Why Struts 2

The new version Struts 2.0 is a combination of the Sturts action framework and Webwork. According to the Struts 2.0.1 release announcement, some key features are:
 
Simplified Design - Programming the abstract classes instead of interfaces is one of design problem of struts1 framework that has been resolved in the struts 2 framework. Most of the Struts 2 classes are based on interfaces and most of its core interfaces are HTTP independent. Struts 2 Action classes are framework independent and are simplified to look as simple POJOs. Framework components are tried to keep loosely coupled.

Simplified Actions - Actions are simple POJOs. Any java class with execute() method can be used as an Action class. Even we don't need to implement interfaces always. Inversion of Control is introduced while developing the action classes. This make the actions to be neutral to the underlying framework .

No more ActionForms - ActionForms feature is no more known to the struts2 framework. Simple JavaBean flavored actions are used to put properties directly. No need to use all String properties.

Simplified testability - Struts 2 Actions are HTTP independent and framework neutral. This enables to test struts applications very easily without resorting to mock objects.

Intelligent Defaults - Most configuration elements have a default value which can be set according to the need. Even there are xml-based default configuration files that can be overridden according to the need.

Improved results - Unlike ActionForwards, Struts 2 Results provide flexibility to create multiple type of outputs and in actual it helps to prepare the response.

Better Tag features - Struts 2 tags enables to add style sheet-driven markup capabilities, so that we can create consistent pages with less code. Struts 2 tags are more capable and result oriented. Struts 2 tag markup can be altered by changing an underlying stylesheet. Individual tag markup can be changed by editing a FreeMarker template. Both JSP and FreeMarker tags are fully supported.

Annotations introduced : Applications in struts 2 can use Java 5 annotations as an alternative to XML and Java properties configuration. Annotations minimize the use of xml.

Stateful Checkboxes - Struts 2 checkboxes do not require special handling for false values.

QuickStart - Many changes can be made on the fly without restarting a web container.

customizing controller - Struts 1 lets to customize the request processor per module, Struts 2 lets to customize the request handling per action, if desired.
Easy Spring integration - Struts 2 Actions are Spring-aware. Just need to add Spring beans!

Easy plugins - Struts 2 extensions can be added by dropping in a JAR. No manual configuration is required!

AJAX support - The AJAX theme gives interactive applications a significant boost.The framework provides a set of tags to help you ajaxify your applications, even on Dojo. The AJAX features include:
AJAX Client Side Validation
Remote form submission support (works with the submit tag as well)
An advanced div template that provides dynamic reloading of partial HTML
An advanced template that provides the ability to load and evaluate JavaScript remotely
An AJAX-only tabbed Panel implementation
A rich pub-sub event model
Interactive auto complete tag

Struts 1.x Vs Struts 2.x

In the following section, we are going to compare the various features between the two frameworks. Struts 2.x is very simple as compared to struts 1.x, few of its excellent features are:

1. Servlet Dependency:
Actions in Struts1 have dependencies on the servlet API since the HttpServletRequest and HttpServletResponse objects are passed to the execute method when an Action is invoked but in case of Struts 2, Actions are not container dependent because they are made simple POJOs. In struts 2, the servlet contexts are represented as simple Maps which allows actions to be tested in isolation. Struts 2 Actions can access the original request and response, if required. However, other architectural elements reduce or eliminate the need to access the HttpServetRequest or HttpServletResponse directly.

2. Action classes
Programming the abstract classes instead of interfaces is one of design issues of struts1 framework that has been resolved in the struts 2 framework. Struts1 Action classes needs to extend framework dependent abstract base class. But in case of Struts 2 Action class may or may not implement interfaces to enable optional and custom services. In case of Struts 2 , Actions are not container dependent because they are made simple POJOs. Struts 2 provides a base ActionSupport class to implement commonly used interfaces. Albeit, the Action interface is not required. Any POJO object with an execute signature can be used as an Struts 2 Action object.

3. Validation
Struts1 and Struts 2 both supports the manual validation via a validate method.Struts1 uses validate method on the ActionForm, or validates through an extension to the Commons Validator. However, 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.

4. Threading Model
In Struts1, Action resources must be thread-safe or synchronized. So Actions are singletons and thread-safe, there should 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 Struts1 Actions and requires extra care to develop. However in case of 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.)

5. Testability
Testing Struts1 applications are a bit complex. A major hurdle to test Struts1 Actions is that the execute method because it exposes the Servlet API. A third-party extension, Struts TestCase, offers a set of mock object for Struts1. But the Struts 2 Actions can be tested by instantiating the Action, setting properties and invoking methods. Dependency Injection support also makes testing simpler. Actions in struts2 are simple POJOs and are framework independent, hence testability is quite easy in struts2.

6. Harvesting Input
Struts1 uses an ActionForm object to capture input. And all ActionForms needs to extend a framework dependent base class. JavaBeans cannot be used as ActionForms, so the developers have to create redundant classes to capture input. However Struts 2 uses Action properties (as input properties independent of underlying framework) that eliminates the need for a second input object, hence reduces redundancy. Additionally in struts2, 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. Even rich object types, including business or domain objects, can be used as input/output objects.

7. Expression Language
Struts1 integrates with JSTL, so it uses the JSTL-EL. The struts1 EL has basic object graph traversal, but relatively weak collection and indexed property support. Struts 2 can also use JSTL, however it supports a more powerful and flexible expression language called "Object Graph Notation Language" (OGNL).

8. Binding values into views
In the view section, Struts1 uses the standard JSP mechanism to bind objects (processed from the model section) into the page context to access. However 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 the reuse of views across a range of types which may have the same property name but different property types.

9. Type Conversion
Usually, Struts1 ActionForm properties are all Strings. Struts1 uses Commons-Beanutils for type conversion. These type converters are per-class and not configurable per instance. However Struts 2 uses OGNL for type conversion. The framework includes converters for basic and common object types and primitives.

10. Control Of Action Execution
Struts1 supports separate Request Processor (lifecycles) for each module, but all the Actions in a module must share the same lifecycle. However 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.