In this tutorial, it shows how to create an own interceptor in Struts 2.

Summary steps :

  1. Create a class implements com.opensymphony.xwork2.interceptor.Interceptor.
  2. Implement the intercept(ActionInvocation invocation) method.
  3. Configure the interceptor in the struts.xml.
  4. Link it to action.
Struts 2 interceptors
Struts 2 comes with many ready interceptors, make sure you check the list of the available Struts 2 interceptors before you create your own interceptor.

A complete example to create an own interceptor :

1. Action

A simple action to forward the user request and print a message.

HelloAction.java

package com.mkyong.common.action;
 
import com.opensymphony.xwork2.ActionSupport;
 
public class HelloAction extends ActionSupport{
 
	public String execute() throws Exception {
 
		System.out.println("HelloAction execute() is called");
		return SUCCESS;
 
	}
}

2. Interceptor

A full interceptor example.

PrintMsgInterceptor.java

package com.mkyong.common.interceptor;
 
import com.opensymphony.xwork2.ActionInvocation;
import com.opensymphony.xwork2.interceptor.Interceptor;
 
public class PrintMsgInterceptor implements Interceptor{
 
        //called during interceptor destruction
	public void destroy() {
		System.out.println("CustomInterceptor destroy() is called...");
	}
 
	//called during interceptor initialization
	public void init() {
		System.out.println("CustomInterceptor init() is called...");
	}
 
	//put interceptor code here
	public String intercept(ActionInvocation invocation) throws Exception {
 
		System.out.println("CustomInterceptor, before invocation.invoke()...");
 
		String result = invocation.invoke();
 
		System.out.println("CustomInterceptor, after invocation.invoke()...");
 
		return result;
	}
 
}

Explanation
The interceptor class must implements the com.opensymphony.xwork2.interceptor.Interceptor interface. During interceptor initialization, init() is called; interceptor destruction, destroy() is called. In last, put all interceptor code that does the work inside the intercept(ActionInvocation invocation) method.

invocation.invoke()
In the interceptor intercept() method, you must called the invocation.invoke() and return it’s result. This is the method responsible for calling the next interceptor or the action. The action will failed to continue without calling the invocation.invoke() method.
destroy() is not reliable
It’s not recommend to put any code inside the destroy(), because this method is not reliable. When your application server is force shutdown or be killed by command, the destroy() will not be called.

3. struts.xml

Configure the interceptor in the struts.xml file.

<?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>
  <package name="default" namespace="/" extends="struts-default">
 
    <interceptors>	
	<interceptor name="printMsgInterceptor" 
	class="com.mkyong.common.interceptor.PrintMsgInterceptor"></interceptor>
 
         <interceptor-stack name="newStack">
     		<interceptor-ref name="printMsgInterceptor"/>
		<interceptor-ref name="defaultStack" />
          </interceptor-stack>
 
    </interceptors>
 
    <action name="helloAction" 
	class="com.mkyong.common.action.HelloAction" >
	<interceptor-ref name="newStack"/>
	<result name="success">pages/hello.jsp</result>
     </action>
 
   </package>
</struts>

4. Demo

During the server initialization, the interceptor init() method is called.

INFO: Overriding property struts.i18n.reload - old value: false new value: true
15 Julai 2010 11:37:42 AM com.opensymphony.xwork2.util.logging.commons.CommonsLogger info
INFO: Overriding property struts.configuration.xml.reload - old value: false new value: true
 
CustomInterceptor init() is called...
 
15 Julai 2010 11:37:42 AM org.apache.coyote.http11.Http11Protocol start
INFO: Starting Coyote HTTP/1.1 on http-8080
15 Julai 2010 11:37:42 AM org.apache.jk.common.ChannelSocket init
INFO: JK: ajp13 listening on /0.0.0.0:8009
15 Julai 2010 11:37:42 AM org.apache.jk.server.JkMain start
INFO: Jk running ID=0 time=0/20  config=null
15 Julai 2010 11:37:42 AM org.apache.catalina.startup.Catalina start
INFO: Server startup in 994 ms

While you access the action via URL : http://localhost:8080/Struts2Example/helloAction.action

INFO: Overriding property struts.i18n.reload - old value: false new value: true
15 Julai 2010 11:37:42 AM com.opensymphony.xwork2.util.logging.commons.CommonsLogger info
INFO: Overriding property struts.configuration.xml.reload - old value: false new value: true
 
CustomInterceptor init() is called...
 
15 Julai 2010 11:37:42 AM org.apache.coyote.http11.Http11Protocol start
INFO: Starting Coyote HTTP/1.1 on http-8080
15 Julai 2010 11:37:42 AM org.apache.jk.common.ChannelSocket init
INFO: JK: ajp13 listening on /0.0.0.0:8009
15 Julai 2010 11:37:42 AM org.apache.jk.server.JkMain start
INFO: Jk running ID=0 time=0/20  config=null
15 Julai 2010 11:37:42 AM org.apache.catalina.startup.Catalina start
INFO: Server startup in 994 ms
 
CustomInterceptor, before invocation.invoke()...
HelloAction execute() is called
CustomInterceptor, after invocation.invoke()...

Reference

  1. Struts 2 interceptor documentation
  2. Struts 2 building your own interceptor
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