XWork Core Concepts

XWork is based on a number of core concepts that helps to explain how the framework works. The core concepts can be broken down into two parts: Architecture Concepts and Terminology.

Architecture Concepts

• Explain Command Driven Architecture (in general)
• Explain the implementation in XWork

Terminology

Actions

Actions are classes that get invoked in response to a request, execute some code and return a Result. Actions implement at a minimum a single method, execute(), that defines the entry point called by the framework. This method allows developers to define a unit of work that will be executed each time the Action is called.

ActionContext

The ActionContext provides access to the execution environment in the form of named objects during an Action invocation. A new ActionContext is created for each invocation allowing developers to access/modify these properties in a thread safe manner. The ActionContext makes a number of properties available that are typically set to appropriate values by the framework. In WebWork 2 for example, the ActionContext session map wraps an underlying HttpSession object. This allows access to environment specific properties without tying the core framework to a specific execution environment. For more information, see ActionContext in Basics.

Interceptors

In XWork, Interceptors are objects that dynamically intercept Action invocations. They provide the developer with the opportunity to define code that can be executed before and/or after the execution of an action. They also have the ability to prevent an action from executing. Interceptors provide developers a way to encapulate common functionality in a re-usable form that can be applied to one or more Actions. See Interceptors for further details.

Stacks

To handle the case where developers want to apply more than a single Interceptor to an Action, Stacks have been introduced. Stacks are an ordered list of Interceptors and/or other Stacks that get applied when an Action is invoked. Stacks centralize the declaration of Interceptors and provide a convenient way to configure mutiple actions.

Results

Results are string constants that Actions return to indicate the status of an Action execution. A standard set of Results are defined by default: error, input, login, none and success. Developers are, of course, free to create their own Results to indicate more application specific cases.

Result Types

Result Types are classes that determine what happens after an Action executes and a Result is returned. Developers are free to create their own Result Types according to the needs of their application or environment. In WebWork 2 for example, Servlet and Velocity Result Types have been created to handle rendering views in web applications.

Packages

Packages are a way to group Actions, Results, Result Types, Interceptors and Stacks into a logical unit that shares a common configuration. Packages are similiar to objects in that they can be extended and have individual parts overridden by "sub" packages.

ValueStack

The ValueStack is a stack implementation built on top of an OGNL core. The OGNL expression language can be used to traverse the stack and retrieve the desired object. The OGNL expression language provides a number of additional features including: automatic type conversion, method invocation and object comparisons. For more information, see the OGNL Website.

Components

XWork provides the ComponentManager interface (and a corresponding implementation in the DefaultComponentManager class) to apply a design pattern known as Inversion of Control (or IoC for short). In a nutshell, the IoC pattern allows a parent object (in this case XWork's ComponentManager instance) to control a client object (usually an action, but it could be any object that implements the appropriate enabler). See Components for further details.

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