HandlerInterceptor

Spring MVC 中拦截器的顶层接口如下：

package org.springframework.web.servlet;import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.springframework.lang.Nullable;public interface HandlerInterceptor {default boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {return true;}default void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, @Nullable ModelAndView modelAndView) throws Exception {}default void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, @Nullable Exception ex) throws Exception {}
}

拦截器的执行

在Spring MVC流程解析一章的最后我们知道，拦截器是封装在HandlerExecutionChain对象中的，而且是一个集合。意味着我们可以有多个拦截器同时作用。
我们再来看看DispatcherServlet的doService方法，其最终调用的是DispatcherServlet的doDispatch方法。该方法的定义如下：

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {HttpServletRequest processedRequest = request;HandlerExecutionChain mappedHandler = null;boolean multipartRequestParsed = false;WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);try {try {ModelAndView mv = null;Object dispatchException = null;try {processedRequest = this.checkMultipart(request);multipartRequestParsed = processedRequest != request;mappedHandler = this.getHandler(processedRequest);if (mappedHandler == null) {this.noHandlerFound(processedRequest, response);return;}HandlerAdapter ha = this.getHandlerAdapter(mappedHandler.getHandler());String method = request.getMethod();boolean isGet = "GET".equals(method);if (isGet || "HEAD".equals(method)) {long lastModified = ha.getLastModified(request, mappedHandler.getHandler());if ((new ServletWebRequest(request, response)).checkNotModified(lastModified) && isGet) {return;}}if (!mappedHandler.applyPreHandle(processedRequest, response)) {return;}mv = ha.handle(processedRequest, response, mappedHandler.getHandler());if (asyncManager.isConcurrentHandlingStarted()) {return;}this.applyDefaultViewName(processedRequest, mv);mappedHandler.applyPostHandle(processedRequest, response, mv);} catch (Exception var20) {dispatchException = var20;} catch (Throwable var21) {dispatchException = new NestedServletException("Handler dispatch failed", var21);}this.processDispatchResult(processedRequest, response, mappedHandler, mv, (Exception)dispatchException);} catch (Exception var22) {this.triggerAfterCompletion(processedRequest, response, mappedHandler, var22);} catch (Throwable var23) {this.triggerAfterCompletion(processedRequest, response, mappedHandler, new NestedServletException("Handler processing failed", var23));}} finally {if (asyncManager.isConcurrentHandlingStarted()) {if (mappedHandler != null) {mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);}} else if (multipartRequestParsed) {this.cleanupMultipart(processedRequest);}}}

太长了，我们截取出关键代码如下：

.....
// mappedHandler就是HandlerExecutionChain对象
// 该方法就是通过HandlerExecutionChain对象的handler对象获取对应的HandlerAdapter
HandlerAdapter ha = this.getHandlerAdapter(mappedHandler.getHandler());
......
// 调用HandlerExecutionChain的applyPreHandle方法，如果返回false，则直接return
if (!mappedHandler.applyPreHandle(processedRequest, response)) {return;
}
......
// 执行HandlerAdapter方法的handle方法（就是执行我们Controller的实际逻辑）
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
......
// 进行一些页面渲染的操作
this.applyDefaultViewName(processedRequest, mv);
// 调用HandlerExecutionChain的applyPostHandle方法
mappedHandler.applyPostHandle(processedRequest, response, mv);
.......

其中只有两个方法与我们的拦截器有关，applyPreHandle、applyPostHandle，这两个方法都是HandlerExecutionChain对象的方法

applyPreHandle方法

boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {HandlerInterceptor[] interceptors = this.getInterceptors();if (!ObjectUtils.isEmpty(interceptors)) {for(int i = 0; i < interceptors.length; this.interceptorIndex = i++) {HandlerInterceptor interceptor = interceptors[i];if (!interceptor.preHandle(request, response, this.handler)) {this.triggerAfterCompletion(request, response, (Exception)null);return false;}}}return true;}

方法内直接调用了所有配置的拦截器的preHandle方法，只要有一个拦截器的preHandle方法返回false，则调用triggerAfterCompletion方法，并直接返回false。之后的拦截器不再执行。

如DispatcherServlet的代码applyPreHandle如果返回false则当前的请求将直接return；

triggerAfterCompletion方法

void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response, @Nullable Exception ex) throws Exception {HandlerInterceptor[] interceptors = this.getInterceptors();if (!ObjectUtils.isEmpty(interceptors)) {for(int i = this.interceptorIndex; i >= 0; --i) {HandlerInterceptor interceptor = interceptors[i];try {interceptor.afterCompletion(request, response, this.handler, ex);} catch (Throwable var8) {logger.error("HandlerInterceptor.afterCompletion threw exception", var8);}}}}

triggerAfterCompletion方法依次调用了所有拦截器的afterCompletion方法。

applyPostHandle方法

void applyPostHandle(HttpServletRequest request, HttpServletResponse response, @Nullable ModelAndView mv) throws Exception {HandlerInterceptor[] interceptors = this.getInterceptors();if (!ObjectUtils.isEmpty(interceptors)) {for(int i = interceptors.length - 1; i >= 0; --i) {HandlerInterceptor interceptor = interceptors[i];interceptor.postHandle(request, response, this.handler, mv);}}}

applyPostHandle方法依次调用了所有拦截器的postHandle方法。

总结

• 如果我们的添加了拦截器，在执行请求之前会先执行preHandle方法，如果返回false，则执行所有拦截器的afterCompletion方法，然后请求直接return。
• 在执行请求之后会执行所有拦截器的postHandle方法
• 如果请求过程中出现异常（见DispatcherServlet源码），在catch异常后调用了triggerAfterCompletion方法。意味着，如果出现异常，则会调用拦截器的afterCompletion方法。

拦截器各个方法的作用

preHandle方法：在请求处理之前执行，可用于请求前的权限认证等操作

postHandle方法：在请求之后执行。可用于请求执行之后的日志记录等操作

afterCompletion方法：主要用于释放当前拦截器使用的资源等操作。

自定义拦截器

自定义拦截器一般通过如下方式来实现

• 直接实现HandlerInterceptor接口。
• 继承HandlerInterceptorAdapter抽象类
  HandlerInterceptorAdapter其实就是一个适配器，jdk1.8之后接口可以定义default方法，HandlerInterceptor的接口定义默认就是一个适配器。所以我们通过直接实现HandlerInterceptor接口只重写其中一个方法也是可以的。

继承HandlerInterceptorAdapter来实现

package com.yyoo.springmvc.interceptor;import org.springframework.web.servlet.HandlerInterceptor;
import org.springframework.web.servlet.ModelAndView;import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;public class MyInterceptor implements HandlerInterceptor {@Overridepublic boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {System.out.println("拦截器 preHandle");return true;}@Overridepublic void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception {System.out.println("拦截器 postHandle");}@Overridepublic void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {System.out.println("拦截器 afterCompletion");}
}

配置拦截器

还记得我们前面实现WebMvcConfigurer的配置类吗？在该类中重写如下代码来配置拦截器。

    @Beanpublic MyInterceptor myInterceptor(){return new MyInterceptor();}@Overridepublic void addInterceptors(InterceptorRegistry registry) {registry.addInterceptor(myInterceptor()).addPathPatterns("/**") // 拦截器作用的url.order(1);// 拦截器的注册顺序}

上一篇：009-Spring MVC Json返回
下一篇：待续