SpringMVC 請求分析

    從SpringMVC容器分析的最後一步能夠看出，DispatcherServlet在onRefresh()方法中初始化了Controller和靜態資源等的映射。如今咱們來分析下Request在DispatcherServlet中是如何分發的。本文以Get請求爲例。

Request處理過程：
1. FrameworkServlet的service()方法接收Request，根據Servlet規範，Servlet容器默認調用Servlet的service()方法處理請求

protected void service(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {
    String method = request.getMethod();
    if (method.equalsIgnoreCase(RequestMethod.PATCH.name())) {
        processRequest(request, response);
    } else {
        // 調用HttpServlet方法處理
       super.service(request, response);
    }
}

2. HttpServlet的service()方法內部分發請求

protected void service(HttpServletRequest req, HttpServletResponse resp)
    throws ServletException, IOException {
    String method = req.getMethod();

    // 調用FrameworkServlet的doGet()方法
    if (method.equals(METHOD_GET)) {
        long lastModified = getLastModified(req);
        if (lastModified == -1) {
            // servlet doesn't support if-modified-since, no reason
            // to go through further expensive logic
            doGet(req, resp);
        } else {
            long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
            if (ifModifiedSince < lastModified) {
                // If the servlet mod time is later, call doGet()
                // Round down to the nearest second for a proper compare
                // A ifModifiedSince of -1 will always be less
                maybeSetLastModified(resp, lastModified);
                doGet(req, resp);
            } else {
                resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
            }
        }
    } else if (method.equals(METHOD_HEAD)) {
        long lastModified = getLastModified(req);
        maybeSetLastModified(resp, lastModified);
        doHead(req, resp);

    } else if (method.equals(METHOD_POST)) {
        doPost(req, resp);

    }
    /* 省略部分代碼 */
}

3. FrameworkServlet的doGet方法處理Request

protected final void doGet(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {
    processRequest(request, response);
}

protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {
    /* 省略部分代碼 */
    try {
         // 調用DispatcherServlet的doService()方法
        doService(request, response);
    }
    catch (ServletException ex) {
        failureCause = ex;
        throw ex;
    }
    /* 省略部分代碼 */
}

4. DispatcherServlet的doService()方法接收Request

protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
    /*省略部分代碼*/

    // 設置request屬性
    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()) {
            // Restore the original attribute snapshot, in case of an include.
            if (attributesSnapshot != null) {
                restoreAttributesAfterInclude(request, attributesSnapshot);
            }
        }
    }
}

5. DispatcherServlet的doDispatch()處理Request，最終由Handler處理請求

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
    /* 省略部分代碼 */
    try {
        ModelAndView mv = null;
        Exception dispatchException = null;

        try {
            processedRequest = checkMultipart(request);
            multipartRequestParsed = (processedRequest != request);
            /*
             * 從已有的handlerMappings中找出相應的handler
             * handlerMappings在initHandlerMappings()方法中初始化
             */
            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;
            }

            // 最後，終於調用相應的方法，並返回視圖
            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);
    }
    /* 省略部分代碼 */
}