Apache Struts had become the de facto standard for web development for several years. Struts2 is the next generation of Apache Struts, developed by Patrick Lightbody. It is a mixture of Struts and WebWork.
When we speak about WebWork we are really referencing two projects – XWork and WebWork. XWork is a generic command framework. It provides many of the core features such as actions, validation and interceptors, and is completely execution context independent. XWork also provides an internal dependency inject mechanism that is used for configuration and factory implementation management.
WebWork, on the other hand, is a completely context dependent. It provides a wrapper around XWork with the context that is needed when working on web applications, along with specific implementations that make web developer easier.
The goal of Struts2 is simple – to make web development easier for the developer. To achieve this goal Struts2 provides features to reduce XML configuration via intelligent defaults, utilizes annotations and provides conventions over configuration.
Actions are now POJOs which increases testability and reduces coupling in the framework, and HTML form field data is converted to proper types for the action to use. Still further decreasing coupling is request processing has been made more modular by allowing a series of interceptors (custom or Struts2 provided) to provide pre-processing and post-processing functionality.
There are a lot of features in Struts 2 which are of interest. I’ll discuss a few here.
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.
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.)
Struts 2 Actions can be tested by instantiating the Action, setting properties, and invoking methods. Dependency Injection support also makes testing simpler.
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.
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 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.
Struts 2 can use JSTL, but the framework also supports a more powerful and flexible expression language called “Object Graph Notation Language” (OGNL).
The complete language guide can be found here.
Binding values into views
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.
Struts 2 uses OGNL for type conversion. The framework includes converters for basic and common object types and primitives.
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 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.