SpringMVC学习记录5

Springmvc流程中的扩展点有很多,可以在很多地方插入自己的代码逻辑达到控制流程的目的.

如果要对Controller的handler方法做统一的处理.我想应该会有很多选择,比如:@ModelAttribute @InitBinder @ExceptionHandler @ControllerAdvice等注解,自己写AOP包裹Controller,Interceptor等等..

我从来没有用过Interceptor,所以最近稍微花了点时间研究了下.

流程

先看看在Springmvc中Interceptor拦截器是在什么时候被调用的.

 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 || mappedHandler.getHandler() == 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 (logger.isDebugEnabled()) {
                         logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
                     }
                     if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
                         return;
                     }
                 }

                 if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                     return;
                 }

                 // Actually invoke the handler.
                 mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

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

                 applyDefaultViewName(request, mv);
                 mappedHandler.applyPostHandle(processedRequest, response, mv);
             }
             catch (Exception ex) {
                 dispatchException = ex;
             }
             processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
         }
         catch (Exception ex) {
             triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
         }
         catch (Error err) {
             triggerAfterCompletionWithError(processedRequest, response, mappedHandler, 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);
                 }
             }
         }
     }

从dispatcherServlet中可以看出大致的工作流程:

第44行是我们比较熟悉的,让HandlerAdapter来调用我们自己写的Controller的handler来处理request.

在此之前的39行是调用包装了handler的HandlerExecutionChain的前置处理方法,第51行是调用了HandlerExecutionChain的后置处理方法.

而HandlerExecutionChain的applyPreHandle与applyPostHandle里面会调用拦截器的相关方法.

HandlerExecutionChain

handler方法会和各种Interceptor包装成HandlerExecutionChain给dispatcherServlet调用,所以有必要看看HandlerExecutionChain里的方法.

 /**
      * Apply preHandle methods of registered interceptors.
      * @return {@code true} if the execution chain should proceed with the
      * next interceptor or the handler itself. Else, DispatcherServlet assumes
      * that this interceptor has already dealt with the response itself.
      */
     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];
                 if (!interceptor.preHandle(request, response, this.handler)) {
                     triggerAfterCompletion(request, response, null);
                     return false;
                 }
                 this.interceptorIndex = i;
             }
         }
         return true;
     }

     /**
      * Apply postHandle methods of registered interceptors.
      */
     void applyPostHandle(HttpServletRequest request, HttpServletResponse response, ModelAndView mv) throws Exception {
         HandlerInterceptor[] interceptors = 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);
             }
         }
     }

     /**
      * Trigger afterCompletion callbacks on the mapped HandlerInterceptors.
      * Will just invoke afterCompletion for all interceptors whose preHandle invocation
      * has successfully completed and returned true.
      */
     void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response, Exception ex)
             throws Exception {

         HandlerInterceptor[] interceptors = 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 ex2) {
                     logger.error("HandlerInterceptor.afterCompletion threw exception", ex2);
                 }
             }
         }
     }

这3个方法刚好对应了HandlerInterceptor里的3个方法:

boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)
        throws Exception;

void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView)
            throws Exception;

void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex)
            throws Exception;

从Interceptor方法名字中我们就可以看出来这3个方法的作用:handler方法之前调用,handler方法之后调用,dispatch之后调用(视图渲染完之后).

如果配置了多个拦截器,从HandlerExecutionChain中的3个方法里也可以看出:

1.如果前置处理的顺序是拦截器1,拦截器2,拦截器3.那后置处理的顺序是拦截器3,拦截器2,拦截器1

2.如果一个拦截器前置处理的时候返回了false.那么后面的拦截器就不需要执行了.比如拦截2前置方法返回false.那拦截器3就不会执行.

3.在2前置处理失败的情况下,..调用过前置处理方法的拦截器的afterCompletion方法会被调用,用来清理资源...从代码中可以看出afterCompletion和后置处理方法是一样,也是拦截器顺序的倒序执行的,不过只有前置方法返回true的拦截器才会触发(因为拦截器3都没触发,所以不需要触发afterCompletion来清理资源).

多个拦截器的顺序

从HandlerExecutionChain中已经可以看出配置多个拦截器时候的调用顺序了...那怎么确定配置的多个拦截器在HandlerExecutionChain中的加载顺序呢?

实践结果

我实践的结果是在XML里配置的顺序就是加载顺序..

     <mvc:interceptors>
         <mvc:interceptor>
             <mvc:mapping path="/**" />
             <bean class="com.labofjet.controller.TestInteceptor1"></bean>
         </mvc:interceptor>

         <mvc:interceptor>
             <mvc:mapping path="/**" />
             <bean class="com.labofjet.controller.TestInteceptor2"></bean>
         </mvc:interceptor>
     </mvc:interceptors>

比如这样就是先进入拦截1,再进入拦截2.

分析思考过程

记录一下我的思考过程,便于以后复习:

从DispatcherServlet的doDispatcher放中会调用这个方法:

 protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
         for (HandlerMapping hm : this.handlerMappings) {
             if (logger.isTraceEnabled()) {
                 logger.trace(
                         "Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'");
             }
             HandlerExecutionChain handler = hm.getHandler(request);
             if (handler != null) {
                 return handler;
             }
         }
         return null;
     }

说明HandlerExecutionChain 来自HandlerMapping,在一般的情况下应该是RequestMappingHandlerMapping.因为RequestMappingHandlerMapping的order是最高的.所以它返回了handler以后就直接作为方法的结果返回了.

