Springboot拦截器使用及其底层源码剖析

　　博主最近看了一下公司刚刚开发的微服务，准备入手从基本的过滤器以及拦截器开始剖析，以及在帮同学们分析一下上次的jetty过滤器源码与本次Springboot中tomcat中过滤器的区别。正题开始，拦截器顾名思义是进行拦截请求的一系列操作。先给大家示例一下使用操作

 @Configuration
 public class WebConfiguration implements WebMvcConfigurer {

     @Override
     public void addInterceptors(InterceptorRegistry registry) {
         registry.addInterceptor(new TstCfg());
     }
 }

配置示例

 /**
  * @title: TstCfg
  * @Author junyu
  * 旧巷里有一个穿着白衬衫笑起来如太阳般温暖我的少年。
  * 记忆里有一个穿着连衣裙哭起来如孩子般讨人喜的女孩。
  * 他说，哪年树弯了腰，人见了老，桃花落了白发梢，他讲的笑话她还会笑，那便是好。
  * 她说，哪年国改了号，坟长了草，地府过了奈何桥，她回头看时他还在瞧，就不算糟。
  * @Date: 2020/7/29 11:53
  * @Version 1.0
  */
 public class TstCfg extends HandlerInterceptorAdapter {

     @Override
     public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
         System.out.println("前");
         return super.preHandle(request, response, handler);
     }

     @Override
     public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception {
         System.out.println("后");
     }

     @Override
     public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {
         System.out.println("一直会出现");
         System.out.println(1/0);
     }
 }

拦截器示例

　　首先我们可能会想到，我们的拦截器是何时装配到拦截器数组中

　　其实就是在springboot启动时执行doCreateBean时，进行调用创建的org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration$EnableWebMvcConfiguration会在这里放入进去所有实现了WebMvcConfigurer接口的类，一共有7个，其中就有我们自己实现了WebMvcConfigurer接口的WebConfiguration类，

　　我们的写的配置类WebConfiguration，继承了WebMvcConfigurer并重写了addInterceptors方法，所以我们的拦截器就在这时候装配进去了。这次知道为什么我们写的配置拦截器的配置示例需要继承------WebMvcConfigurer，我们当然也可以去继承已经实现了这个类的其他类，因为都可以去添加拦截器，博主亲试过，所以就不贴图了！

　　 好了，拦截器已经添加完了，那什么时候调用我们拦截器呢？一步一步脚印来，当浏览器请求我们地址的 时候，分一下几步：

　第一步：tomcat容器首先会接受到请求，这里将会走DispatcherServlet，看到这个大家都熟悉了。

第二步：当然不会先走我们的拦截器了，我们的拦截器是在Springboot框架进行管理的，现在还在servlet，所以会先走到filter过滤器这一步，来贴图：官方代码太长，一屏截不下，前面有一个创建过滤器链的过程：等下次在给大家讲一下jetty的过滤器链与tomcat的过滤器链的区别

ApplicationFilterChain filterChain = ApplicationFilterFactory.createFilterChain(request, wrapper, servlet);

第三步：所以一旦连过滤器都没通过的话，会直接return回去，不会再进行拦截器的调用。来贴代码，过滤器通过后如何调用我们拦截器的

 private void internalDoFilter(ServletRequest request,
                                   ServletResponse response)
         throws IOException, ServletException {
         //这里将会调用所有过滤器链的过滤器，不做重点讲解了，看看下面拦截器的调用
         // Call the next filter if there is one
         if (pos < n) {
             ApplicationFilterConfig filterConfig = filters[pos++];
             try {
                 Filter filter = filterConfig.getFilter();

                 if (request.isAsyncSupported() && "false".equalsIgnoreCase(
                         filterConfig.getFilterDef().getAsyncSupported())) {
                     request.setAttribute(Globals.ASYNC_SUPPORTED_ATTR, Boolean.FALSE);
                 }
                 if( Globals.IS_SECURITY_ENABLED ) {
                     final ServletRequest req = request;
                     final ServletResponse res = response;
                     Principal principal =
                         ((HttpServletRequest) req).getUserPrincipal();

                     Object[] args = new Object[]{req, res, this};
                     SecurityUtil.doAsPrivilege ("doFilter", filter, classType, args, principal);
                 } else {
                     filter.doFilter(request, response, this);
                 }
             } catch (IOException | ServletException | RuntimeException e) {
                 throw e;
             } catch (Throwable e) {
                 e = ExceptionUtils.unwrapInvocationTargetException(e);
                 ExceptionUtils.handleThrowable(e);
                 throw new ServletException(sm.getString("filterChain.filter"), e);
             }
             return;
         }

         // We fell off the end of the chain -- call the servlet instance
         try {
             if (ApplicationDispatcher.WRAP_SAME_OBJECT) {
                 lastServicedRequest.set(request);
                 lastServicedResponse.set(response);
             }

             if (request.isAsyncSupported() && !servletSupportsAsync) {
                 request.setAttribute(Globals.ASYNC_SUPPORTED_ATTR,
                         Boolean.FALSE);
             }
             // Use potentially wrapped request from this point
             if ((request instanceof HttpServletRequest) &&
                     (response instanceof HttpServletResponse) &&
                     Globals.IS_SECURITY_ENABLED ) {
                 final ServletRequest req = request;
                 final ServletResponse res = response;
                 Principal principal =
                     ((HttpServletRequest) req).getUserPrincipal();
                 Object[] args = new Object[]{req, res};
                 SecurityUtil.doAsPrivilege("service",
                                            servlet,
                                            classTypeUsedInService,
                                            args,
                                            principal);
             } else {
                 //过滤器终于完事了，现在终于开始正式调用我们的方法了，我们看看service方法做了什么吧！
                 servlet.service(request, response);
             }
         } catch (IOException | ServletException | RuntimeException e) {
             throw e;
         } catch (Throwable e) {
             e = ExceptionUtils.unwrapInvocationTargetException(e);
             ExceptionUtils.handleThrowable(e);
             throw new ServletException(sm.getString("filterChain.servlet"), e);
         } finally {
             if (ApplicationDispatcher.WRAP_SAME_OBJECT) {
                 lastServicedRequest.set(null);
                 lastServicedResponse.set(null);
             }
         }
     }    

