spring mvc DispatcherServlet詳解以前傳---前端控制器架構

前端控制器是整個MVC框架中最爲核心的一塊，它主要用來攔截符合要求的外部請求，並把請求分發到不一樣的控制器去處理，根據控制器處理後的結果，生成相應的響應發送到客戶端。前端控制器既能夠使用Filter實現（Struts2採用這種方式），也能夠使用Servlet來實現(spring MVC框架)。

 

 

DispatcherServlet 做爲前置控制器是web服務器的入口，是spring mvc最重要的一個類，經過它的生命週期能夠加深對web服務器的理解。

servlet的生命週期

首先咱們回憶一下servlet的生命週期：

Servlet生命週期分爲三個階段：【http://www.cnblogs.com/cuiliang/archive/2011/10/21/2220671.html】

　　1，初始化階段  調用init()方法。Servlet被裝載後，Servlet容器建立一個Servlet實例而且調用Servlet的init()方法進行初始化。在Servlet的整個生命週期內，init()方法只被調用一次。

　　2，響應客戶請求階段　　調用service()方法

　　3，終止階段　　調用destroy()方法

 

Servlet初始化階段：

　　在下列時刻Servlet容器裝載Servlet：

　　　　1，Servlet容器啓動時自動裝載某些Servlet，實現它只須要在web.XML文件中的<Servlet></Servlet>之間添加以下代碼：　　

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

　　　　2，在Servlet容器啓動後，客戶首次向Servlet發送請求

　　　　3，Servlet類文件被更新後，從新裝載Servlet

DispatcherServlet的結構

複習了上述知識後咱們來看看DispatcherServlet的結構：

DispatcherServlet繼承自抽象類：FrameworkServlet，間接繼承了HttpServlet （FrameworkServlet繼承自HttpServletBean，而HttpServletBean繼承自HttpServlet ）

Servlet的初始化

    protected void initStrategies(ApplicationContext context) {
        initMultipartResolver(context); //文件上傳解析，若是請求類型是multipart將經過MultipartResolver進行文件上傳解析；
        initLocaleResolver(context);    //本地化解析
        initThemeResolver(context);　　　//主題解析
        initHandlerMappings(context);   //經過HandlerMapping，將請求映射處處理器
        initHandlerAdapters(context);   //經過HandlerAdapter支持多種類型的處理器
        initHandlerExceptionResolvers(context); //若是執行過程當中遇到異常將交給HandlerExceptionResolver來解析
        initRequestToViewNameTranslator(context); //直接解析請求到視圖名
        initViewResolvers(context);      //經過ViewResolver解析邏輯視圖名到具體視圖實現
        initFlashMapManager(context);   //flash映射管理器
    }

 

servlet如何處理請求：

servlet的service方法處理http請求。

FrameworkServlet.java 定義了servlet的service和destroy方法，以下所示：

    /**
     * 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);
        }
    }

 

咱們知道http請求類型有七種（外加一個option選項），定義以下：

public enum RequestMethod {

GET, HEAD, POST, PUT, PATCH, DELETE, OPTIONS, TRACE

}

FrameworkServlet的service()處理不一樣的請求，咱們以常見的post來講明：

    /**
     * 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 抽象定義了處理流程，留待子類來實現該方法，完成具體的請求處理。

/**
     * 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;

具體實現以下：

    /**
     * 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);
            }
        }
    }

重頭戲，做爲請求分發器的實現：

功能：1. 把請求分發到handler（按照配置順序獲取servlet的映射關係獲取handler）；2. 根據servlet已安裝的  HandlerAdapters 去查詢第一個能處理的handler；3. handler激發處理請求

    /**
     * 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銷燬

    /**
     * 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();
        }
    }

小結：

本文因篇章限制，僅僅介紹了請求處理的流程，沒有對代碼進行深刻的分析，接下來的文章將從細微處着手，分析spring的代碼之美。