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 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

<?xml version="1.0" encoding="UTF-8" ?>
<!--
/*
 * $Id$
 *
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *  http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
-->

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

<struts>
    <bean class="com.opensymphony.xwork2.ObjectFactory" name="xwork" />
    <bean type="com.opensymphony.xwork2.ObjectFactory" name="struts" class="org.apache.struts2.impl.StrutsObjectFactory" />

    <bean type="com.opensymphony.xwork2.ActionProxyFactory" name="xwork" class="com.opensymphony.xwork2.DefaultActionProxyFactory"/>
    <bean type="com.opensymphony.xwork2.ActionProxyFactory" name="struts" class="org.apache.struts2.impl.StrutsActionProxyFactory"/>

    <bean type="com.opensymphony.xwork2.util.ObjectTypeDeterminer" name="tiger" class="com.opensymphony.xwork2.util.GenericsObjectTypeDeterminer"/>
    <bean type="com.opensymphony.xwork2.util.ObjectTypeDeterminer" name="notiger" class="com.opensymphony.xwork2.util.DefaultObjectTypeDeterminer"/>
    <bean type="com.opensymphony.xwork2.util.ObjectTypeDeterminer" name="struts" class="com.opensymphony.xwork2.util.DefaultObjectTypeDeterminer"/>

    <bean type="org.apache.struts2.dispatcher.mapper.ActionMapper" name="struts" class="org.apache.struts2.dispatcher.mapper.DefaultActionMapper" />
    <bean type="org.apache.struts2.dispatcher.mapper.ActionMapper" name="composite" class="org.apache.struts2.dispatcher.mapper.CompositeActionMapper" />
    <bean type="org.apache.struts2.dispatcher.mapper.ActionMapper" name="restful" class="org.apache.struts2.dispatcher.mapper.RestfulActionMapper" />
    <bean type="org.apache.struts2.dispatcher.mapper.ActionMapper" name="restful2" class="org.apache.struts2.dispatcher.mapper.Restful2ActionMapper" />

    <bean type="org.apache.struts2.dispatcher.multipart.MultiPartRequest" name="struts" class="org.apache.struts2.dispatcher.multipart.JakartaMultiPartRequest" scope="default" optional="true"/>
    <bean type="org.apache.struts2.dispatcher.multipart.MultiPartRequest" name="jakarta" class="org.apache.struts2.dispatcher.multipart.JakartaMultiPartRequest" scope="default" optional="true" />

    <bean type="org.apache.struts2.views.TagLibrary" name="s" class="org.apache.struts2.views.DefaultTagLibrary" />

    <bean class="org.apache.struts2.views.freemarker.FreemarkerManager" name="struts" optional="true"/>
    <bean class="org.apache.struts2.views.velocity.VelocityManager" name="struts" optional="true" />

    <bean class="org.apache.struts2.components.template.TemplateEngineManager" />
    <bean type="org.apache.struts2.components.template.TemplateEngine" name="ftl" class="org.apache.struts2.components.template.FreemarkerTemplateEngine" />
    <bean type="org.apache.struts2.components.template.TemplateEngine" name="vm" class="org.apache.struts2.components.template.VelocityTemplateEngine" />
    <bean type="org.apache.struts2.components.template.TemplateEngine" name="jsp" class="org.apache.struts2.components.template.JspTemplateEngine" />

    <bean type="com.opensymphony.xwork2.util.XWorkConverter" name="xwork1" class="com.opensymphony.xwork2.util.XWorkConverter" />
    <bean type="com.opensymphony.xwork2.util.XWorkConverter" name="struts" class="com.opensymphony.xwork2.util.AnnotationXWorkConverter" />

    <bean type="com.opensymphony.xwork2.TextProvider" name="xwork1" class="com.opensymphony.xwork2.TextProviderSupport" />
    <bean type="com.opensymphony.xwork2.TextProvider" name="struts" class="com.opensymphony.xwork2.TextProviderSupport" />

 <bean type="org.apache.struts2.components.UrlRenderer" name="struts" class="org.apache.struts2.components.ServletUrlRenderer"/>
 
