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摘要: 原创出处 http://blog.csdn.net/u012410733/article/details/51920055 「carl.zhao」欢迎转载，保留摘要，谢谢！

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根据web.xml配置我们知道.DispatcherServlet这个类是Spring MVC的关键类。Spring的方法命名很有趣，在Spring进行DI注入的时候,也就是当Application调用getBean方法的时候。getBean方法调用的是doGetBean.而Spring MVC在处理request对象的时候调用的是doService方法。

调用时序图：

这里写图片描述

通过上面这张时序调用图我们可以看到Dispatcher最终用使用反射调用Method中的invoke方法。而这个方法就是Controller当中的定义的@RequestMapping方法。可以看看之前我的Blog – Spring MVC 之 @RequestMapping。整个从用户request，到服务器response都可以串联起来。下面我们来具体看一看是怎么调用的。

1、doService – Spring MVC处理请求入口

我们来看看处于DispatcherServlet的doService方法。

/**
 * Exposes the DispatcherServlet-specific request attributes and delegates to {@link #doDispatch}
 * for the actual dispatching.
 */
@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
    if (logger.isDebugEnabled()) {
        String resumed = WebAsyncUtils.getAsyncManager(request).hasConcurrentResult() ? " resumed" : "";
        logger.debug("DispatcherServlet with name '" + getServletName() + "'" + resumed +
                " processing " + request.getMethod() + " request for [" + getRequestUri(request) + "]");
    }

    // Keep a snapshot of the request attributes in case of an include,
    // to be able to restore the original attributes after the include.
    Map<String, Object> attributesSnapshot = null;
    if (WebUtils.isIncludeRequest(request)) {
        attributesSnapshot = new HashMap<String, Object>();
        Enumeration<?> attrNames = request.getAttributeNames();
        while (attrNames.hasMoreElements()) {
            String attrName = (String) attrNames.nextElement();
            if (this.cleanupAfterInclude || attrName.startsWith("org.springframework.web.servlet")) {
                attributesSnapshot.put(attrName, request.getAttribute(attrName));
            }
        }
    }

    // 1、提供框架对象处理程序和视图对象。(也就是Spring MVC中特殊的几个bean)
    request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
    // 2、spring mvc支持i18n
    request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
    // 3、spring mvc支持主题
    request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
    request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());

    // 4、springn mvc支持POST/Redirect/GET模式问题
    FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
    if (inputFlashMap != null) {
        request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
    }
    request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
    request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);

    try {
        // 5、spring mvc分发前端request
        doDispatch(request, response);
    }
    finally {
        if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
            // Restore the original attribute snapshot, in case of an include.
            if (attributesSnapshot != null) {
                restoreAttributesAfterInclude(request, attributesSnapshot);
            }
        }
    }
}

这个方法主要的功能实现都在是doDispatch方法中。我们来看一看这个方法。

2、doDispatch – 进行request分发处理的整个流程

下面我们来看看同样处于DispatcherServlet中的doDispatch方法。

/**
 * Process the actual dispatching to the handler.
 * <p>The handler will be obtained by applying the servlet's HandlerMappings in order.
 * The HandlerAdapter will be obtained by querying the servlet's installed HandlerAdapters
 * to find the first that supports the handler class.
 * <p>All HTTP methods are handled by this method. It's up to HandlerAdapters or handlers
 * themselves to decide which methods are acceptable.
 * @param request current HTTP request
 * @param response current HTTP response
 * @throws Exception in case of any kind of processing failure
 */
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 {
            // 步骤1、检查是否是请求是否是multipart（如文件上传）,如果是将通过MultipartResolver解析
            processedRequest = checkMultipart(request);
            multipartRequestParsed = (processedRequest != request);

            // 步骤2、请求到处理器（页面控制器）的映射，通过HandlerMapping进行映射
            mappedHandler = getHandler(processedRequest);
            if (mappedHandler == null || mappedHandler.getHandler() == null) {
                noHandlerFound(processedRequest, response);
                return;
            }

            // 步骤3、处理器适配，即将我们的处理器包装成相应的适配器（从而支持多种类型的处理器）
            HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

            // 步骤4、如果处理程序支持,处理header中包含last-modified
            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;
            }

            // 步骤5、真正调用适配器执行处理器（这里是调用Controller的入口）
            mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

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

            // 步骤6、由ViewResolver解析View（viewResolver.resolveViewName(viewName, locale))
            applyDefaultViewName(request, mv);
            // 步骤7、执行HandlerInterceptor链,并执行post处理
            mappedHandler.applyPostHandle(processedRequest, response, mv);
        }
        catch (Exception ex) {
            dispatchException = ex;
        }
        // 步骤8、渲染视图(包含对异常的处理)
        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);
            }
        }
    }
}

这里写图片描述这样已经拿到了handler这个信息了。其实这个类包装了handler也就是之前文章提到了的HandlerMethod.

