springmvc拦截器入门及其执行顺序源码分析

springmvc拦截器是偶尔会用到的一个功能，本案例来演示一个较简单的springmvc拦截器的使用，并通过源码来分析拦截器的执行顺序的控制。
具体操作步骤为：
1、maven项目引入spring依赖
2、配置web.xml中的DispatcherServlet
3、准备两个拦截器，并在springmvc配置文件中进行配置管理
4、准备业务类，该类转发到一个JSP页面，并在页面做后台打印
5、测试发送请求到业务类，查看执行顺序
6、源码分析
7、总结以及代码附件
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下面开始开发！
1、maven项目引入spring依赖

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<dependencies>         <dependency>             <groupId>org.springframework</groupId>             <artifactId>spring-context</artifactId>             <version>5.0.2.RELEASE</version>         </dependency>           <dependency>             <groupId>org.springframework</groupId>             <artifactId>spring-web</artifactId>             <version>5.0.2.RELEASE</version>         </dependency>           <dependency>             <groupId>org.springframework</groupId>             <artifactId>spring-webmvc</artifactId>             <version>5.0.2.RELEASE</version>         </dependency>           <dependency>             <groupId>javax.servlet</groupId>             <artifactId>servlet-api</artifactId>             <version>2.5</version>             <scope>provided</scope>         </dependency>           <dependency>             <groupId>javax.servlet.jsp</groupId>             <artifactId>jsp-api</artifactId>             <version>2.0</version>             <scope>provided</scope>         </dependency>     </dependencies>

2、配置web.xml中的DispatcherServlet

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<!-- 前端控制器（加载classpath:springmvc.xml 服务器启动创建servlet） -->     <servlet>         <servlet-name>dispatcherServlet</servlet-name>         <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>         <!-- 配置初始化参数，创建完DispatcherServlet对象，加载springmvc.xml配置文件 -->         <init-param>             <param-name>contextConfigLocation</param-name>             <param-value>classpath:springmvc.xml</param-value>         </init-param>         <load-on-startup>1</load-on-startup>     </servlet>     <servlet-mapping>         <servlet-name>dispatcherServlet</servlet-name>         <url-pattern>/</url-pattern>     </servlet-mapping>

3、准备两个拦截器，并在springmvc配置文件中进行配置管理
两个拦截器代码如下：

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public class MyInterceptor1 implements HandlerInterceptor {           public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) {         System.out.println("==1-1====前置拦截器1 执行======");         return true; //ture表示放行     }       public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) {         System.out.println("==1-2=====后置拦截器1 执行======");     }       public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) {         System.out.println("==1-3======最终拦截器1 执行======");     } }

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public class MyInterceptor2 implements HandlerInterceptor {       public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) {         System.out.println("==2-1====前置拦截器2 执行======");         return true; //ture表示放行     }       public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) {         System.out.println("==2-2=====后置拦截器2 执行======");     }       public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) {         System.out.println("==2-3======最终拦截器2 执行======");     } }

springmvc配置如下(此处只挑选拦截器配置，具体代码参见附件)：

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<!--配置拦截器-->     <mvc:interceptors>         <!--配置拦截器-->         <mvc:interceptor>             <mvc:mapping path="/**" />             <bean class="com.itheima.interceptor.MyInterceptor1" />         </mvc:interceptor>           <mvc:interceptor>             <mvc:mapping path="/**" />             <bean class="com.itheima.interceptor.MyInterceptor2" />         </mvc:interceptor>       </mvc:interceptors>

注意配置顺序拦截器1先于拦截器2，并且都拦截所有的方法
4、准备业务类，该类转发到一个JSP页面，并在页面做后台打印

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@Controller public class BizController {     @RequestMapping("testBiz")     public String showUserInfo(Integer userId, Model model){         System.out.println(">>>>>业务代码执行-查询用户ID为:"+ userId);         User user = new User(userId);         user.setName("宙斯");         model.addAttribute("userInfo",user);         return "user_detail";     } }

响应的页面如下：

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<html> <head>     <title>detail</title> </head> <body>     用户详情：     ${userInfo.id}:${userInfo.name}       <%System.out.print(">>>>>jsp页面的输出为：");%>     <%System.out.println(((User)request.getAttribute("userInfo")).getName());%> </body> </html>

5、测试发送请求到业务类，查看执行顺序
启动项目后，在浏览器中访问/testBiz?userId=1，会在后台打印如下内容：

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==1-1====前置拦截器1 执行====== ==2-1====前置拦截器2 执行====== >>>>>业务代码执行-查询用户ID为:1 ==2-2=====后置拦截器2 执行====== ==1-2=====后置拦截器1 执行====== >>>>>jsp页面的输出为：宙斯 ==2-3======最终拦截器2 执行====== ==1-3======最终拦截器1 执行======

6、源码分析
通过打印结果会发现，执行顺序为：拦截器1的前置>拦截器2的前置>业务代码>拦截器2后置>拦截器1后置>拦截器2最终>拦截器1最终
我们可以通过源码来分析一下为何是该打印顺序。
最关键的为DispatcherServlet的核心方法——doDispatch。
当浏览器发送/testBiz?userId=1的请求时，会经过DispatcherServlet的doDispatch方法，省略掉部分代码，贴出如下（注释有说明）：

