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Interceptors
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Added by victorsosa, last edited by Ted Husted on Feb 12, 2007 (view change)

| The default Interceptor stack is designed to serve the needs of most applications. Most applications will not need to add Interceptors or change the Interceptor stack.

Many Actions share common concerns. Some Actions need input validated. Other Actions may need a file upload to be pre-processed. Another Action might need protection from a double submit. Many Actions need drop-down lists and other controls pre-populated before the page displays.

The Struts Action Framework makes it easy to share solutions to these concerns using an "Interceptor" strategy. When you request a resource that maps to an "action", the framework invokes the Action object. But, before the Action is executed, the invocation can be intercepted by another object. After the Action executes, the invocation could be intercepted again. Unsurprisingly, we call these objects "Interceptors."

## Understanding Interceptors

Interceptors can execute code before and after an Action is invoked. Most of the framework's core functionality is implemented as Interceptors. Features like double-submit guards, type conversion, object population, validation, file upload, page preparation, and more, are all implemented with the help of Interceptors. Each and every Interceptor is pluggable, so you can decide exactly which features an Action needs to support.

Interceptors can be configured on a per-action basis. Your own custom Interceptors can be mixed-and-matched with the Interceptors bundled with the framework. Interceptors "set the stage" for the Action classes, doing much of the "heavy lifting" before the Action executes.

Action Lifecyle

In some cases, an Interceptor might keep an Action from firing, because of a double-submit or because validation failed. Interceptors can also change the state of an Action before it executes.

The Interceptors are defined in a stack that specifies the execution order. In some cases, the order of the Interceptors on the stack can be very important.

## Configuring Interceptors

struts.xml

    <package name="default" extends="struts-default">
       <interceptors>
           <interceptor name="timer" class=".."/>
           <interceptor name="logger" class=".."/>
       </interceptors>

       <action name="login"
          class="tutorial.Login">
            <interceptor-ref name="timer"/>
            <interceptor-ref name="logger"/>
             <result name="input">login.jsp</result>
             <result name="success"
                type="redirect-action">/secure/home</result>
       </action>
    </package>

## Stacking Interceptors

With most web applications, we find ourselves wanting to apply the same set of Interceptors over and over again. Rather than reiterate the same list of Interceptors, we can bundle these Interceptors together using an Interceptor Stack.

struts.xml

    <package name="default" extends="struts-default">
       <interceptors>
            <interceptor name="timer" class=".."/>
            <interceptor name="logger" class=".."/>
            <interceptor-stack name="myStack">
               <interceptor-ref name="timer"/>
               <interceptor-ref name="logger"/>
            </interceptor-stack>
        </interceptors>

    <action name="login"
         class="tutuorial.Login">
             <interceptor-ref name="myStack"/>
             <result name="input">login.jsp</result>
             <result name="success"
                 type="redirect-action">/secure/home</result>
    </action>
    </package>

Looking inside `struts-default.xml`, we can see how it's done.

### The Default Configuration

struts-default.xml

    <!DOCTYPE struts PUBLIC
        "-//Apache Software Foundation//DTD Struts Configuration 2.0//EN"
        "http://struts.apache.org/dtds/struts-2.0.dtd">

    <!-- // START SNIPPET: struts-default -->
    <struts>
        <package name="struts-default">
            <result-types>
                <result-type name="chain" class="com.opensymphony.xwork.ActionChainResult"/>
                <result-type name="dispatcher" class="org.apache.struts2.dispatcher.ServletDispatcherResult"
                             default="true"/>
                <result-type name="freemarker" class="org.apache.struts2.views.freemarker.FreemarkerResult"/>
                <result-type name="httpheader" class="org.apache.struts2.dispatcher.HttpHeaderResult"/>
                <result-type name="jasper" class="org.apache.struts2.views.jasperreports.JasperReportsResult"/>
                <result-type name="redirect" class="org.apache.struts2.dispatcher.ServletRedirectResult"/>
                <result-type name="redirect-action"
                             class="org.apache.struts2.dispatcher.ServletActionRedirectResult"/>
                <result-type name="stream" class="org.apache.struts2.dispatcher.StreamResult"/>
                <result-type name="velocity" class="org.apache.struts2.dispatcher.VelocityResult"/>
                <result-type name="xslt" class="org.apache.struts2.views.xslt.XSLTResult"/>
            </result-types>