调用servlet的service方法

其实最终它会调用到DispatcherServlet的doDispatch方法

 protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
         HttpServletRequest processedRequest = request;
         HandlerExecutionChain mappedHandler = null;
         boolean multipartRequestParsed = false;

         WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

         try {
             ModelAndView mv = null;
             Exception dispatchException = null;

             try {
                 processedRequest = checkMultipart(request);
                 multipartRequestParsed = (processedRequest != request);

                 // Determine handler for the current request.
                 mappedHandler = getHandler(processedRequest);
                 if (mappedHandler == null) {
                     noHandlerFound(processedRequest, response);
                     return;
                 }

                 // Determine handler adapter for the current request.
                 HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

                 // Process last-modified header, if supported by the handler.
                 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;
                     }
                 }
                 //所有拦截器开始在调用方法前拦截，如果你拦截器中返回false，则直接return不会再调用该方法！下面有源代码
                 if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                     return;
                 }

                 // Actually invoke the handler.
                 //底层进行invoke反射，调用当前请求的方法，不用再往里面看了
                 mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

                 if (asyncManager.isConcurrentHandlingStarted()) {
                     return;
                 }

                 applyDefaultViewName(processedRequest, mv);
                 //调用拦截器的postHandle，下面有源代码
                 mappedHandler.applyPostHandle(processedRequest, response, mv);
             }
             catch (Exception ex) {
                 dispatchException = ex;
             }
             catch (Throwable err) {
                 // As of 4.3, we're processing Errors thrown from handler methods as well,
                 // making them available for @ExceptionHandler methods and other scenarios.
                 dispatchException = new NestedServletException("Handler dispatch failed", err);
             }
             //该方法中多做了一些逻辑，其实最后也调用了triggerAfterCompletion方法，最终调用拦截器方法的afterCompletion方法
             processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
         }
         catch (Exception ex) {
             //所以不管是否出现异常，拦截器方法的afterCompletion方法是一定会调用的！
             triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
         }
         catch (Throwable err) {
             //所以不管是否出现异常，拦截器方法的afterCompletion方法是一定会调用的！
             triggerAfterCompletion(processedRequest, response, mappedHandler,
                     new NestedServletException("Handler processing failed", err));
         }
         finally {
             if (asyncManager.isConcurrentHandlingStarted()) {
                 // Instead of postHandle and afterCompletion
                 if (mappedHandler != null) {
                     mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
                 }
             }
             else {
                 // Clean up any resources used by a multipart request.
                 if (multipartRequestParsed) {
                     cleanupMultipart(processedRequest);
                 }
             }
         }
     }

doDispatch方法，拦截器重点

现在终于开始了我们拦截器的方法了，一个一个来：

 boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {
         HandlerInterceptor[] interceptors = getInterceptors();
         if (!ObjectUtils.isEmpty(interceptors)) {
             for (int i = 0; i < interceptors.length; i++) {
                 HandlerInterceptor interceptor = interceptors[i];
                 //调用所有拦截器的preHandle方法
                 if (!interceptor.preHandle(request, response, this.handler)) {
                     //就算preHandle方法没有通过，仍然会调用这个triggerAfterCompletion方法。
                     triggerAfterCompletion(request, response, null);
                     return false;
                 }
                 this.interceptorIndex = i;
             }
         }
         return true;
     }

applyPreHandle

 void applyPostHandle(HttpServletRequest request, HttpServletResponse response, @Nullable ModelAndView mv)
             throws Exception {

         HandlerInterceptor[] interceptors = getInterceptors();
         if (!ObjectUtils.isEmpty(interceptors)) {
             for (int i = interceptors.length - 1; i >= 0; i--) {
                 HandlerInterceptor interceptor = interceptors[i];
                 //调用拦截器的postHandle方法，
                 interceptor.postHandle(request, response, this.handler, mv);
             }
         }
     }

applyPostHandle

 void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response, @Nullable Exception ex)
             throws Exception {

         HandlerInterceptor[] interceptors = getInterceptors();
         if (!ObjectUtils.isEmpty(interceptors)) {
             for (int i = this.interceptorIndex; i >= 0; i--) {
                 HandlerInterceptor interceptor = interceptors[i];
                 try {
                     //调用拦截器的afterCompletion方法，不管是否异常都会进行调用，但是如果该方法报异常，会被抓住。
                     //不会影响程序正常运行，只会打印出来
                     interceptor.afterCompletion(request, response, this.handler, ex);
                 }
                 catch (Throwable ex2) {
                     logger.error("HandlerInterceptor.afterCompletion threw exception", ex2);
                 }
             }
         }
     }

triggerAfterCompletion

下面这个就是打印了一下，但是不会影响我们的请求响应回去：

还是会正常响应回客户端：

好了，到此拦截器的实现以及源码分析流程到此结束，本来想给大家从Springboot的reflash方法开始解析拦截器，但是内容太多了，不仅跑题而且博主也一时半会给大家无法讲解明白。

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