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Understanding Interceptors
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Added by victorsosa, last edited by Ted Husted on Sep 04, 2006 (view change)

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 `struts.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 `struts.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.
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, `struts-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

1. 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).
2. Declare the class in your XML configuration file (`struts.xml`) using the element `<interceptor />` nested within `<interceptors />`.
3. Create stacks of Interceptors, using the `<interceptor-stack />` element (optional).
4. 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 `invoke`method executes the next Interceptor in the stack or, if this is the last Interceptor, the appropriate method of the Action class. The `invoke`method 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.

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