Understanding 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."

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.struts.action2.dispatcher.ServletDispatcherResult"
                         default="true"/>
            <result-type name="freemarker" class="org.apache.struts.action2.views.freemarker.FreemarkerResult"/>
            <result-type name="httpheader" class="org.apache.struts.action2.dispatcher.HttpHeaderResult"/>
            <result-type name="jasper" class="org.apache.struts.action2.views.jasperreports.JasperReportsResult"/>
            <result-type name="redirect" class="org.apache.struts.action2.dispatcher.ServletRedirectResult"/>
            <result-type name="redirect-action"
                         class="org.apache.struts.action2.dispatcher.ServletActionRedirectResult"/>
            <result-type name="stream" class="org.apache.struts.action2.dispatcher.StreamResult"/>
            <result-type name="velocity" class="org.apache.struts.action2.dispatcher.VelocityResult"/>
            <result-type name="xslt" class="org.apache.struts.action2.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.struts.action2.interceptor.WebWorkConversionErrorInterceptor"/>
            <interceptor name="external-ref" class="com.opensymphony.xwork.interceptor.ExternalReferencesInterceptor"/>
            <interceptor name="execAndWait" class="corg.apache.struts.action2.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.struts.action2.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 WebWork Inversion of Control stack
                 Note: WebWork's IoC is deprecated - please
                 look at alternatives such as Sprint -->
            <interceptor-stack name="componentStack">
                <interceptor-ref name="component"/>
                <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>
</xwork>
<!-- // END SNIPPET: struts-default -->

Since we included struts-default.xml in our strits.xml, all the predefined interceptors and stacks are available for us to use in our actions.

timer	clocks how long the action (including nested interceptors and view) takes to execute
logger	logs the action being executed
chain	makes the previous action's properties available to the current action. Used to make action chaining (reference: Result Types)
static-params	sets the parameters defined in `xwork.xml` onto the action. These are the `<param />` tags that are direct children of the `<action />` tag
params	sets the request (POST and GET) parameters onto the action class. We have seen an example of this in TODO
model-driven	if the action implements `ModelDriven`, pushes the `getModel()` result onto the Value Stack
component	enables and makes registered components available to the actions. (reference: IoC & Components)
token	checks for valid token presence in action, prevents duplicate form submission
token-session	same as above, but storing the submitted data in session when handed an invalid token;
validation	performs validation using the validators defined in {`Action}-validation.xml` (reference: Validation).
workflow	calls the validate method in your action class. If action errors created then it returns the `INPUT` view. Good to use together with the validation interceptor (reference: Validation)
servlet-config	give access to `HttpServletRequest` and `HttpServletResponse` (think twice before using this since this ties you to the Servlet API)
prepare	allows you to programmatic access to your Action class before the parameters are set on it.
conversionError	Adds field errors if any type-conversion errors occurred\
execAndWait	Spawns a separate thread to execute the action
fileUpload	Sets uploaded files as action files (File objects)

In addition to the prepackaged Interceptors, action-default.xml includes prepackaged combinations in named Interceptor Stacks.

Building Your Own Interceptor

If the stock Interceptors aren't enough, you can implement custom Interceptors to solve a new problem or and old problem differently.

Suppose several pages in an application would like to to display a greeting that changes according to the time of day. A good way to solve this use case is to use an Interceptor to place the greeting in the session on each request. As soon as the time of day changes, so would the greeting. As an Interceptor, the greeting can be created automatically, so that the Actions need not be bothered.

Implementing a Greeting Interceptor

Create an Interceptor class by implementing the com.opensymphony.xwork.interceptor.Interceptor interface (bundled in the xwork-1.1.jar provided with the framework distribution).
Declare the class in your XML configuration file (action.xml) using the element <interceptor /> nested within <interceptors />.
Create stacks of Interceptors, using the <interceptor-stack /> element (optional).
Determine which Interceptors are used by which mapping, using <interceptor-ref /> or <default-interceptor-ref />. The <interceptor-ref> defines the interceptors to be used in a specific action. The <default-interceptor-ref /> determines the default interceptor stack to be used by all actions that do not specify their own <interceptor-ref />.

Coding the Interceptor class

GreetingInterceptor.java

package tutorial;

import java.util.Calendar;
import com.opensymphony.xwork.interceptor.Interceptor;
import com.opensymphony.xwork.ActionInvocation;

public class GreetingInterceptor implements Interceptor {
  public void init() { }
  public void destroy() { }
  public String intercept(ActionInvocation invocation) throws Exception {
    Calendar calendar = Calendar.getInstance();
    int hour = calendar.get(Calendar.HOUR_OF_DAY);
    String greeting = (hour < 6) ? "Good evening" :
        ((hour < 12) ? "Good morning":
 	((hour < 18) ? "Good afternoon": "Good evening"));

     invocation.getInvocationContext().getSession().put("greeting", greeting);

     String result = invocation.invoke();

     return result;
  }
}

Coding the configuration

struts.xml

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

<struts>
  <include file="struts-default.xml" />

  <package name="default" extends="struts-default">
     <interceptors>
       <interceptor name="greeting" class="tutorial.GreetingInterceptor" />
     </interceptors>

     <action name="greeting" class="tutorial.GreetingAction">
 	<interceptor-ref name="greeting" />
 	<result name="success" type="velocity">greeting.vm</result>
      </action>

      <!-- other action mappings -->

   </package>
 </struts>

Coding the Action class

GreetingAction.java

package tutorial;

import com.opensymphony.xwork.ActionSupport;

public class GreetingAction extends ActionSupport {
  public String execute() throws Exception
  {
    return SUCCESS;
  }
}

Coding the result page

Understanding Interceptors

<html>
  <head>
    <title>Understanding Interceptors</title>
   </head>
   <body>
     #set ($ses = $req.getSession())
     <p><b>${ses.getAttribute('greeting')}!</b></p>
   </body>
 </html>

How the code works

Let's take a look at our Interceptor class first. An Interceptor must implement com.opensymphony.xwork.interceptor.Interceptor which expects three methods.

init() is called during interceptor initialization,
destroy() is called during destruction, and most importantly,
- intercept(ActionInvocation invocation) is where we place the code that does the work.

The Greeting Interceptor returns the result from invocation.invoke. The invokemethod executes the next Interceptor in the stack or, if this is the last Interceptor, the appropriate method of the Action class. The invokemethod grants the Interceptor the power to short-circuiting the Action Invocation. Instead of calling invoke, the Interceptor can return a result String and bypass any remaining Interceptors on the stack and the Action's execute method.

An Interceptor can also execute code after the Action method executes. Just place more code after the invocation.invoke call. Control returns to each Interceptor on the stack in reverse order. When the call to invoke returns, you can continue processing.

Many Interceptors execute code only before or after the Action, but some do execute code in both places. To make coding Interceptors a little easier, the framework provides an abstract class that implements the "before and after" behaviour. Instead of calling invoke}] itself, an Interceptor can extend {{com.opensymphony.xwork.interceptor.AroundInterceptor and implement the methods before(ActionInvocation invocation) and after(ActionInvocation dispatcher, String result).

Summary

The Interceptor strategy promotes reusability and simplicity. Interceptors make it easier to separate concerns, so that we can solve one problem at a time. The best part is that each application, and each Action, controls exactly what Interceptors are used in what order.

Next	Onward to Understanding Validation
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Child pages

Understanding Interceptors

Configuring Interceptors

Stacking Interceptors

The Default Configuration

Building Your Own Interceptor

Implementing a Greeting Interceptor

Coding the Interceptor class

Coding the configuration

Coding the Action class

Coding the result page

How the code works

Summary