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    Struts 2 – Interceptors

    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, etc. In fact, as Struts2 emphasizes much of its functionality on interceptors, it is not likely to have 7 or 8 interceptors assigned per action.

    Struts2 Framework 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 −

    Sr.No 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 look into Struts 2 documentation for complete detail on the abovementioned 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. At the 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>
    < constantname="struts.devMode"value="true"/>
    
    < packagename="helloworld"extends="struts-default">
    < actionname="hello"
    class="com.ducatindia.struts2.HelloWorldAction"
    method="execute">
    < interceptor-refname="params"/>
    < interceptor-refname="timer"/>
    < resultname="success">/HelloWorld.jsp< /result>
    < /action>
    < /package>
    < /struts>
    

    Create Custom Interceptors

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

    publicinterfaceInterceptorextendsSerializable{
    void destroy();
    voidinit();
    Stringintercept(ActionInvocation invocation)
    throwsException;
    }
    

    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 threadsafe, 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 nooperation implementation of the init() and destroy() methods.

    Create Interceptor Class

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

    package com.ducatindia.struts2;
    
    importjava.util.*;
    import com.opensymphony.xwork2.ActionInvocation;
    import com.opensymphony.xwork2.interceptor.AbstractInterceptor;
    
    publicclassMyInterceptorextendsAbstractInterceptor{
    
    publicString intercept(ActionInvocation invocation)throwsException{
    
    /* 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;
    }
    }
    

    Create Action Class

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

    package com.ducatindia.struts2;
    
    import com.opensymphony.xwork2.ActionSupport;
    
    publicclassHelloWorldActionextendsActionSupport{
    privateString name;
    
    publicString execute()throwsException{
    System.out.println("Inside action....");
    return"success";
    }
    
    publicStringgetName(){
    return name;
    }
    
    publicvoidsetName(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:propertyvalue="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>
    < formaction="hello">
    < labelfor="name">Please enter your name< /label>< br/>
    < inputtype="text"name="name"/>
    < inputtype="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 insstruts.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>
    < constantname="struts.devMode"value="true"/>
    < packagename="helloworld"extends="struts-default">
    
    < interceptors>
    < interceptorname="myinterceptor"
    class="com.ducatindia.struts2.MyInterceptor"/>
    < /interceptors>
    
    < actionname="hello"
    class="com.ducatindia.struts2.HelloWorldAction"
    method="execute">
    < interceptor-refname="params"/>
    < interceptor-refname="myinterceptor"/>
    < resultname="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-appxmlns: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>
    

    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 strutsdefault.xml file −

    < interceptor-stackname="basicStack">
    < interceptor-refname="exception"/>
    < interceptor-refname="servlet-config"/>
    < interceptor-refname="prepare"/>
    < interceptor-refname="checkbox"/>
    < interceptor-refname="params"/>
    < interceptor-refname="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 −

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

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