            <interceptors>
                <interceptor name="alias" class="com.opensymphony.xwork.interceptor.AliasInterceptor"/>
                <interceptor name="autowiring"
                             class="com.opensymphony.xwork.spring.interceptor.ActionAutowiringInterceptor"/>
                <interceptor name="chain" class="com.opensymphony.xwork.interceptor.ChainingInterceptor"/>
                <interceptor name="component" class="com.opensymphony.xwork.interceptor.component.ComponentInterceptor"/>
                <interceptor name="conversionError"
                             class="org.apache.struts2.interceptor.WebWorkConversionErrorInterceptor"/>
                <interceptor name="external-ref" class="com.opensymphony.xwork.interceptor.ExternalReferencesInterceptor"/>
                <interceptor name="execAndWait" class="corg.apache.struts2.interceptor.ExecuteAndWaitInterceptor"/>
                <interceptor name="exception" class="com.opensymphony.xwork.interceptor.ExceptionMappingInterceptor"/>
                <interceptor name="fileUpload" class="com.opensymphony.webwork.interceptor.FileUploadInterceptor"/>
                <interceptor name="i18n" class="com.opensymphony.xwork.interceptor.I18nInterceptor"/>
                <interceptor name="logger" class="com.opensymphony.xwork.interceptor.LoggingInterceptor"/>
                <interceptor name="model-driven" class="com.opensymphony.xwork.interceptor.ModelDrivenInterceptor"/>
                <interceptor name="params" class="com.opensymphony.xwork.interceptor.ParametersInterceptor"/>
                <interceptor name="prepare" class="com.opensymphony.xwork.interceptor.PrepareInterceptor"/>
                <interceptor name="static-params" class="com.opensymphony.xwork.interceptor.StaticParametersInterceptor"/>
                <interceptor name="servlet-config" class="org.apache.struts.action2.interceptor.ServletConfigInterceptor"/>
                <interceptor name="sessionAutowiring"
                             class="org.apache.struts2.spring.interceptor.SessionContextAutowiringInterceptor"/>
                <interceptor name="timer" class="com.opensymphony.xwork.interceptor.TimerInterceptor"/>
                <interceptor name="token" class="com.opensymphony.webwork.interceptor.TokenInterceptor"/>
                <interceptor name="token-session"
                             class="com.opensymphony.webwork.interceptor.TokenSessionStoreInterceptor"/>
                <interceptor name="validation" class="com.opensymphony.xwork.validator.ValidationInterceptor"/>
                <interceptor name="workflow" class="com.opensymphony.xwork.interceptor.DefaultWorkflowInterceptor"/>

                <!-- Basic stack -->
                <interceptor-stack name="basicStack">
                    <interceptor-ref name="exception"/>
                    <interceptor-ref name="servlet-config"/>
                    <interceptor-ref name="prepare"/>
                    <interceptor-ref name="static-params"/>
                    <interceptor-ref name="params"/>
                    <interceptor-ref name="conversionError"/>
                </interceptor-stack>

                <!-- Sample validation and workflow stack -->
                <interceptor-stack name="validationWorkflowStack">
                    <interceptor-ref name="basicStack"/>
                    <interceptor-ref name="validation"/>
                    <interceptor-ref name="workflow"/>
                </interceptor-stack>

                <!-- Sample file upload stack -->
                <interceptor-stack name="fileUploadStack">
                    <interceptor-ref name="fileUpload"/>
                    <interceptor-ref name="basicStack"/>
                </interceptor-stack>

                <!-- Sample model-driven stack  -->
                <interceptor-stack name="modelDrivenStack">
                    <interceptor-ref name="model-driven"/>
                    <interceptor-ref name="basicStack"/>
                </interceptor-stack>

                <!-- Sample action chaining stack -->
                <interceptor-stack name="chainStack">
                    <interceptor-ref name="chain"/>
                    <interceptor-ref name="basicStack"/>
                </interceptor-stack>

                <!-- Sample i18n stack -->
                <interceptor-stack name="chainStack">
                    <interceptor-ref name="i18n"/>
                    <interceptor-ref name="basicStack"/>
                </interceptor-stack>

                <!-- Sample execute and wait stack.
                     Note: execAndWait should always be the *last* interceptor. -->
                <interceptor-stack name="executeAndWaitStack">
                    <interceptor-ref name="basicStack"/>
                    <interceptor-ref name="execAndWait"/>
                </interceptor-stack>

