Struts Framework

Struts1 vs Struts2

Feature	Struts 1	Struts 2
Action classes	Struts 1 requires Action classes to extend an abstract base class. A common problem in Struts 1 is programming to abstract classes instead of interfaces.	An Struts 2 Action may implement an Action interface, along with other interfaces to enable optional and custom services. Struts 2 provides a base ActionSupport class to implement commonly used interfaces. Albeit, the Action interface is not required. Any POJO object with a execute signature can be used as an Struts 2 Action object.
Threading Model	Struts 1 Actions are singletons and must be thread-safe since there will only be one instance of a class to handle all requests for that Action. The singleton strategy places restrictions on what can be done with Struts 1 Actions and requires extra care to develop. Action resources must be thread-safe or synchronized.	Struts 2 Action objects are instantiated for each request, so there are no thread-safety issues. (In practice, servlet containers generate many throw-away objects per request, and one more object does not impose a performance penalty or impact garbage collection.)
Servlet Dependency	Struts 1 Actions have dependencies on the servlet API since the HttpServletRequest and HttpServletResponse is passed to the execute method when an Action is invoked.	Struts 2 Actions are not coupled to a container. Most often the servlet contexts are represented as simple Maps, allowing Actions to be tested in isolation. Struts 2 Actions can still access the original request and response, if required. However, other architectural elements reduce or eliminate the need to access the HttpServetRequest or HttpServletResponse directly.
Testability	A major hurdle to testing Struts 1 Actions is that the execute method exposes the Servlet API. A third-party extension, Struts TestCase, offers a set of mock object for Struts 1.	Struts 2 Actions can be tested by instantiating the Action, setting properties, and invoking methods. Dependency Injection support also makes testing simpler.
Harvesting Input	Struts 1 uses an ActionForm object to capture input. Like Actions, all ActionForms must extend a base class. Since  other JavaBeans cannot be used as ActionForms, developers often create redundant classes to capture input. DynaBeans can used as an alternative to creating conventional ActionForm classes, but, here too, developers may be redescribing existing JavaBeans.	Struts 2 uses Action properties as input properties, eliminating the need for a second input object. Input properties may be rich object types which may have their own properties. The Action properties can be accessed from the web page via the taglibs. Struts 2 also supports the ActionForm pattern, as well as POJO form objects and POJO Actions. Rich object types, including business or domain objects, can be used as input/output objects. The ModelDriven feature simplifies taglb references to POJO input objects.
Expression Language	Struts 1 integrates with JSTL, so it uses the JSTL EL. The EL has basic object graph traversal, but relatively weak collection and indexed property support.	Struts 2 can use JSTL, but the framework also supports a more powerful and flexible expression language called "Object Graph Notation Language" (OGNL).
Binding values into views	Struts 1 uses the standard JSP mechanism for binding objects into the page context for access.	Struts 2 uses a "ValueStack" technology so that the taglibs can access values without coupling your view to the object type it is rendering. The ValueStack strategy allows reuse of views across a range of types which may have the same property name but different property types.
Type Conversion	Struts 1 ActionForm properties are usually all Strings. Struts 1 uses Commons-Beanutils for type conversion. Converters are per-class, and not configurable per instance.	Struts 2 uses OGNL for type conversion. The framework includes converters for basic and common object types and primitives.
Validation	Struts 1 supports manual validation via a validate method on the ActionForm, or through an extension to the Commons Validator. Classes can have different validation contexts for the same class, but cannot chain to validations on sub-objects.	Struts 2 supports manual validation via the validate method and the XWork Validation framework. The Xwork Validation Framework supports chaining validation into sub-properties using the validations defined for the properties class type and the validation context.
Control Of Action Execution	Struts 1 supports separate Request Processors (lifecycles) for each module, but all the Actions in the module must share the same lifecycle.	Struts 2 supports creating different lifecycles on a per Action basis via Interceptor Stacks. Custom stacks can be created and used with different Actions, as needed.