     @Override
     public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
         Object handler = getHandlerInternal(request);
         if (handler == null) {
             handler = getDefaultHandler();
         }
         if (handler == null) {
             return null;
         }
         // Bean name or resolved handler?
         if (handler instanceof String) {
             String handlerName = (String) handler;
             handler = getApplicationContext().getBean(handlerName);
         }
         return getHandlerExecutionChain(handler, request);
     }

L15说明HandlerExecutionChain来自getHandlerExecutionChain方法.

     protected HandlerExecutionChain getHandlerExecutionChain(Object handler, HttpServletRequest request) {
         HandlerExecutionChain chain = (handler instanceof HandlerExecutionChain ?
                 (HandlerExecutionChain) handler : new HandlerExecutionChain(handler));
         chain.addInterceptors(getAdaptedInterceptors());

         String lookupPath = this.urlPathHelper.getLookupPathForRequest(request);
         for (MappedInterceptor mappedInterceptor : this.mappedInterceptors) {
             if (mappedInterceptor.matches(lookupPath, this.pathMatcher)) {
                 chain.addInterceptor(mappedInterceptor.getInterceptor());
             }
         }

         return chain;
     }

interceptor来么来自adaptedInterceptors要么来自mappedInterceptors.

打断点发现是来自mappedInterceptors,而adaptedInterceptors是给我们自己扩展去加载自定义拦截器用的.

再思考下:

RequestMappingHandlerMapping是初始化Springmvc的时候加载的bean,Controller的handler方法是Springmvc初始化的时候就找出来并包装好的.所以Interceptor没有理由不在Springmvc初始化的时候找出来并包装好.毕竟这handler和interceptor功能都差不多....

我粗略看了下RequestMappingHandlerMapping在初始化的时候似乎只找到了2个地方可以参与Spring声明周期:

第一个是:

 /**
      * Detects handler methods at initialization.
      */
     @Override
     public void afterPropertiesSet() {
         initHandlerMethods();
     }

     /**
      * Scan beans in the ApplicationContext, detect and register handler methods.
      * @see #isHandler(Class)
      * @see #getMappingForMethod(Method, Class)
      * @see #handlerMethodsInitialized(Map)
      */
     protected void initHandlerMethods() {
         if (logger.isDebugEnabled()) {
             logger.debug("Looking for request mappings in application context: " + getApplicationContext());
         }

         String[] beanNames = (this.detectHandlerMethodsInAncestorContexts ?
                 BeanFactoryUtils.beanNamesForTypeIncludingAncestors(getApplicationContext(), Object.class) :
                 getApplicationContext().getBeanNamesForType(Object.class));

         for (String beanName : beanNames) {
             if (!beanName.startsWith(SCOPED_TARGET_NAME_PREFIX) &&
                     isHandler(getApplicationContext().getType(beanName))){
                 detectHandlerMethods(beanName);
             }
         }
         handlerMethodsInitialized(getHandlerMethods());
     }

afterPropertiesSet这里是为了找到所有handler方法.

第二个是:

     /**
      * Initializes the interceptors.
      * @see #extendInterceptors(java.util.List)
      * @see #initInterceptors()
      */
     @Override
     protected void initApplicationContext() throws BeansException {
         extendInterceptors(this.interceptors);
         detectMappedInterceptors(this.mappedInterceptors);
         initInterceptors();
     }

这里initApplicationContext就是初始化interceptor用的.

前面实践打断点发现是mappedInterceptors,所以是调用detectMappedInterceptors来加载我定义的拦截器的.

    /**
     * Detect beans of type {@link MappedInterceptor} and add them to the list of mapped interceptors.
     * <p>This is called in addition to any {@link MappedInterceptor}s that may have been provided
     * via {@link #setInterceptors}, by default adding all beans of type {@link MappedInterceptor}
     * from the current context and its ancestors. Subclasses can override and refine this policy.
     * @param mappedInterceptors an empty list to add {@link MappedInterceptor} instances to
     */
    protected void detectMappedInterceptors(List<MappedInterceptor> mappedInterceptors) {
        mappedInterceptors.addAll(
                BeanFactoryUtils.beansOfTypeIncludingAncestors(
                        getApplicationContext(), MappedInterceptor.class, true, false).values());
    }

这个方法其实就是把所有MappedInterceptor类型的类找出来而已...我们定义的类型不是MappedInterceptor的子类..想必有什么地方把我们定义的Interceptor放到了MappedInterceptor类中.那是哪里呢? 我没有找到.......可能需要找到XML中字符串<mvc:interceptors>是怎么解析的...

不过这里已经得到了足够的信息,至少可以知道Interceptor读出来以后并没有进行什么排序,所以读出来的顺序应该就是List<MappedInterceptor> mappedInterceptors的顺序.

总结

简单总结一下

拦截器的流程:

请求 -> DispatcherServlet -> 各个拦截器定义顺序来执行前置拦截方法 -> 各个拦截器定义顺序倒序来执行后置拦截方法 -> 处理视图 -> 拦截器定义倒序顺序回收资源

RequestMappingHandlerMapping:

找到所有MappedInterceptor拦截器并设置到属性里.

找到handler方法并设置到属性里.

当有请求到DispatcherServlet的时候把2者结合成HandlerExecutionChain丢给DispatcherServlet去调用.