spring mvc DispatcherServlet详解之一---前端控制器架构

前端控制器是整个MVC框架中最为核心的一块，它主要用来拦截符合要求的外部请求，并把请求分发到不同的控制器去处理，根据控制器处理后的结果，生成相应的响应发送到客户端。前端控制器既可以使用Filter实现（Struts2采用这种方式），也可以使用Servlet来实现(spring MVC框架)。

 

DispatcherServlet 作为前置控制器是web服务器的入口，是spring mvc最重要的一个类，通过它的生命周期可以加深对web服务器的理解。

servlet的生命周期

首先我们回忆一下servlet的生命周期：

Servlet生命周期分为三个阶段：【http://www.cnblogs.com/cuiliang/archive/2011/10/21/2220671.html】

　　1，初始化阶段  调用init()方法。Servlet被装载后，Servlet容器创建一个Servlet实例并且调用Servlet的init()方法进行初始化。在Servlet的整个生命周期内，init()方法只被调用一次。

　　2，响应客户请求阶段　　调用service()方法

　　3，终止阶段　　调用destroy()方法

 

Servlet初始化阶段：

　　在下列时刻Servlet容器装载Servlet：

　　　　1，Servlet容器启动时自动装载某些Servlet，实现它只需要在web.XML文件中的<Servlet></Servlet>之间添加如下代码：　　

<loadon-startup>1</loadon-startup> 

　　　　2，在Servlet容器启动后，客户首次向Servlet发送请求

　　　　3，Servlet类文件被更新后，重新装载Servlet

DispatcherServlet的结构

复习了上述知识后我们来看看DispatcherServlet的结构：

DispatcherServlet继承自抽象类：FrameworkServlet，间接继承了HttpServlet （FrameworkServlet继承自HttpServletBean，而HttpServletBean继承自HttpServlet ）

Servlet的初始化

    protected void initStrategies(ApplicationContext context) {        initMultipartResolver(context); //文件上传解析，如果请求类型是multipart将通过MultipartResolver进行文件上传解析；        initLocaleResolver(context);    //本地化解析        initThemeResolver(context);　　　//主题解析        initHandlerMappings(context);   //通过HandlerMapping，将请求映射到处理器        initHandlerAdapters(context);   //通过HandlerAdapter支持多种类型的处理器        initHandlerExceptionResolvers(context); //如果执行过程中遇到异常将交给HandlerExceptionResolver来解析        initRequestToViewNameTranslator(context); //直接解析请求到视图名        initViewResolvers(context);      //通过ViewResolver解析逻辑视图名到具体视图实现        initFlashMapManager(context);   //flash映射管理器    }

 

servlet如何处理请求：

servlet的service方法处理http请求。

FrameworkServlet.java 定义了servlet的service和destroy方法，如下所示：

    /**     * Override the parent class implementation in order to intercept PATCH     * requests.     */    @Override    protected void service(HttpServletRequest request, HttpServletResponse response)            throws ServletException, IOException {        String method = request.getMethod();        if (method.equalsIgnoreCase(RequestMethod.PATCH.name())) {            processRequest(request, response);        }        else {            super.service(request, response);        }    }

 

我们知道http请求类型有七种（外加一个option选项），定义如下：

public enum RequestMethod {GET, HEAD, POST, PUT, PATCH, DELETE, OPTIONS, TRACE}

FrameworkServlet的service()处理不同的请求，我们以常见的post来说明：

    /**     * Process this request, publishing an event regardless of the outcome.     * <p>The actual event handling is performed by the abstract     * {@link #doService} template method.     */    protected final void processRequest(HttpServletRequest request, HttpServletResponse response)            throws ServletException, IOException {        long startTime = System.currentTimeMillis();        Throwable failureCause = null;        LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();        LocaleContext localeContext = buildLocaleContext(request);        RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();        ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);        asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());        initContextHolders(request, localeContext, requestAttributes);        try {            doService(request, response);        }        catch (ServletException ex) {            failureCause = ex;            throw ex;        }        catch (IOException ex) {            failureCause = ex;            throw ex;        }        catch (Throwable ex) {            failureCause = ex;            throw new NestedServletException("Request processing failed", ex);        }        finally {            resetContextHolders(request, previousLocaleContext, previousAttributes);            if (requestAttributes != null) {                requestAttributes.requestCompleted();            }            if (logger.isDebugEnabled()) {                if (failureCause != null) {                    this.logger.debug("Could not complete request", failureCause);                }                else {                    if (asyncManager.isConcurrentHandlingStarted()) {                        logger.debug("Leaving response open for concurrent processing");                    }                    else {                        this.logger.debug("Successfully completed request");                    }                }            }            publishRequestHandledEvent(request, startTime, failureCause);        }    }

FrameworkServlet 抽象定义了处理流程，留待子类来实现该方法，完成具体的请求处理。

/**     * Subclasses must implement this method to do the work of request handling,     * receiving a centralized callback for GET, POST, PUT and DELETE.     * <p>The contract is essentially the same as that for the commonly overridden     * {@code doGet} or {@code doPost} methods of HttpServlet.     * <p>This class intercepts calls to ensure that exception handling and     * event publication takes place.     * @param request current HTTP request     * @param response current HTTP response     * @throws Exception in case of any kind of processing failure     * @see javax.servlet.http.HttpServlet#doGet     * @see javax.servlet.http.HttpServlet#doPost     */    protected abstract void doService(HttpServletRequest request, HttpServletResponse response)            throws Exception;

具体实现如下：

    /**     * 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));                }            }        }        // Make framework objects available to handlers and view objects.        request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());        request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);        request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);        request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());        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 {            doDispatch(request, response);        }        finally {            if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {                return;            }            // Restore the original attribute snapshot, in case of an include.            if (attributesSnapshot != null) {                restoreAttributesAfterInclude(request, attributesSnapshot);            }        }    }

重头戏，作为请求分发器的实现：

功能：1. 把请求分发到handler（按照配置顺序获取servlet的映射关系获取handler）；2. 根据servlet已安装的  HandlerAdapters 去查询第一个能处理的handler；3. handler激发处理请求

    /**     * 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 {                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;                }                try {                    // Actually invoke the handler.                    mv = ha.handle(processedRequest, response, mappedHandler.getHandler());                }                finally {                    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                mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);                return;            }            // Clean up any resources used by a multipart request.            if (multipartRequestParsed) {                cleanupMultipart(processedRequest);            }        }    }

servlet销毁

    /**     * Close the WebApplicationContext of this servlet.     * @see org.springframework.context.ConfigurableApplicationContext#close()     */    @Override    public void destroy() {        getServletContext().log("Destroying Spring FrameworkServlet '" + getServletName() + "'");        // Only call close() on WebApplicationContext if locally managed...        if (this.webApplicationContext instanceof ConfigurableApplicationContext && !this.webApplicationContextInjected) {            ((ConfigurableApplicationContext) this.webApplicationContext).close();        }    }