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## com.opensymphony.xwork2.interceptor
Interface Interceptor

All Superinterfaces:
  Serializable

All Known Implementing Classes:
  AbstractInterceptor, ActionAutowiringInterceptor, AliasInterceptor, AnnotationParameterFilterIntereptor, AnnotationWorkflowInterceptor, ChainingInterceptor, ConversionErrorInterceptor, DefaultWorkflowInterceptor, ExceptionMappingInterceptor, I18nInterceptor, LoggingInterceptor, MethodFilterInterceptor, MockInterceptor, ModelDrivenInterceptor, ParameterFilterInterceptor, ParameterRemoverInterceptor, ParametersInterceptor, PrepareInterceptor, ScopedModelDrivenInterceptor, StaticParametersInterceptor, TimerInterceptor, ValidationInterceptor

    public interface Interceptor

extends Serializable

An interceptor is a stateless class that follows the interceptor pattern, as found in `javax.servlet.Filter` and in AOP languages.

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.

Interceptors must be stateless and not assume that a new instance will be created for each request or Action. Interceptors may choose to either short-circuit the `ActionInvocation` execution and return a return code (such as `Action.SUCCESS`), or it may choose to do some processing before and/or after delegating the rest of the procesing using `ActionInvocation.invoke()`.

Interceptor's parameter could be overriden through the following ways :-

Method 1:

     <action name="myAction" class="myActionClass">
         <interceptor-ref name="exception"/>
         <interceptor-ref name="alias"/>
         <interceptor-ref name="params"/>
         <interceptor-ref name="servletConfig"/>
         <interceptor-ref name="prepare"/>
         <interceptor-ref name="i18n"/>
         <interceptor-ref name="chain"/>
         <interceptor-ref name="modelDriven"/>
         <interceptor-ref name="fileUpload"/>
         <interceptor-ref name="staticParams"/>
         <interceptor-ref name="params"/>
         <interceptor-ref name="conversionError"/>
         <interceptor-ref name="validation">
         <param name="excludeMethods">myValidationExcudeMethod</param>
         </interceptor-ref>
         <interceptor-ref name="workflow">
         <param name="excludeMethods">myWorkflowExcludeMethod</param>
         </interceptor-ref>
     </action>

Method 2:

     <action name="myAction" class="myActionClass">
       <interceptor-ref name="defaultStack">
         <param name="validation.excludeMethods">myValidationExcludeMethod</param>
         <param name="workflow.excludeMethods">myWorkflowExcludeMethod</param>
       </interceptor-ref>
     </action>

In the first method, the whole default stack is copied and the parameter then changed accordingly.

In the second method, the 'interceptor-ref' refer to an existing interceptor-stack, namely defaultStack in this example, and override the validator and workflow interceptor excludeMethods typically in this case. Note that in the 'param' tag, the name attribute contains a dot (.) the word before the dot(.) specifies the interceptor name whose parameter is to be overridden and the word after the dot (.) specifies the parameter itself. Essetially it is as follows :-

        <interceptor-name>.<parameter-name>

Note also that in this case the 'interceptor-ref' name attribute is used to indicate an interceptor stack which makes sense as if it is referring to the interceptor itself it would be just using Method 1 describe above.

Nested Interceptor param overriding

Interceptor stack parameter overriding could be nested into as many level as possible, though it would be advisable not to nest it too deep as to avoid confusion, For example,

     <interceptor name="interceptor1" class="foo.bar.Interceptor1" />
     <interceptor name="interceptor2" class="foo.bar.Interceptor2" />
     <interceptor name="interceptor3" class="foo.bar.Interceptor3" />
     <interceptor name="interceptor4" class="foo.bar.Interceptor4" />
     <interceptor-stack name="stack1">
         <interceptor-ref name="interceptor1" />
     </interceptor-stack>
     <interceptor-stack name="stack2">
         <interceptor-ref name="intercetor2" />
         <interceptor-ref name="stack1" />
     </interceptor-stack>
     <interceptor-stack name="stack3">
         <interceptor-ref name="interceptor3" />
         <interceptor-ref name="stack2" />
     </interceptor-stack>
     <interceptor-stack name="stack4">
         <interceptor-ref name="interceptor4" />
         <interceptor-ref name="stack3" />
      </interceptor-stack>

Assuming the interceptor has the following properties

Interceptor | property
Interceptor1 | param1
Interceptor2 | param2
Interceptor3 | param3
Interceptor4 | param4

We could override them as follows :-

        <action ... >
            <!-- to override parameters of interceptor located directly in the stack  -->
            <interceptor-ref name="stack4">
               <param name="interceptor4.param4"> ... </param>
            </interceptor-ref>
        </action>

        <action ... >
            <!-- to override parameters of interceptor located under nested stack -->
            <interceptor-ref name="stack4">
                <param name="stack3.interceptor3.param3"> ... </param>
                <param name="stack3.stack2.interceptor2.param2"> ... </param>
                <param name="stack3.stack2.stack1.interceptor1.param1"> ... </param>
            </interceptor-ref>
        </action>

Version:
  $Date$ $Id$
Author:
  Jason Carreira, tmjee

Method Summary
`void` | `destroy()` Called to let an interceptor clean up any resources it has allocated.
`void` | `init()` Called after an interceptor is created, but before any requests are processed using `intercept` , giving the Interceptor a chance to initialize any needed resources.
`String` | `intercept(ActionInvocation invocation)` Allows the Interceptor to do some processing on the request before and/or after the rest of the processing of the request by the `ActionInvocation` or to short-circuit the processing and just return a String return code.

Method Detail

### destroy

    void destroy()

  Called to let an interceptor clean up any resources it has allocated.

### init

    void init()

  Called after an interceptor is created, but before any requests are processed using `intercept` , giving the Interceptor a chance to initialize any needed resources.

### intercept

    String intercept(ActionInvocation invocation)
                     throws Exception

  Allows the Interceptor to do some processing on the request before and/or after the rest of the processing of the request by the `ActionInvocation` or to short-circuit the processing and just return a String return code.

  Parameters:
    `invocation` - the action invocation
  Returns:
    the return code, either returned from `ActionInvocation.invoke()`, or from the interceptor itself.
  Throws:
    `Exception` - any system-level error, as defined in `Action.execute()`.

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