(ApplicationEvent evt) {
if (evt instanceof BlackListEvent) {
// notify appropriate person...
}
}
}
Of course, this particular example could probably be
implemented in better ways (perhaps by using AOP features), but it
should be sufficient to illustrate the basic event mechanism.
3.8.3. Convenient access to low-level resources
For optimal usage and understanding of application contexts, users
should generally familiarize themselves with Spring's
Resource abstraction, as described
in the chapter entitled
Chapter 4, Resources.
An application context is a
ResourceLoader,
able to be used to load
Resources. A
Resource is essentially a
java.net.URL on steroids (in fact, it just wraps and
uses a URL where appropriate), which can be used to obtain low-level
resources from almost any location in a transparent fashion, including
from the classpath, a filesystem location, anywhere describable with a
standard URL, and some other variations. If the resource location string
is a simple path without any special prefixes, where those resources
come from is specific and appropriate to the actual application context
type.
A bean deployed into the application context may implement the
special marker interface,
ResourceLoaderAware, to be
automatically called back at initialization time with the application
context itself passed in as the
ResourceLoader.
A bean may also expose properties of type
Resource, to be used to access static resources, and
expect that they will be injected into it like any other properties. The
person deploying the bean may specify those
Resource
properties as simple String paths, and rely on a special JavaBean
PropertyEditor that is automatically registered
by the context, to convert those text strings to actual
Resource
objects.
The location path or paths supplied to an
ApplicationContext
constructor are actually resource strings, and in simple form are
treated appropriately to the specific context implementation (
ClassPathXmlApplicationContext treats a simple location path as a
classpath location), but may also be used with special prefixes to force
loading of definitions from the classpath or a URL, regardless of the
actual context type.
3.8.4. Convenient ApplicationContext instantiation for web applications
As opposed to the
BeanFactory, which will
often be created programmatically,
ApplicationContext
instances can be created declaratively using for example a
ContextLoader. Of course you can also create
ApplicationContext instances programmatically
using one of the
ApplicationContext implementations.
First, let's examine the
ContextLoader mechanism
and its implementations.
The
ContextLoader mechanism comes in two
flavors: the
ContextLoaderListener and the
ContextLoaderServlet. They both have the same
functionality but differ in that the listener version cannot be reliably used in
Servlet 2.3 containers. Since the Servlet 2.4 specification, servlet context
listeners are required to execute immediately after the servlet context for the
web application has been created and is available to service the first request
(and also when the servlet context is about to be shut down): as such a
servlet context listener is an ideal place to initialize the Spring
ApplicationContext. It is up to you
as to which one you use, but all things being equal you should probably
prefer
ContextLoaderListener; for more information on
compatibility, have a look at the Javadoc for the
ContextLoaderServlet.
You can register an
ApplicationContext using the
ContextLoaderListener as follows:
<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>/WEB-INF/daoContext.xml /WEB-INF/applicationContext.xml</param-value>
</context-param>
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>
<!-- or use the ContextLoaderServlet instead of the above listener
<servlet>
<servlet-name>context</servlet-name>
<servlet-class>org.springframework.web.context.ContextLoaderServlet</servlet-class>
<load-on-startup>1</load-on-startup>
</servlet>
-->
The listener inspects the
'contextConfigLocation' parameter. If the parameter does not exist,
the listener will use
/WEB-INF/applicationContext.xml as a default.
When it
does exist, it will separate the String using
predefined delimiters (comma, semi-colon and whitespace) and use the values as
locations where application contexts will be searched for. Ant-style path patterns
are supported as well: e.g.
/WEB-INF/*Context.xml
(for all files whose name ends with "Context.xml", residing in the "WEB-INF" directory)
or
/WEB-INF/**/*Context.xml (for all such files in any subdirectory
of "WEB-INF").
The
ContextLoaderServlet can be used instead of
the
ContextLoaderListener. The servlet will use the
'contextConfigLocation' parameter just as the listener does.
3.9. Glue code and the evil singleton
The majority of the code inside an application is best written in a
DI style, where that code is served out of a Spring IoC container, has its own
dependencies supplied by the container when it is created, and is
completely unaware of the container. However, for the small glue layers of
code that are sometimes needed to tie other code together, there is
sometimes a need for singleton (or quasi-singleton) style access to a
Spring IoC container. For example,
third party code may try to construct new objects directly (
Class.forName()
style), without the ability to force it to get these objects out of a
Spring IoC container. If the object constructed by the
third party code is just a small stub or proxy, which then uses a singleton style access to a
Spring IoC container to get a real object to delegate to, then inversion of control has
still been achieved for the majority of the code (the object coming out of the
container); thus most code is still unaware of the container or how it is accessed, and
remains uncoupled from other code, with all ensuing benefits. EJBs may also use this stub/proxy
approach to delegate to a plain Java implementation object, coming out of
a Spring IoC container. While the Spring IoC container itself ideally does not have to be a
singleton, it may be unrealistic in terms of memory usage or initialization times (when
using beans in the Spring IoC container such as a Hibernate
SessionFactory) for each bean to use its own, non-singleton
Spring IoC container.
As another example, in complex J2EE apps with multiple layers
(various JAR files, EJBs, and WAR files packaged as an EAR),
with each layer having its own Spring IoC container definition
(effectively forming a hierarchy), the preferred approach when
there is only one web-app (WAR) in the top hierarchy is to simply
create one composite Spring IoC container from the multiple XML definition
files from each layer. All of the various Spring IoC container implementations
may be constructed from multiple definition files in this fashion.
However, if there are multiple sibling web-applications at the root
of the hierarchy, it is problematic to create a
Spring IoC container for each web-application
which consists of mostly identical bean definitions from lower layers,
as there may be issues due to increased memory usage, issues with
creating multiple copies of beans which take a long time to initialize
(for example a Hibernate
SessionFactory), and
possible issues due to side-effects. As an alternative, classes such as
ContextSingletonBeanFactoryLocator
or
SingletonBeanFactoryLocator
may be used to demand-load multiple hierarchical (that is one container is the parent of
another) Spring IoC container instances in a singleton fashion,
which may then be used as the parents of the web-application Spring IoC
container instances. The result is that bean definitions for lower layers are
loaded only as needed, and loaded only once.
3.9.1. Using the Singleton-helper classes
You can see a detailed example of their usage in
SingletonBeanFactoryLocator
and
ContextSingletonBeanFactoryLocator
by viewing their respective Javadocs.
As mentioned in the
chapter on EJBs, the
Spring convenience base classes for EJBs normally use a non-singleton
BeanFactoryLocator implementation, which is
easily replaced by the use of
SingletonBeanFactoryLocator
and
ContextSingletonBeanFactoryLocator if there is
a need.