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protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {         //...         try {             try {                 ModelAndView mv = null;                 Object dispatchException = null;                   try {                     processedRequest = this.checkMultipart(request);                     multipartRequestParsed = processedRequest != request;                     //1.获取执行链                     mappedHandler = this.getHandler(processedRequest);                     if (mappedHandler == null) {                         this.noHandlerFound(processedRequest, response);                         return;                     }                     //2.获取处理器适配器                     HandlerAdapter ha = this.getHandlerAdapter(mappedHandler.getHandler());                       //...                     //3.执行前置拦截器                     if (!mappedHandler.applyPreHandle(processedRequest, response)) {                         return;                     }                     //4.执行业务handler                     mv = ha.handle(processedRequest, response, mappedHandler.getHandler());                     if (asyncManager.isConcurrentHandlingStarted()) {                         return;                     }                     this.applyDefaultViewName(processedRequest, mv);                     //5.执行后置拦截器                     mappedHandler.applyPostHandle(processedRequest, response, mv);                 } catch (Exception var20) {                     dispatchException = var20;                 } catch (Throwable var21) {                     dispatchException = new NestedServletException("Handler dispatch failed", var21);                 }                 //6.处理页面响应，并执行最终拦截器                 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 {             //...         }     }

其中，第一步中"获取执行链"，执行链内容可以截图查看：
<ignore_js_op>
可以看到我们自定义的两个拦截器按顺序保存。
        在doDispatch方法中，我们注释的第3、5、6步骤是对拦截器的执行处理，现在分别来查看第3、5、6步骤的源码。
第三步骤源码如下：

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//3.执行前置拦截器中的详细代码     boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {         //获得本次请求对应的所有拦截器         HandlerInterceptor[] interceptors = this.getInterceptors();         if (!ObjectUtils.isEmpty(interceptors)) {             //按照拦截器顺序依次执行每个拦截器的preHandle方法.             //并且，interceptorIndex值会一次 + 1 (该值是给后面的最终拦截器使用的)             for(int i = 0; i < interceptors.length; this.interceptorIndex = i++) {                 HandlerInterceptor interceptor = interceptors[/color][i][color=black];                 //只要每个拦截器不返回false,则继续执行，否则执行最终拦截器                 if (!interceptor.preHandle(request, response, this.handler)) {                     this.triggerAfterCompletion(request, response, (Exception)null);                     return false;                 }             }         }         //最终返回true         return true;     }

我们可以看到拦截器的preHandler方法是按拦截器顺序执行的。紧接着贴出第5步的后置拦截器源码：

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//5.执行后置拦截器     void applyPostHandle(HttpServletRequest request, HttpServletResponse response, @Nullable ModelAndView mv) throws Exception {         //获得本次请求对应的所有拦截器         HandlerInterceptor[] interceptors = this.getInterceptors();         if (!ObjectUtils.isEmpty(interceptors)) {             //按倒叙执行每个拦截器的postHandle方法——所以我们看到先执行的拦截器2的postHandle，再执行拦截器1的postHandle             for(int i = interceptors.length - 1; i >= 0; --i) {                 HandlerInterceptor interceptor = interceptors[/color][color=black];                 interceptor.postHandle(request, response, this.handler, mv);             }         }     }

会发现，后置处理是按照拦截器顺序倒叙处理的。最后，我们贴出执行最终拦截器方法的代码：

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private void processDispatchResult(HttpServletRequest request, HttpServletResponse response, @Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv, @Nullable Exception exception) throws Exception {     //...       if (mv != null && !mv.wasCleared()) {         //处理响应         this.render(mv, request, response);         if (errorView) {             WebUtils.clearErrorRequestAttributes(request);         }     } else if (this.logger.isDebugEnabled()) {         this.logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + this.getServletName() + "': assuming HandlerAdapter completed request handling");     }       if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {         if (mappedHandler != null) {             //6、执行拦截器的最终方法们             mappedHandler.triggerAfterCompletion(request, response, (Exception)null);         }       } }

其中，有一个render()方法，该方法会直接处理完response。再后则是触发triggerAfterCompletion方法：

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//6、执行拦截器的[align=left][color=#b00000]最终[/color][/align]方法     void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response, @Nullable Exception ex) throws Exception {         HandlerInterceptor[] interceptors = this.getInterceptors();         if (!ObjectUtils.isEmpty(interceptors)) {             //倒叙执行每个拦截器(interceptorIndex为前置拦截器动态计算)的afterCompletion方法             for(int i = this.interceptorIndex; i >= 0; --i) {                 HandlerInterceptor interceptor = interceptors[/color][/i][color=black][i];                   try {                     interceptor.afterCompletion(request, response, this.handler, ex);                 } catch (Throwable var8) {                     logger.error("HandlerInterceptor.afterCompletion threw exception", var8);                 }             }         }     }

由此可以看到，拦截器的最终方法的执行也是按照倒叙来执行的，而且是在响应之后。
7、总结
        我们可以从源码中看到，拦截器类似于对我们业务方法的环绕通知效果，并且是通过循环集合来控制每个拦截器方法的执行顺序。
源码如下： <ignore_js_op> mvc_interceptor.zip (13.49 KB, 下载次数: 106) 

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