SpringMVC源碼分析（二）

　　上一篇博文SpringMVC源碼分析（一）中已經對SpringMVC中的Servlet（GenericServlet，HttpServlet，HttpServletBean，FrameworkServlet，DispatcherServlet）源碼進行分析，今天接着分析SpringMVC請求處理過程。

　　SpringMVC請求處理過程主要由DispatchServlet來負責完成，FrameworkServlet是DispatchServlet的父類，先看一下FrameWorkServlet的處理請求過程。
　　
　　當接收到請求時會調用service方法，代碼如下：

    @Override
    protected void service(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        HttpMethod httpMethod = HttpMethod.resolve(request.getMethod());
        if (HttpMethod.PATCH == httpMethod || httpMethod == null) {
            processRequest(request, response);//如果請求方式是PATCH時直接調用
        }
        else {
            super.service(request, response);//調用父類的service方法，也就是HttpServlet的service方法
        }
    }

　　service方法中會根據請求類型選擇處理方式，比如get請求就會調用doGet()方法，
FrameworkServlet中重寫doGet()方法，代碼如下：

@Override
    protected final void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        processRequest(request, response);
    }

　　所以最後會調用processRequest處理請求，processRequest是FrameworkServlet類處理請求的核心方法。

protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        long startTime = System.currentTimeMillis();
        Throwable failureCause = null;
        //獲取LocalContextHolder中保存LocalContext（保存了本地化信息，比如zh-cn）
        LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
        //獲取當前請求的LocalContext（保存了本地化信息，比如zh-cn）
        LocaleContext localeContext = buildLocaleContext(request);
        //獲取RequestContextHolder保存的RequestAttributes
        RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
        //獲取當前請求的RequestAttributes
        ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);

        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
        asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());
        //將當前的LocalContext和RequestAttributes分別放置到LocalContextHolder和RequestContextHolder中
        initContextHolders(request, localeContext, requestAttributes);

        try {
            //實際處理請求的入口
            doService(request, response);//在DispatchServlet中實現
        }
        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 {
            //恢復原來的LocalContext和ServletRequestAttributes到LocalContextHolder和RequestContextHolder中
            resetContextHolders(request, previousLocaleContext, previousAttributes);
            if (requestAttributes != null) {
                requestAttributes.requestCompleted();//執行完這個方法後isRequestActive()返回值爲false
            }

            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");
                    }
                }
            }
            //發送servletRequestHandlerEvent消息
            publishRequestHandledEvent(request, response, startTime, failureCause);
        }
    }

　　ServletRequestAttributes中封裝了我們平時使用setAttribute和getAttribute方法，根據scope參數來判斷是request還是session。

    @Override
    public void setAttribute(String name, Object value, int scope) {
        if (scope == SCOPE_REQUEST) {
            if (!isRequestActive()) {//上面調用了requestCompleted之後就會變爲false也就是不進行操作了
                throw new IllegalStateException(
                        "Cannot set request attribute - request is not active anymore!");
            }
            this.request.setAttribute(name, value);
        }
        else {
            HttpSession session = getSession(true);
            this.sessionAttributesToUpdate.remove(name);
            session.setAttribute(name, value);
        }
    }

　　publishRequestHandledEvent方法是請求處理結束後會發出消息，無論請求是否成功都會發出。

　　接下來繼續看service()處理請求，service方法會調用doService方法，doService是個模板方法，DispatcherServlet中重寫了這個方法，我們轉到ServletDispatcherServlet上分析處理請求過程，DispatcherServlet是Spring 的最核心的類。整個處理請求的過程的頂層設計都在這裏。我們來看看DispatcherServlet的doService方法。

@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;
        //當include請求時把request中的atrribute備份一份
        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(DEFAULT_STRATEGIES_PREFIX)) {
                    attributesSnapshot.put(attrName, request.getAttribute(attrName));
                }
            }
        }

        // Make framework objects available to handlers and view objects.
        request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());//設置webApplication
        request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);//設置localResolver
        request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);//設置themeResolver
        request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());//設置themeResource

        FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);//用於redirect中attribute的數據傳遞
        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 {
            //處理請求的核心代碼
            doDispatch(request, response);
        }
        finally {
            if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
                // Restore the original attribute snapshot, in case of an include.
                if (attributesSnapshot != null) {
                    //還原request中的attribute
                    restoreAttributesAfterInclude(request, attributesSnapshot);
                }
            }
        }
    }

　　繼續追蹤doDispatch方法，doDispatch主要的任務是根據request找到Handler，根據handler找到相對應的HandlerAdapter，用HandlerAdapter處理Handler,最後把結果通過processDispatchResult()返回，實現代碼如下：

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);//檢查是否是上傳文件的請求
                multipartRequestParsed = (processedRequest != request);
                // Determine handler for the current request.
                mappedHandler = getHandler(processedRequest);//通過請求找到handler（這裏的handler就是我們說的Controller中處理請求的方法）
                if (mappedHandler == null || mappedHandler.getHandler() == null) {
                    noHandlerFound(processedRequest, response);//沒有找到handler則調用此方法處理
                    return;
                }