    <!--  Only have static injections -->
    <bean class="com.opensymphony.xwork2.ObjectFactory" static="true" />
    <bean class="com.opensymphony.xwork2.util.XWorkConverter" static="true" />
    <bean class="com.opensymphony.xwork2.util.OgnlValueStack" static="true" />
    <bean class="org.apache.struts2.dispatcher.Dispatcher" static="true" />
    <bean class="org.apache.struts2.components.Include" static="true" />
    <bean class="org.apache.struts2.dispatcher.FilterDispatcher" static="true" />
    <bean class="org.apache.struts2.views.util.ContextUtil" static="true" />
    <bean class="org.apache.struts2.views.util.UrlHelper" static="true" />

    <package name="struts-default" abstract="true">
        <result-types>
            <result-type name="chain" class="com.opensymphony.xwork2.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="redirect" class="org.apache.struts2.dispatcher.ServletRedirectResult"/>
            <result-type name="redirectAction" 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-type name="plainText" class="org.apache.struts2.dispatcher.PlainTextResult" />
        </result-types>

        <interceptors>
            <interceptor name="alias" class="com.opensymphony.xwork2.interceptor.AliasInterceptor"/>
            <interceptor name="autowiring" class="com.opensymphony.xwork2.spring.interceptor.ActionAutowiringInterceptor"/>
            <interceptor name="chain" class="com.opensymphony.xwork2.interceptor.ChainingInterceptor"/>
            <interceptor name="conversionError" class="org.apache.struts2.interceptor.StrutsConversionErrorInterceptor"/>
            <interceptor name="createSession" class="org.apache.struts2.interceptor.CreateSessionInterceptor" />
            <interceptor name="debugging" class="org.apache.struts2.interceptor.debugging.DebuggingInterceptor" />
            <interceptor name="externalRef" class="com.opensymphony.xwork2.interceptor.ExternalReferencesInterceptor"/>
            <interceptor name="execAndWait" class="org.apache.struts2.interceptor.ExecuteAndWaitInterceptor"/>
            <interceptor name="exception" class="com.opensymphony.xwork2.interceptor.ExceptionMappingInterceptor"/>
            <interceptor name="fileUpload" class="org.apache.struts2.interceptor.FileUploadInterceptor"/>
            <interceptor name="i18n" class="com.opensymphony.xwork2.interceptor.I18nInterceptor"/>
            <interceptor name="logger" class="com.opensymphony.xwork2.interceptor.LoggingInterceptor"/>
            <interceptor name="modelDriven" class="com.opensymphony.xwork2.interceptor.ModelDrivenInterceptor"/>
            <interceptor name="scopedModelDriven" class="com.opensymphony.xwork2.interceptor.ScopedModelDrivenInterceptor"/>
            <interceptor name="params" class="com.opensymphony.xwork2.interceptor.ParametersInterceptor"/>
            <interceptor name="prepare" class="com.opensymphony.xwork2.interceptor.PrepareInterceptor"/>
            <interceptor name="staticParams" class="com.opensymphony.xwork2.interceptor.StaticParametersInterceptor"/>
            <interceptor name="scope" class="org.apache.struts2.interceptor.ScopeInterceptor"/>
            <interceptor name="servletConfig" class="org.apache.struts2.interceptor.ServletConfigInterceptor"/>
            <interceptor name="sessionAutowiring" class="org.apache.struts2.spring.interceptor.SessionContextAutowiringInterceptor"/>
            <interceptor name="timer" class="com.opensymphony.xwork2.interceptor.TimerInterceptor"/>
            <interceptor name="token" class="org.apache.struts2.interceptor.TokenInterceptor"/>
            <interceptor name="tokenSession" class="org.apache.struts2.interceptor.TokenSessionStoreInterceptor"/>
            <interceptor name="validation" class="org.apache.struts2.interceptor.validation.AnnotationValidationInterceptor"/>
            <interceptor name="workflow" class="com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor"/>
            <interceptor name="store" class="org.apache.struts2.interceptor.MessageStoreInterceptor" />
            <interceptor name="checkbox" class="org.apache.struts2.interceptor.CheckboxInterceptor" />
            <interceptor name="profiling" class="org.apache.struts2.interceptor.ProfilingActivationInterceptor" />
            <interceptor name="roles" class="org.apache.struts2.interceptor.RolesInterceptor" />
            <interceptor name="jsonValidation" class="org.apache.struts2.interceptor.validation.JSONValidationInterceptor" />
            <interceptor name="annotationWorkflow" class="com.opensymphony.xwork2.interceptor.annotations.AnnotationWorkflowInterceptor" />

