Struts2 Interceptor的原理

1.Struts2的interceptor是配置在xml文件中的:
    interceptor: interceptor标签，必须包含名称和类名
    interceptors: package标签的子标签，只用来包含interceptor-ref、
                    interceptor-stack，无其它作用
    interceptor-ref: interceptor标签别名，可以指向interceptor，也可以指向
                        interceptor-stack
    interceptor-stack: intercepor标签的集合,只包含interceptor-ref
    default-interceptor-ref: action默认的interceptor集合
dtd描述代码:

<!ELEMENT interceptors (interceptor|interceptor-stack)+>

<!ELEMENT interceptor (param*)>
<!ATTLIST interceptor
    name CDATA #REQUIRED
    class CDATA #REQUIRED
>

<!ELEMENT interceptor-stack (interceptor-ref*)>
<!ATTLIST interceptor-stack
    name CDATA #REQUIRED
>

<!ELEMENT interceptor-ref (param*)>
<!ATTLIST interceptor-ref
    name CDATA #REQUIRED
>

<!ELEMENT default-interceptor-ref (#PCDATA)>
<!ATTLIST default-interceptor-ref
    name CDATA #REQUIRED
>

2.Struts2的Interceptor是以责任链模式被调用的，包括ActionInvocation、Interceptor接口，以下为一个示意例子:

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

interface ActionInvocation{ 
    public String invoke(); 
} 

interface Interceptor{ 
	public String intercept(ActionInvocation invocation); 
} 

class SampleActionInvocation implements ActionInvocation{ 

    private Iterator interceptors = null; 
    private Action action = null; 

    void setInterceptors(List list){ 
           interceptors = list.iterator(); 
    } 

    void setAction(Action action){ 
          this.action = action; 
    } 
    
     public String invoke(){ 
        //do interceptor       
        if(interceptors.hasNext()){ 
           Interceptor interceptor = (Interceptor)interceptors.next(); 
           interceptor.intercept(this); 
        }else{ 
           //do action 
           action.execute(); 
        }
        return null;
     } 
} 

class SampleInterceptor implements Interceptor{ 
      public String intercept(ActionInvocation invocation){ 
          System.out.println(this.toString()+" before intercept"); 
          invocation.invoke(); 
          System.out.println(this.toString()+" after intercept"); 
          return null; 
      } 
} 

class Action{ 
   void execute(){ 
       System.out.println(this.toString()+" excute "); 
   } 
} 

public class TestMain{ 
   public static void main(String[] args){ 
       SampleActionInvocation sai = new SampleActionInvocation();
       List list = new ArrayList();
       list.add(new SampleInterceptor());
       list.add(new SampleInterceptor());
       sai.setInterceptors(list); 
       sai.setAction(new Action()); 
       sai.invoke();
   } 
}