                // Determine handler adapter for the current request.
                HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());//通過handler找到HandlerAdapter

                // Process last-modified header, if supported by the handler.
                String method = request.getMethod();//獲取請求方式
                boolean isGet = "GET".equals(method);
                if (isGet || "HEAD".equals(method)) {
                    //處理GET、HEAD請求的Last-Modified
                    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;
                    }
                }
                //執行相應Interceptor的preHandler方法
                if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                    return;
                }
                //handlerAdapter使用handler處理請求
                // Actually invoke the handler.
                mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
                if (asyncManager.isConcurrentHandlingStarted()) {
                    return;
                }
                //當View爲空時，比如我們寫的Controller中處理請求的方法爲空時，根據request的設置view
                applyDefaultViewName(processedRequest, mv);
                //執行相應Intercerptor的postHandler方法
                mappedHandler.applyPostHandle(processedRequest, response, mv);
            }
            catch (Exception ex) {
                dispatchException = ex;
            }
            catch (Throwable err) {

                dispatchException = new NestedServletException("Handler dispatch failed", err);
            }
            //返回處理結果，包括異常處理、渲染頁面、發出通知出發Interceptor的afterCompletion方法
            processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
        }
        catch (Exception ex) {
            triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
        }
        catch (Throwable err) {
            triggerAfterCompletion(processedRequest, response, mappedHandler,
                    new NestedServletException("Handler processing failed", err));
        }
        finally {
            //判斷是否執行異步請求
            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.         //如果是上傳請求刪除請求的資源
                if (multipartRequestParsed) {
                    cleanupMultipart(processedRequest);
                }
            }
        }
    }

可以看出doDispatch方法中的核心工作就是
　　1.根據當前Request找到handler；
　　2.根據Handler找到對應的HandlerAdapter；
　　3.使用HandlerAdapter處理handler
　　4.調用processDispatchResult方法處理上面請求之後的結果

補充幾個重要的概念：
　　Handler：也就是處理器，他直接對應着MVC的Controller層，他的表現形式很多，標註了@RequestMapping的所有方法都可以看出一個handler，只要能處理實際請求的就可以是Handler。
　　HandlerMapping：用來查找Handler的，SpringMVC的請求很多，每個請求都需要一個handler來處理，收到的請求由那個handler來處理就由HandlerMapping來確定。
　　HandlerAdapter：他就是一個適配器，SpringMVC中handler的形式可以任意，主要能處理請求就OK，但是Servlet處理請求的方法結構是固定的。如何讓固定的servlet處理方法調用靈活的handler來處理就由HandlerAdapter來實現。

　　接着分析handlerAdpter中調用handler方法處理請求。

public interface HandlerAdapter {
    //判斷是否可以用某個handler
    boolean supports(Object handler);
    //具體使用Handler處理請求
    ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception;
    //返回LastModified的值
    long getLastModified(HttpServletRequest request, Object handler);
}

我們只要實現HandlerAdapter接口就可以處理請求，SimpleControllerHandlerAdapter實現了HandlerAdapter，實現代碼如下：

public class SimpleControllerHandlerAdapter implements HandlerAdapter {
    //判斷是否可以用某個handler
    @Override
    public boolean supports(Object handler) {
        return (handler instanceof Controller);
    }
    @Override
    public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
            throws Exception {
        //調用Controller接口中的handleRequest方法，我們如果繼承了Controller接口就需要重寫這個方法，然後在裏面處理邏輯即可。
        return ((Controller) handler).handleRequest(request, response);
    }
    @Override
    public long getLastModified(HttpServletRequest request, Object handler) {
        if (handler instanceof LastModified) {
            return ((LastModified) handler).getLastModified(request);
        }
        return -1L;
    }
}

　　請求處理完的結果會封裝在ModelAndView中，我們通過processDispatchResult方法把結果渲染到頁面中，實現代碼如下：

private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
            HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) throws Exception {

        boolean errorView = false;
        //判斷是否有異常
        if (exception != null) {
            if (exception instanceof ModelAndViewDefiningException) {
                logger.debug("ModelAndViewDefiningException encountered", exception);
                //異常處理
                mv = ((ModelAndViewDefiningException) exception).getModelAndView();
            }
            else {
                Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
                mv = processHandlerException(request, response, handler, exception);
                errorView = (mv != null);
            }
        }

        // Did the handler return a view to render?
        if (mv != null && !mv.wasCleared()) {
            render(mv, request, response);//調用此方法進行頁面渲染
            if (errorView) {
                WebUtils.clearErrorRequestAttributes(request);
            }
        }
        else {
            if (logger.isDebugEnabled()) {
                logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + getServletName() +
                        "': assuming HandlerAdapter completed request handling");
            }
        }

        if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
            // Concurrent handling started during a forward
            return;
        }
        //發出請求處理完成的通知，出發Interceptor的afterCompletion
        if (mappedHandler != null) {
            mappedHandler.triggerAfterCompletion(request, response, null);
        }
    }

　　在render方法中會去查找視圖解析器，然後轉化成View類型的視圖（比如jsp，html，freemaker等）顯示在頁面上。

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