            <!-- Basic stack -->
            <interceptor-stack name="basicStack">
                <interceptor-ref name="exception"/>
                <interceptor-ref name="servletConfig"/>
                <interceptor-ref name="prepare"/>
                <interceptor-ref name="checkbox"/>
                <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 JSON validation stack -->
            <interceptor-stack name="jsonValidationWorkflowStack">
                <interceptor-ref name="basicStack"/>
                <interceptor-ref name="validation">
                    <param name="excludeMethods">input,back,cancel</param>
                </interceptor-ref>
                <interceptor-ref name="jsonValidation"/>
                <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="modelDriven"/>
                <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="i18nStack">
                <interceptor-ref name="i18n"/>
                <interceptor-ref name="basicStack"/>
            </interceptor-stack>

            <!-- An example of the params-prepare-params trick. This stack
                 is exactly the same as the defaultStack, except that it
                 includes one extra interceptor before the prepare interceptor:
                 the params interceptor.

                 This is useful for when you wish to apply parameters directly
                 to an object that you wish to load externally (such as a DAO
                 or database or service layer), but can't load that object
                 until at least the ID parameter has been loaded. By loading
                 the parameters twice, you can retrieve the object in the
                 prepare() method, allowing the second params interceptor to
                 apply the values on the object. -->
            <interceptor-stack name="paramsPrepareParamsStack">
                <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="checkbox"/>
                <interceptor-ref name="staticParams"/>
                <interceptor-ref name="params"/>
                <interceptor-ref name="conversionError"/>
                <interceptor-ref name="validation">
                    <param name="excludeMethods">input,back,cancel</param>
                </interceptor-ref>
                <interceptor-ref name="workflow">
                    <param name="excludeMethods">input,back,cancel</param>
                </interceptor-ref>
            </interceptor-stack>

            <!-- A complete stack with all the common interceptors in place.
                 Generally, this stack should be the one you use, though it
                 may do more than you need. Also, the ordering can be
                 switched around (ex: if you wish to have your servlet-related
                 objects applied before prepare() is called, you'd need to move
                 servlet-config interceptor up.

                 This stack also excludes from the normal validation and workflow
                 the method names input, back, and cancel. These typically are
                 associated with requests that should not be validated.
                 -->
            <interceptor-stack name="defaultStack">
                <interceptor-ref name="exception"/>
                <interceptor-ref name="alias"/>
                <interceptor-ref name="servletConfig"/>
                <interceptor-ref name="prepare"/>
                <interceptor-ref name="i18n"/>
                <interceptor-ref name="chain"/>
                <interceptor-ref name="debugging"/>
                <interceptor-ref name="profiling"/>
                <interceptor-ref name="scopedModelDriven"/>
                <interceptor-ref name="modelDriven"/>
                <interceptor-ref name="fileUpload"/>
                <interceptor-ref name="checkbox"/>
                <interceptor-ref name="staticParams"/>
                <interceptor-ref name="params">
                  <param name="excludeParams">dojo\..*</param>
                </interceptor-ref>
                <interceptor-ref name="conversionError"/>
                <interceptor-ref name="validation">
                    <param name="excludeMethods">input,back,cancel,browse</param>
                </interceptor-ref>
                <interceptor-ref name="workflow">
                    <param name="excludeMethods">input,back,cancel,browse</param>
                </interceptor-ref>
            </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>

            <!-- Sample execute and wait stack.
                 Note: execAndWait should always be the *last* interceptor. -->
            <interceptor-stack name="executeAndWaitStack">
                <interceptor-ref name="execAndWait">
                    <param name="excludeMethods">input,back,cancel</param>
                </interceptor-ref>
                <interceptor-ref name="defaultStack"/>
                <interceptor-ref name="execAndWait">
                    <param name="excludeMethods">input,back,cancel</param>
                </interceptor-ref>
            </interceptor-stack>

