模拟Struts2的AOP实现

在Struts2中有拦截器的概念，通过它的拦截器可以拦截Action。Struts2的拦截器是通过AOP来实现的，在Spring也有类似的概念。下面的我们先来比较一下Struts2和Spring中AOP的东西。

AOP概念	Struts2	Spring
JoinPoint	Action中方法的执行	符合条件方法的执行
Pointcut	Action（不能自己指定）	可以通过正则或AspectJ表达式来指定
Advice	Before、After、Around	Before、After、Around
Aspect	拦截器	拦截器

     从上面的比较中可以看到，Struts2的AOP功能比较单一，只能拦截Action类中的方法。Spring的AOP是通过JDK动态代理或者CGLib来生成目标对象的代理对象，然后将增强功能（Aspect【包括了Advice和Pointcut】）织入到符合条件（Pointcut）的类的方法（JoinPoint）上。

     Struts2的AOP实现跟Filter的实现差不多，它有一系列的拦截器，称为拦截器栈，通过这些拦截器栈通过ActionInvocation的调度可以在Action中方法执行之前或执行做一些操作。原理图如下：

       下面，用代码来模拟一下它的实现

/*** Action、Interceptor调度器* @author zyb* @since 2013-6-2 下午1:22:05*/public interface ActionInvocation {String invoke();}

/*** Action、Interceptor调度器默认实现* @author zyb* @since 2013-6-2 下午1:21:14*/public class DefaultActionInvoation implements ActionInvocation {private int index;private Action action;private List<Interceptor> interceptors = new ArrayList<Interceptor>();public void addInterceptor(Interceptor interceptor) {this.interceptors.add(interceptor);}public void setAction(Action action) {this.action = action;}@Overridepublic String invoke() {String result = "";// 如果所有的拦截器已经执行完，则执行Action中的方法if (index == interceptors.size()) {result = action.execute();} else {Interceptor interceptor = interceptors.get(index);index++;result = interceptor.intercept(this);}return result;}}

/*** 拦截器* @author zyb* @since 2013-6-2 下午1:23:35*/public interface Interceptor {String intercept(ActionInvocation invocation);}

public class ExceptionInterceptor implements Interceptor {@Overridepublic String intercept(ActionInvocation invocation) {System.out.println("ExceptionInterceptor");return invocation.invoke();}}

public class I18NInterceptor implements Interceptor {@Overridepublic String intercept(ActionInvocation invocation) {System.out.println("I18NInterceptor");return invocation.invoke();}}

/*** 环绕拦截器* @author zyb* @since 2013-6-2 下午1:23:25*/public class AroundInterceptor implements Interceptor {@Overridepublic String intercept(ActionInvocation invocation) {System.out.println("before:" + this.getClass());String result = invocation.invoke();System.out.println("after:" + this.getClass());return result;}}

/*** 环绕拦截器* @author zyb* @since 2013-6-2 下午1:21:24*/public class AroundInterceptor01 implements Interceptor {@Overridepublic String intercept(ActionInvocation invocation) {System.out.println("before:" + this.getClass());String result = invocation.invoke();System.out.println("after:" + this.getClass());return result;}}

/*** Action接口* @author zyb* @since 2013-6-2 下午1:21:55*/public interface Action {String execute();}

public class MyAction implements Action {@Overridepublic String execute() {System.out.println("execute...");return "success:" + getClass();}}

public class Test {public static void main(String[] args) {Interceptor exptionInterceptor = new ExceptionInterceptor();Interceptor i18nInterceptor = new I18NInterceptor();Interceptor aroundInterceptor = new AroundInterceptor();Interceptor aroundInterceptor01 = new AroundInterceptor01();DefaultActionInvoation actionInvocation = new DefaultActionInvoation();actionInvocation.addInterceptor(exptionInterceptor);actionInvocation.addInterceptor(i18nInterceptor);actionInvocation.addInterceptor(aroundInterceptor);actionInvocation.addInterceptor(aroundInterceptor01);Action action = new MyAction();actionInvocation.setAction(action);String result = actionInvocation.invoke();System.out.println("Action result:" + result);}}

      Struts2就是这样实现AOP的，比起Spring的AOP实现，简单多了。^_^

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