拦截器（interceptor）概念

拦截器是动态拦截action调用的对象，类似于servlet中的过滤器。在执行action中的业务逻辑处理方法之前，struts会首先执行struts.xml中引用的拦截器。烂拦截器是struts2中的中一个重要的特性，struts2框架大多是核心都是围绕拦截器来实现的：避免表单重复提交，类型转换，登录验证，文件上传等。

struts2工作流程图

拦截器小结

拦截器也是一种类，类中有一个方法，但是该类可以自动调用，在执行action之前或者之后。拦截器首次实现了AOP的变成思想，是一种可插拔式的编程方式，拦截器也是方法调用的一种改进，方法可以在系统开启前就开始执行了。

如何创建一个自定义的拦截器

1.自定义一个类实现interceptor接口，重写intercept方法

public class LoginInterceptor implements Interceptor {@Overridepublic void destroy() {}@Overridepublic void init() {}@Overridepublic String intercept(ActionInvocation invocation) throws Exception {//获取当前访问Action的URLString actionName = invocation.getProxy().getActionName();//如果当前访问Action的URL是"loginAction_login"表示此时还没有Sesion，需要放行if(!"studentAction_login".equals(actionName)){//从Session中获取当前用户对象Student stu = SessionContextUitl.getStu();//如果Session不存在，跳转到登录页面if(stu==null){return "login";}}return invocation.invoke();//放行}}

2.配置struts.xml中参数

        <!-- 自定义拦截器  --><interceptors><!-- <interceptor name="execAndWait" class="com.it.system.ExecAndWaitInterceptorEx"></interceptor> --><interceptor name="logini" class="com.it.interceptor.LoginInterceptor"></interceptor><!-- 自定义一个拦截器栈，加入默认拦截器和自定义的拦截器  --><interceptor-stack name="myi"><!-- 防止自定义拦截器覆盖默认拦截器 --><interceptor-ref name="defaultStack"></interceptor-ref><interceptor-ref name="logini"></interceptor-ref></interceptor-stack></interceptors><!-- 启用默认拦截器栈  程序一旦加载，自动加载的拦截器  --><default-interceptor-ref name="myi"></default-interceptor-ref> 

3.设置全局的result处理

<global-results><result name="login">/login.jsp</result></global-results>

此时程序运行结束，实现了一个简单的拦截器。

问题<interceptor name="execAndWait" ></interceptor>

启用的这个exexAndWait有点问题，可能会出现ActionContext和ServletActionContext出现nullpointException异常，是因为AcitonContext是ThreadLocal的，而execAndWait新开线程的时候并没有把父线程中的ActionContext传递给子线程，所以导致空指针异常。解决方案重写strut.xml中提供的execAndWait。

ActionInvocationEx.java

import com.opensymphony.xwork2.ActionContext;
import com.opensymphony.xwork2.ActionEventListener;
import com.opensymphony.xwork2.ActionInvocation;
import com.opensymphony.xwork2.ActionProxy;
import com.opensymphony.xwork2.Result;
import com.opensymphony.xwork2.interceptor.PreResultListener;
import com.opensymphony.xwork2.util.ValueStack;public class ActionInvocationEx implements ActionInvocation {/*** */private static final long serialVersionUID = 2434502343414625665L;private final ActionInvocation mActionInvocation;private final ActionContext context;public ActionInvocationEx(ActionInvocation aActionInvocation,ActionContext aContext){mActionInvocation = aActionInvocation;context = aContext;}public Object getAction() {return mActionInvocation.getAction();}public boolean isExecuted() {return mActionInvocation.isExecuted();}public ActionContext getInvocationContext() {return mActionInvocation.getInvocationContext();}public ActionProxy getProxy() {return mActionInvocation.getProxy();}public Result getResult() throws Exception {return mActionInvocation.getResult();}public String getResultCode() {return mActionInvocation.getResultCode();}public void setResultCode(String resultCode) {mActionInvocation.setResultCode(resultCode);}public ValueStack getStack() {return mActionInvocation.getStack();}public void addPreResultListener(PreResultListener listener) {mActionInvocation.addPreResultListener(listener);}public String invoke() throws Exception {return mActionInvocation.invoke();}public String invokeActionOnly() throws Exception {return mActionInvocation.invokeActionOnly();}public void setActionEventListener(ActionEventListener listener) {mActionInvocation.setActionEventListener(listener);}public void init(ActionProxy proxy) {mActionInvocation.init(proxy);}public ActionInvocation serialize() {return mActionInvocation.serialize();}public ActionInvocation deserialize(ActionContext actionContext) {return mActionInvocation.deserialize(actionContext);}/*** @return the context*/public ActionContext getContext() {return context;}}

2.ExecAndWaitInterceptorEx.java

import org.apache.struts2.interceptor.BackgroundProcess;
import org.apache.struts2.interceptor.ExecuteAndWaitInterceptor;import com.opensymphony.xwork2.ActionContext;
import com.opensymphony.xwork2.ActionInvocation;public class ExecAndWaitInterceptorEx extends ExecuteAndWaitInterceptor {/*** */private static final long serialVersionUID = 8829373762598564300L;/*** {@inheritDoc}*/@Overrideprotected BackgroundProcess getNewBackgroundProcess(String arg0, ActionInvocation arg1, int arg2) {ActionInvocationEx aActionInvocationEx = new ActionInvocationEx(arg1,ActionContext.getContext());return new BackgroundProcessEx(arg0, aActionInvocationEx, arg2);}private class BackgroundProcessEx extends BackgroundProcess {public BackgroundProcessEx(String threadName,ActionInvocation invocation, int threadPriority) {super(threadName, invocation, threadPriority);}private static final long serialVersionUID = -9069896828432838638L;/*** {@inheritDoc}* @throws InterruptedException */@Overrideprotected void beforeInvocation() throws InterruptedException {ActionInvocationEx aActionInvocationEx = (ActionInvocationEx)this.invocation;ActionContext context = aActionInvocationEx.getContext();ActionContext.setContext(context);}/*** {@inheritDoc}*/@Overrideprotected void afterInvocation() {ActionContext.setContext(null);}}
}