       </interceptors>

        <default-interceptor-ref name="defaultStack"/>
    </package>

</struts>

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.
Cookie Interceptor	cookie	Inject cookie with a certain configurable name / value into action. (Since 2.0.7.)
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	servletConfig	Provide access to Maps representing HttpServletRequest and HttpServletResponse.
Static Parameters Interceptor	staticParams	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	tokenSession	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

Since 2.0.7, Interceptors and Results with hyphenated names were converted to camelCase. (The former model-driven is now modelDriven.) The original hyphenated names are retained as "aliases" until Struts 2.1.0. For clarity, the hyphenated versions are not listed here, but might be referenced in prior versions of the documentation.

Method Filtering

An abstract Interceptor that is applied to filter method names 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.xwork2.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

----------------------------------------------------------------------------------

Points to remember about interceptor

-->The default Interceptor stack is designed to serve the needs of most applications. Most applications will not need to add (register) Interceptors or change the Interceptor stack.
Therefore we don't need to explicitly register default interceptors eith <interceptor-ref> before using them.

->Since there are lot of things to do in FilterDispatcher.However code reside in FilterDispatcher would be modularize into diffrent interceptors plugged in and out in struts.xml file.Hence new code for action processing can be added without recompiling the framework

-->Custom Interceptor must be register before use.However there is no need to register default interceptor

->Unlike actions, interceptors are reused across requests and need to be thread-safe, especially the intercept() method.

-->It should be noted that you can register more than one interceptors inside <package> tag and same time you can call more than one interceptors inside the <action> tag. You can call same interceptor with the different actions.

Interceptors are conceptually the same as servlet filters or the JDKs Proxy class. Interceptors allow for crosscutting functionality to be implemented separately from the action as well as the framework. You can achieve the following using interceptors:

Providing preprocessing logic before the action is called.
Providing postprocessing logic after the action is called.
Catching exceptions so that alternate processing can be performed.

Many of the features provided in the Struts2 framework are implemented using interceptors; examples include exception handling, file uploading, lifecycle callbacks and validation etc. In fact, as Struts2 bases much of its functionality on interceptors, it is not unlikely to have 7 or 8 interceptors assigned per action.

Some common Interceptors:

Struts 2 framework provides a good list of out-of-the-box interceptors that come preconfigured and ready to use. Few of the important interceptors are listed below:

SN	Interceptor & Description
1	alias Allows parameters to have different name aliases across requests.
2	checkbox Assists in managing check boxes by adding a parameter value of false for check boxes that are not checked.
3	conversionError Places error information from converting strings to parameter types into the action's field errors.
4	createSession Automatically creates an HTTP session if one does not already exist.
5	debugging Provides several different debugging screens to the developer.
6	execAndWait Sends the user to an intermediary waiting page while the action executes in the background.
7	exception Maps exceptions that are thrown from an action to a result, allowing automatic exception handling via redirection.
8	fileUpload Facilitates easy file uploading.
9	i18n Keeps track of the selected locale during a user's session.
10	logger Provides simple logging by outputting the name of the action being executed.
11	params Sets the request parameters on the action.
12	prepare This is typically used to do pre-processing work, such as setup database connections.
13	profile Allows simple profiling information to be logged for actions.
14	scope Stores and retrieves the action's state in the session or application scope.
15	ServletConfig Provides the action with access to various servlet-based information.
16	timer Provides simple profiling information in the form of how long the action takes to execute.
17	token Checks the action for a valid token to prevent duplicate formsubmission.
18	validation Provides validation support for actions

Please loook into Struts 2 documentation for complete detail on above mentioned interceptors. But I will show you how to use an interceptor in general in your Struts application.

How to use Interceptors?