MVC nice article with Observable pattern

4 down vote accepted

First, you need to understand the Observer pattern. The basic idea is that you define an object that notifies interested parties when it changes. (Note that the Observer pattern is a bit more general - Observables notify Observers about "some event". For MVC, that event is "something changed") First you define a contract between the OBSERVABLE and OBSERVER package aaa; // AN OBSERVER INTERFACE // This is a contract between an interested party (the OBSERVER) and //   the thing it would like to know has changed (the OBSERVABLE) // The OBSERVABLE will call this method whenever its data changes public interface SomethingChangedListener {     void somethingChanged(String name, Object newValue); } Then you define the OBSERVABLE package aaa; import java.util.ArrayList; import java.util.List; // An OBSERVABLE class public class Person {     // STEP 1: keep track of "who cares"     //         outsiders with interest implement the observer interface     //         and register with the person to indicate that they care     private List<SomethingChangedListener> listeners = new ArrayList<SomethingChangedListener>();     public void addSomethingChangedListener(SomethingChangedListener scl) {         listeners.add(scl);     }     public void removeSomethingChangedListener(SomethingChangedListener scl) {         listeners.remove(scl);     }     // STEP 2: be able to notify those observers by calling a method in the observer interface     protected void fireSomethingChanged(String name, Object newValue) {         for (SomethingChangedListener scl : listeners) {             scl.somethingChanged(name, newValue);         }     }     // STEP 3: whenever the data changes, notify the observers     private String name;     private int age;     public int getAge() {         return age;     }     public void setAge(int age) {         this.age = age;         fireSomethingChanged("age", name);     }     public String getName() {         return name;     }     public void setName(String name) {         this.name = name;         fireSomethingChanged("name", name);     } } This allows any other object to "listen" for changes on the thing it's interested in, as long as it implements that observer interface. For example: package aaa; public class Test1 {     private static class TestObserver implements SomethingChangedListener {         @Override public void somethingChanged(String name, Object newValue) {             System.out.println("Property '" + name + "' changed to '" + newValue + "'");         }     }     public static void main(String[] args) {         Person p = new Person();         p.addSomethingChangedListener(new TestObserver());         p.setName("Scott");         p.setAge(43);     } } Creates an instance of the observable (the Person), registers an observer (TestObserver), and then interacts with the Observable. When run, we see Property 'name' changed to 'Scott' Property 'age' changed to 'Scott' So far so good. Now let's call the Person our MODEL. The MODEL represents the data that we want to manipulate and view. We can do this in a "user interface". The user interface can be, but is not limited to: A Graphical User Interface (GUI) A Command-Line Interface (CLI) A Web Application (HTML, JSP, ASP, etc) A Web Service (SOAP, REST, etc) The code for each of these types of user interfaces (UIs) can be drastically different, but the concepts are the same. The user interface (UI) allows a user (a person or another computer for example) to see information about the model and make changes to that information. The "View" is the part of the UI that displays the information for the user. It reads data from the model and formats it in some way to present it. If the model changes, the View must be updated. To accomplish this, it registers observers with the model. Those observers simply refresh the relevant parts of the presentation in the View. Now what happens if the user wants to make a change? We define a "Controller" in the UI as code that interprets user interaction with that UI. For example, if the user types in the "name" field, the controller may interpret that as "change the value of 'name' to the text the user has typed. The controller makes a person.setName(textFromField); call to update the model. Remember what that setName() method does? It notifies the observers of the change. This will cause the UI to update its view. Note that "view" and "controller" do not need to be separate classes; they're often combined. It's really the roles of "view" (the part of the UI that displays model data) and "controller" (the part of the UI that interprets user interaction and updates the model) that are important to understand. In some settings, such as Web Applications, the view and controller are very separate. The controller interprets the HTTP requests that are made to the server and updates the mode. The view renders HTML responses to the user. (If you're doing an AJAX application, the design is a bit more similar to a GUI) The cool thing about the MVC separation (Model vs UI) is that you can add or remove UIs from the Model at any time, and can have multiple UIs on the same model. If data is changed in one UI, all other UIs are updated to reflect the change. Cool, eh?