                <!-- A complete stack with all the common interceptors in place.
                     Generally, this stack should be the one you use, though it
                     may process additional stuff you don't need, which could
                     lead to some performance problems. Also, the ordering can be
                     switched around (ex: if you wish to have your components
                     before prepare() is called, you'd need to move the component
                     interceptor up -->
                <interceptor-stack name="defaultStack">
                    <interceptor-ref name="exception"/>
                    <interceptor-ref name="alias"/>
                    <interceptor-ref name="prepare"/>
                    <interceptor-ref name="servlet-config"/>
                    <interceptor-ref name="i18n"/>
                    <interceptor-ref name="chain"/>
                    <interceptor-ref name="model-driven"/>
                    <interceptor-ref name="fileUpload"/>
                    <interceptor-ref name="static-params"/>
                    <interceptor-ref name="params"/>
                    <interceptor-ref name="conversionError"/>
                    <interceptor-ref name="validation"/>
                    <interceptor-ref name="workflow"/>
                </interceptor-stack>

                <!-- The completeStack is here for backwards compatibility for
                     applications that still refer to the defaultStack by the
                     old name -->
                <interceptor-stack name="completeStack">
                    <interceptor-ref name="defaultStack"/>
                </interceptor-stack>
            </interceptors>

            <default-interceptor-ref name="defaultStack"/>
        </package>
    </struts>
    <!-- // END SNIPPET: struts-default -->

Since the `struts-default.xml` is included in the application's configuration by default, all of the predefined interceptors and stacks are available "out of the box".

## Framework Interceptors

Interceptor classes are also defined using a key-value pair specified in the Struts configuration file. The names specified below come specified in struts-default.xml. If you extend the `struts-default` package, then you can use the names below. Otherwise, they must be defined in your package with a name-class pair specified in the <interceptors> tag.

Interceptor | Name | Description
--- | --- | ---
Alias Interceptor | alias | Converts similar parameters that may be named differently between requests.
Chaining Interceptor | chain | Makes the previous Action's properties available to the current Action. Commonly used together with <result type="chain"> (in the previous Action).
Checkbox Interceptor | checkbox | Adds automatic checkbox handling code that detect an unchecked checkbox and add it as a parameter with a default (usually 'false') value. Uses a specially named hidden field to detect unsubmitted checkboxes. The default unchecked value is overridable for non-boolean value'd checkboxes. Minor source formatting and cleaned out an empty directory.
Conversion Error Interceptor | conversionError | Adds conversion errors from the ActionContext to the Action's field errors
Create Session Interceptor | createSession | Create an HttpSession automatically, useful with certain Interceptors that require a HttpSession to work properly (like the TokenInterceptor)
DebuggingInterceptor | debugging | Provides several different debugging screens to provide insight into the data behind the page.
Execute and Wait Interceptor | execAndWait | Executes the Action in the background and then sends the user off to an intermediate waiting page.
Exception Interceptor | exception | Maps exceptions to a result.
File Upload Interceptor | fileUpload | An Interceptor that adds easy access to file upload support.
I18n Interceptor | i18n | Remembers the locale selected for a user's session.
Logger Interceptor | logger | Outputs the name of the Action.
Message Store Interceptor | store | Store and retrieve action messages / errors / field errors for action that implements ValidationAware interface into session.
Model Driven Interceptor | model-driven | If the Action implements ModelDriven, pushes the `getModel` Result onto the Value Stack.
Scoped Model Driven Interceptor | scoped-model-driven | If the Action implements ScopedModelDriven, the interceptor retrieves and stores the model from a scope and sets it on the action calling `setModel`.
Parameters Interceptor | params | Sets the request parameters onto the Action.
Prepare Interceptor | prepare | If the Action implements Preparable, calls its `prepare` method.
Scope Interceptor | scope | Simple mechanism for storing Action state in the session or application scope.
Servlet Config Interceptor | servlet-config | Provide access to Maps representing HttpServletRequest and HttpServletResponse.
Static Parameters Interceptor | static-params | Sets the `struts.xml` defined parameters onto the action. These are the <param> tags that are direct children of the <action> tag.
Roles Interceptor | roles | Action will only be executed if the user has the correct JAAS role.
Timer Interceptor | timer | Outputs how long the Action takes to execute (including nested Interceptors and View)
Token Interceptor | token | Checks for valid token presence in Action, prevents duplicate form submission.
Token Session Interceptor | token-session | Same as Token Interceptor, but stores the submitted data in session when handed an invalid token
Validation Interceptor | validation | Performs validation using the validators defined in action-validation.xml
Workflow Interceptor | workflow | Calls the `validate` method in your Action class. If Action errors are created then it returns the `INPUT` view.
Parameter Filter Interceptor | N/A | Removes parameters from the list of those available to Actions
Profiling Interceptor | profiling | Activate profiling through parameter

### Method Filtering

An abstract `Interceptor` that is applied to selectively according to specified included/excluded method lists.