Let us see how to use an already existing interceptor to our "Hello World" program. We will use the timer interceptor whose purpose is to measure how long it took to execute an action method. Same time I'm using params interceptor whose purpose is to send the request parameters to the action. You can try your example without using this interceptor and you will find that name property is not being set because parameter is not able to reach to the action.
We will keep HelloWorldAction.java, web.xml, HelloWorld.jsp and index.jsp files as they have been created in Examples chapter but let us modify the struts.xml file to add an interceptor as follows

 
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE struts PUBLIC
   "-//Apache Software Foundation//DTD Struts Configuration 2.0//EN"
   "http://struts.apache.org/dtds/struts-2.0.dtd">
<struts>
   <constant name="struts.devMode" value="true" />
   <package name="helloworld" extends="struts-default">
      <action name="hello" 
         class="com.tutorialspoint.struts2.HelloWorldAction"
         method="execute">
         <interceptor-ref name="params"/>
         <interceptor-ref name="timer" />
         <result name="success">/HelloWorld.jsp</result>
      </action>
   </package>
</struts>

Right click on the project name and click Export > WAR File to create a War file. Then deploy this WAR in the Tomcat's webapps directory. Finally, start Tomcat server and try to access URL http://localhost:8080/HelloWorldStruts2/index.jsp. This will give you following screen:

Hello World Input

Now enter any word in the given text box and click Say Hello button to execute the defined action. Now if you will check the log generated, you will find following text:

 
INFO: Server startup in 3539 ms
27/08/2011 8:40:53 PM 
com.opensymphony.xwork2.util.logging.commons.CommonsLogger info
INFO: Executed action [//hello!execute] took 109 ms.

Here bottom line is being generated because of timer interceptor which is telling that action took total 109ms to be executed.

Create Custom Interceptors

Using custom interceptors in your application is an elegant way to provide cross-cutting application features. Creating a custom interceptor is easy; the interface that needs to be extended is the following Interceptor interface:

 
public interface Interceptor extends Serializable{
   void destroy();
   void init();
   String intercept(ActionInvocation invocation)
   throws Exception;
}

As the names suggest, the init() method provides a way to initialize the interceptor, and the destroy() method provides a facility for interceptor cleanup. Unlike actions, interceptors are reused across requests and need to be thread-safe, especially the intercept() method.
The ActionInvocation object provides access to the runtime environment. It allows access to the action itself and methods to invoke the action and determine whether the action has already been invoked.
If you have no need for initialization or cleanup code, the AbstractInterceptor class can be extended. This provides a default no-operation implementation of the init() and destroy() methods.

Create Interceptor Class:

Let us create following MyInterceptor.java in Java Resources > src folder:

 
package com.tutorialspoint.struts2;

import java.util.*;
import com.opensymphony.xwork2.ActionInvocation;
import com.opensymphony.xwork2.interceptor.AbstractInterceptor;

public class MyInterceptor extends AbstractInterceptor {

   public String intercept(ActionInvocation invocation)throws Exception{

      /* let us do some pre-processing */
      String output = "Pre-Processing"; 
      System.out.println(output);

      /* let us call action or next interceptor */
      String result = invocation.invoke();

      /* let us do some post-processing */
      output = "Post-Processing"; 
      System.out.println(output);

      return result;

   }
}

As you notice, actual action will be executed using the interceptor by invocation.invoke() call. So you can do some pre-processing and some post-processing based on your requirement.
The framework itself starts the process by making the first call to the ActionInvocation object's invoke(). Each time invoke() is called, ActionInvocation consults its state and executes whichever interceptor comes next. When all of the configured interceptors have been invoked, the invoke() method will cause the action itself to be executed. Following digram shows the same concept through a request flow:

Create Action Class:

Let us create a java file HelloWorldAction.java under Java Resources > src with a package name com.tutorialspoint.struts2 with the contents given below.

package com.tutorialspoint.struts2;

import com.opensymphony.xwork2.ActionSupport;

public class HelloWorldAction extends ActionSupport{
   private String name;

   public String execute() throws Exception {
      System.out.println("Inside action....");
      return "success";
   }  

   public String getName() {
      return name;
   }

   public void setName(String name) {
      this.name = name;
   }
}

This is a same class which we have seen in previous examples. We have standard getters and setter methods for the "name" property and an execute method that returns the string "success".

Create a View

Let us create the below jsp file HelloWorld.jsp in the WebContent folder in your eclipse project.