3、handle – 调用Controller的入口

这个方法位于org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter#handle()中。

public final ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
        throws Exception {
    return handleInternal(request, response, (HandlerMethod) handler);
}

其实由上面传过来的handler就是HandlerMethod.就样就合Controller关联了起来。看到这个方法我们下一步就是跟进handleInternal这个方法中去看看。

4、handleInternal – 判断Controller是否使用了@SessionAttributes

这个方法位于org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter#handleInternal()中。RequestMappingHandlerAdapter这个类非常重要,我们可以先来看一下它的属性。这里写图片描述 Spring MVC不是有支持的方法参数，以及方法返回参数。看到这个类信息，我们就是通过自定义方法参数与自定义方法返回参数来对Spring MVC进行扩展。后面我会用我们项目中的实例来讲解这个功能。这里主要是想给大家打开一个思路。扩展解决问题的方法。下面我们看看这个方法的具体实现:

@Override
protected final ModelAndView handleInternal(HttpServletRequest request,
        HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {

    if (getSessionAttributesHandler(handlerMethod).hasSessionAttributes()) {
        // Always prevent caching in case of session attribute management.
        checkAndPrepare(request, response, this.cacheSecondsForSessionAttributeHandlers, true);
    }
    else {
        // Uses configured default cacheSeconds setting.
        checkAndPrepare(request, response, true);
    }

    // Execute invokeHandlerMethod in synchronized block if required.
    if (this.synchronizeOnSession) {
        HttpSession session = request.getSession(false);
        if (session != null) {
            Object mutex = WebUtils.getSessionMutex(session);
            synchronized (mutex) {
                return invokeHandleMethod(request, response, handlerMethod);
            }
        }
    }

    // 1、invoke handlerMethod
    return invokeHandleMethod(request, response, handlerMethod);
}

调用invoke handlerMethod就是调用Controller具体处理这个请求的方法,我们接着往下看。

5、invokeHandleMethod – 真正分发request处理(异步处理),并创建ModelAndView包装类

这个方法位于org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter#invokeHandleMethod()中。

/**
 * Invoke the {@link RequestMapping} handler method preparing a {@link ModelAndView}
 * if view resolution is required.
 */
private ModelAndView invokeHandleMethod(HttpServletRequest request,
        HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {

    ServletWebRequest webRequest = new ServletWebRequest(request, response);

    WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
    ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);
    ServletInvocableHandlerMethod requestMappingMethod = createRequestMappingMethod(handlerMethod, binderFactory);

    // ModelAndView包装类
    ModelAndViewContainer mavContainer = new ModelAndViewContainer();
    mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));
    modelFactory.initModel(webRequest, mavContainer, requestMappingMethod);
    mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect);

    // DispatcherServlet,就是用于分发请求。而AsyncWebRequest就是异步处理页面请求
    AsyncWebRequest asyncWebRequest = WebAsyncUtils.createAsyncWebRequest(request, response);
    asyncWebRequest.setTimeout(this.asyncRequestTimeout);

    // 异步请求处理器
    final WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
    asyncManager.setTaskExecutor(this.taskExecutor);
    asyncManager.setAsyncWebRequest(asyncWebRequest);
    asyncManager.registerCallableInterceptors(this.callableInterceptors);
    asyncManager.registerDeferredResultInterceptors(this.deferredResultInterceptors);

    if (asyncManager.hasConcurrentResult()) {
        Object result = asyncManager.getConcurrentResult();
        mavContainer = (ModelAndViewContainer) asyncManager.getConcurrentResultContext()[0];
        asyncManager.clearConcurrentResult();

        if (logger.isDebugEnabled()) {
            logger.debug("Found concurrent result value [" + result + "]");
        }
        requestMappingMethod = requestMappingMethod.wrapConcurrentResult(result);
    }
    //
    requestMappingMethod.invokeAndHandle(webRequest, mavContainer);

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

    return getModelAndView(mavContainer, modelFactory, webRequest);
}

6、invokeAndHandle – 调用请求处理，并对ModelAndView包装类进行值添加

这个方法位于org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle()

/**
 * Invokes the method and handles the return value through a registered
 * {@link HandlerMethodReturnValueHandler}.
 *
 * @param webRequest the current request
 * @param mavContainer the ModelAndViewContainer for this request
 * @param providedArgs "given" arguments matched by type, not resolved
 */
public final void invokeAndHandle(ServletWebRequest webRequest,
        ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception {

    // 看名字我们就知道这个方法是干什么的了，没错，继续进去。
    Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);

    setResponseStatus(webRequest);

    if (returnValue == null) {
        if (isRequestNotModified(webRequest) || hasResponseStatus() || mavContainer.isRequestHandled()) {
            mavContainer.setRequestHandled(true);
            return;
        }
    }
    else if (StringUtils.hasText(this.responseReason)) {
        mavContainer.setRequestHandled(true);
        return;
    }

    mavContainer.setRequestHandled(false);

    try {
        this.returnValueHandlers.handleReturnValue(returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
    }
    catch (Exception ex) {
        if (logger.isTraceEnabled()) {
            logger.trace(getReturnValueHandlingErrorMessage("Error handling return value", returnValue), ex);
        }
        throw ex;
    }
}

7、invokeForRequest – 利用反射调用Contoller处理Request的方法

public final Object invokeForRequest(NativeWebRequest request, ModelAndViewContainer mavContainer,
        Object... providedArgs) throws Exception {

    // 1、获取这个方法的请求参数
    Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);
    if (logger.isTraceEnabled()) {
        StringBuilder sb = new StringBuilder("Invoking [");
        sb.append(getBeanType().getSimpleName()).append(".");
        sb.append(getMethod().getName()).append("] method with arguments ");
        sb.append(Arrays.asList(args));
        logger.trace(sb.toString());
    }
    // 2、利用反射调用Contoller中的方法
    Object returnValue = invoke(args);
    if (logger.isTraceEnabled()) {
        logger.trace("Method [" + getMethod().getName() + "] returned [" + returnValue + "]");
    }
    return returnValue;
}

我们可以看看args的值。这里写图片描述这些值就是Controller对应的请求参数。那个invoke方法我们就没有必要分析了。以后进入的断点也是进入Controller的指定处理对应的URL的方法中。这样是不是@RequestMapping与Controller之间的关系，也就是页面请求到进入到Controller是不是理清了。后面就是怎么把页面渲染出来的了。这个问题会在以后的blog中分析的。

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【carl.zhao】Spring MVC DispatcherServlet