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.