Setable parameters are as follows:

- excludeMethods - methods name to be excluded
- includeMethods - methods name to be included

NOTE: If method name are available in both includeMethods and excludeMethods, it will still be considered as an included method. In short includeMethods takes precedence over excludeMethods.

Interceptors that extends this capability would be :-

- TokenInterceptor
- TokenSessionStoreInterceptor
- DefaultWorkflowInterceptor
- ValidationInterceptor

### Interceptor Parameter Overriding

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="servlet-config"/>
        <interceptor-ref name="prepare"/>
        <interceptor-ref name="i18n"/>
        <interceptor-ref name="chain"/>
        <interceptor-ref name="model-driven"/>
        <interceptor-ref name="fileUpload"/>
        <interceptor-ref name="static-params"/>
        <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 refer to an existing interceptor-stack, namely default-stack in this example, and override the validator and workflow interceptor excludeMethods typically in this case. Note that in the tag, the name attribute contains a dot (.) the word before the dot(.) specifies the interceptor name whose parameter is to be overriden 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 name attribute is used to indicate an interceptor stack which makes sense as if it is refering to the interceptor itself it would be just using Method 1 describe above.

### Order of Interceptor Execution

Interceptors provide an excellent means to wrap before/after processing. The concept reduces code duplication (think AOP).

    <interceptor-stack name="xaStack">
      <interceptor-ref name="thisWillRunFirstInterceptor"/>
      <interceptor-ref name="thisWillRunNextInterceptor"/>
      <interceptor-ref name="followedByThisInterceptor"/>
      <interceptor-ref name="thisWillRunLastInterceptor"/>
    </interceptor-stack>

Note that some Interceptors will interrupt the stack/chain/flow ... so the order is very important.

Interceptors implementing `com.opensymphony.xwork.interceptor.PreResultListener` will run after the Action executes but before the Result executes.

    thisWillRunFirstInterceptor
      thisWillRunNextInterceptor
        followedByThisInterceptor
          thisWillRunLastInterceptor
            MyAction1
            MyAction2 (chain)
            MyPreResultListener
            MyResult (result)
          thisWillRunLastInterceptor
        followedByThisInterceptor
      thisWillRunNextInterceptor
    thisWillRunFirstInterceptor

## FAQ

- How do we configure an Interceptor to be used with every Action?
- How do we get access to the session?
- How can we access the HttpServletRequest?
- How can we access the HttpServletResponse?
- How can we access request parameters passed into an Action?
- How do we access static parameters from an Action?
- Can we access an Action's Result?
- How do I obtain security details (JAAS)?
- Why isn't our Prepare interceptor being executed?
- How do we upload files?

## Next: Action Chaining

Children Show Children Hide Children
Servlet Config Interceptor (Apache Struts 2 Documentation)
I18n Interceptor (Apache Struts 2 Documentation)
Scope Interceptor (Apache Struts 2 Documentation)
Token Session Interceptor (Apache Struts 2 Documentation)
Create Session Interceptor (Apache Struts 2 Documentation)
Workflow Interceptor (Apache Struts 2 Documentation)
Exception Interceptor (Apache Struts 2 Documentation)
Non-IoC version of OpenSessionInViewInterceptor (Apache Struts 2 Documentation)
Alias Interceptor (Apache Struts 2 Documentation)
Parameters Interceptor (Apache Struts 2 Documentation)
Model Driven Interceptor (Apache Struts 2 Documentation)
Conversion Error Interceptor (Apache Struts 2 Documentation)
Prepare Interceptor (Apache Struts 2 Documentation)
Parameter Filter Interceptor (Apache Struts 2 Documentation)
Validation Interceptor (Apache Struts 2 Documentation)
Timer Interceptor (Apache Struts 2 Documentation)
Chaining Interceptor (Apache Struts 2 Documentation)
Static Parameters Interceptor (Apache Struts 2 Documentation)
Token Interceptor (Apache Struts 2 Documentation)
File Upload Interceptor (Apache Struts 2 Documentation)
Execute and Wait Interceptor (Apache Struts 2 Documentation)
Logger Interceptor (Apache Struts 2 Documentation)
Message Store Interceptor (Apache Struts 2 Documentation)
Checkbox Interceptor (Apache Struts 2 Documentation)
Scoped Model Driven Interceptor (Apache Struts 2 Documentation)
Roles Interceptor (Apache Struts 2 Documentation)
DebuggingInterceptor (Apache Struts 2 Documentation)
Profiling Interceptor (Apache Struts 2 Documentation)