Interceptors

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.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 -->

By including, struts-default.xml in an application's struts.xml, we make all the predefined interceptors and stacks available.

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 Interceptora that require a HttpSession to work properly (like the TokenInterceptor)
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.
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

Method Filtering

Error formatting macro: snippet: java.lang.NullPointerException

Interceptor Parameter Overriding

Error formatting macro: snippet: java.lang.NullPointerException

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

Child pages

Understanding Interceptors

Configuring Interceptors

Stacking Interceptors

The Default Configuration

Framework Interceptors

Method Filtering

Interceptor Parameter Overriding

Order of Interceptor Execution

FAQ

Next: Result Types