<%@ page contentType="text/html; charset=UTF-8" %>
<%@ taglib prefix="s" uri="/struts-tags" %>
<html>
<head>
<title>Hello World</title>
</head>
<body>
   Hello World, <s:property value="name"/>
</body>
</html>

Create main page:

We also need to create index.jsp in the WebContent folder. This file will serve as the initial action URL where a user can click to tell the Struts 2 framework to call the a defined method of the HelloWorldAction class and render the HelloWorld.jsp view.

<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
   pageEncoding="ISO-8859-1"%>
<%@ taglib prefix="s" uri="/struts-tags"%>
   <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" 
"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<title>Hello World</title>
</head>
<body>
   <h1>Hello World From Struts2</h1>
   <form action="hello">
      <label for="name">Please enter your name</label><br/>
      <input type="text" name="name"/>
      <input type="submit" value="Say Hello"/>
   </form>
</body>
</html>

The hello action defined in the above view file will be mapped to the HelloWorldAction class and its execute method using struts.xml file.

Configuration Files

Now we need to register our interceptor and then call it as we had called default interceptor in previous example. To register a newly defined interceptor, the <interceptors>...</interceptors> tags are placed directly under the <package> tag ins struts.xml file. You can skip this step for a default interceptors as we did in our previous example. But here let us register and use it as follows:

 
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE struts PUBLIC
    "-//Apache Software Foundation//DTD Struts Configuration 2.0//EN"
    "http://struts.apache.org/dtds/struts-2.0.dtd">

<struts>
   <constant name="struts.devMode" value="true" />
   <package name="helloworld" extends="struts-default">

      <interceptors>
         <interceptor name="myinterceptor"
            class="com.tutorialspoint.struts2.MyInterceptor" />
      </interceptors>

      <action name="hello" 
         class="com.tutorialspoint.struts2.HelloWorldAction" 
         method="execute">
         <interceptor-ref name="params"/>
         <interceptor-ref name="myinterceptor" />
         <result name="success">/HelloWorld.jsp</result>
      </action>

   </package>
</struts>

It should be noted that you can register more than one interceptors inside <package> tag and same time you can call more than one interceptors inside the <action> tag. You can call same interceptor with the different actions.
The web.xml file needs to be created under the WEB-INF folder under WebContent as follows:

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xmlns="http://java.sun.com/xml/ns/javaee" 
   xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
   xsi:schemaLocation="http://java.sun.com/xml/ns/javaee 
   http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
   id="WebApp_ID" version="3.0">
   
   <display-name>Struts 2</display-name>
   <welcome-file-list>
      <welcome-file>index.jsp</welcome-file>
   </welcome-file-list>
   <filter>
      <filter-name>struts2</filter-name>
      <filter-class>
         org.apache.struts2.dispatcher.FilterDispatcher
      </filter-class>
   </filter>

   <filter-mapping>
      <filter-name>struts2</filter-name>
      <url-pattern>/*</url-pattern>
   </filter-mapping>
</web-app>

Now enter any word in the given text box and click Say Hello button to execute the defined action. Now if you will check the log generated, you will find following text at the bottom:

 
Pre-Processing
Inside action....
Post-Processing

Stacking multiple Interceptors:

As you can imagine, having to configure multiple interceptor for each action would quickly become extremely unmanageable. For this reason, interceptors are managed with interceptor stacks. Here is an example, directly from the struts-default.xml file:

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

The above stake is called basicStack and can be used in your configuration as shown below. This configuration node is placed under the <package .../> node. Each <interceptor-ref .../> tag references either an interceptor or an interceptor stack that has been configured before the current interceptor stack. It is therefore very important to ensure that the name is unique across all interceptor and interceptor stack configurations when configuring the initial interceptors and interceptor stacks.
We have already seen how to apply interceptor to the action, applying interceptor stacks is no different. In fact, we use exactly the same tag:

 
<action name="hello" class="com.tutorialspoint.struts2.MyAction">
   <interceptor-ref name="basicStack"/>
   <result>view.jsp</result>
</action

The above registration of "basicStack" will register complete stake of all the six interceptors with hello action. This should be noted that interceptors are executed in the order, in which they have been configured. For example, in above case, exception will be executed first, second would be servlet-config and so on.

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Universe For Developers

Monday 18 June 2012

Interceptor achieve Modularity