Struts2 Core Developers Guide-PreResultListener

1.主要功能&触发时间
该监听器主要在action的execute方法执行完以后，result.execute()方法执行前调用。
接口如下

public interface PreResultListener {
    void beforeResult(ActionInvocation invocation, String resultCode);
}

在DefaultActionInvocation对应的调用如下

//判断是否还有拦截器未执行，如果还有则继续执行拦截器链
//这里通过把DefaultActionInvocation对象本身往后续拦截器中传递来实现interceptors这个interceptor的迭代。
if (interceptors.hasNext()) {
    			final InterceptorMapping interceptor = (InterceptorMapping) interceptors.next();
    			UtilTimerStack.profile("interceptor: "+interceptor.getName(), 
    					new UtilTimerStack.ProfilingBlock<String>() {
							public String doProfiling() throws Exception {
				    			resultCode = interceptor.getInterceptor().intercept(DefaultActionInvocation.this);
				    			return null;
							}
    			});
    		} else {
                           //所有拦截器执行完成调用action的执行方法。
    			resultCode = invokeActionOnly();
    		}

    		// this is needed because the result will be executed, then control will return to the Interceptor, which will
    		// return above and flow through again
                  //调用preResultListeners的内容
    		if (!executed) {
    			if (preResultListeners != null) {
    				for (Iterator iterator = preResultListeners.iterator();
    					iterator.hasNext();) {
    					PreResultListener listener = (PreResultListener) iterator.next();
    					
    					String _profileKey="preResultListener: ";
    					try {
    						UtilTimerStack.push(_profileKey);
    						listener.beforeResult(this, resultCode);
    					}
    					finally {
    						UtilTimerStack.pop(_profileKey);
    					}
    				}
    			}

    			// now execute the result, if we're supposed to
//调用result.execute方法.
    			if (proxy.getExecuteResult()) {
    				executeResult();
    			}

    			executed = true;
    		}

2.注册方法
通过调用 invocation.addPreResultListener具体代码如下(摘自struts2 docs)

public class MyAction extends ActionSupport {
     ...
     public String execute() throws Exception {
         ActionInvocation invocation = ActionContext.getActionInvocation();
         invocation.addPreResultListener(new PreResultListener() {
              public void beforeResult(ActionInvocation invocation, 
                                       String resultCode) {
                  // perform operation necessary before Result execution
              }
         });
     }
     ...
  }