Invocation.invoke () in Interceptors (Interceptor)

What do you mean by Invocation.invoke () in a custom validation interceptor (Interceptor) in Struts2.

[Java]   View plain copy package com.xjtu.interceptor;      import  com.opensymphony.xwork2.actioncontext;   import com.opensymphony.xwork2.actioninvocation;    import com.opensymphony.xwork2.interceptor.interceptor;      Public class  PermissionInterceptor implements Interceptor {           /**       *        */        private static final long serialVersionUID = 1L;           @Override        public void destroy ()  {           // TODO Auto-generated  method stub          }            @Override  &nbsp     public void init ()  {            // TODO Auto-generated method stub           }           @Override        public  string intercept (actioninvocation invocation)  throws Exception {           system.out.println ("entry blocker");            object user = actioncontext.getcontext (). GetSession (). Get ("user") ;           system.out.println (user);            if  (user != null)  {                return invocation.invoke ();            }           actioncontext.getcontext () . put ("message",  "you are not authorized to perform the operation. ");           return " Success ";        }     }  

[Java] view plain copy package com.xjtu.soft;          public class Helloworldaction {private String message;       Public String GetMessage () {return message;       The public void Setmessage (String message) {this.message = message; Public String Execute () {System.out.println ("execute Execute");