SpringMVC源碼（三）- DispatcherServlet請求分析

一、service請求（servlet請求轉換為Http請求）

二、doService請求（request中添加SpringMVC初始化的九大件資訊）

三、doDispatch

HandlerExecutionChain結構和HandlerInterceptor方法調用

一、service請求（servlet請求轉換為Http請求）

DispatcherServlet作為一個Servlet，那麼當有請求到Tomcat等Servlet伺服器時，會調用其service方法。再調用到其父類GenericServlet的service方法，HttpServlet中實作，如下（開始請求的調用）：

@Override
public void service(ServletRequest req, ServletResponse res)
        throws ServletException, IOException {
    HttpServletRequest  request;
    HttpServletResponse response;
    try {
        request = (HttpServletRequest) req;
        response = (HttpServletResponse) res;
    } catch (ClassCastException e) {
        throw new ServletException(lStrings.getString("http.non_http"));
    }
    service(request, response);
}

HttpServlet層：将request和response類型進行轉換後，繼續調用service方法：

protected void service(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {

    String method = req.getMethod();
    
    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;
            try {
                ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
            } catch (IllegalArgumentException iae) {
                // Invalid date header - proceed as if none was set
                ifModifiedSince = -1;
            }
            if (ifModifiedSince < (lastModified / 1000 * 1000)) {
                // 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);
    } else if (method.equals(METHOD_PUT)) {
        doPut(req, resp);
    } else if (method.equals(METHOD_DELETE)) {
        doDelete(req, resp);
    } else if (method.equals(METHOD_OPTIONS)) {
        doOptions(req,resp);
    } else if (method.equals(METHOD_TRACE)) {
        doTrace(req,resp);
    } else {
        String errMsg = lStrings.getString("http.method_not_implemented");
        Object[] errArgs = new Object[1];
        errArgs[0] = method;
        errMsg = MessageFormat.format(errMsg, errArgs);
        resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
    }
}

根據調用的Http請求的方式，調用具體的底層（FrameworkServlet層）方法，get、post請求等都會有相同的處理,比如doGet如下：

@Override
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 {

    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 | 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();
        }
        logResult(request, response, failureCause, asyncManager);
        publishRequestHandledEvent(request, response, startTime, failureCause);
    }
}

主要的核心邏輯為doService，但是Tomcat是使用線程池的方式接受來自用戶端的請求的，目前請求中可能帶有Locate（國際化參數資訊），那麼需要使用ThreadLocal在請求前記錄參數資訊，在請求之後finally中将參數恢複回去，不會影響到下一個請求。Spring經常會這樣進行處理，比如AopContext等處理Aop切面資訊。

二、doService請求（request中添加SpringMVC初始化的九大件資訊）

@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
    logRequest(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<>();
        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());
    request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
    request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
    request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());

    if (this.flashMapManager != null) {
        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);
            }
        }
    }
}

1、列印請求日志

2、請求中添加屬性（WebApplicationContext容器，i18n解析器，主題解析器，主題，重定向屬性處理）

3、核心方法 doDispatch

三、doDispatch

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
    HttpServletRequest processedRequest = request;
    HandlerExecutionChain mappedHandler = null;
    boolean multipartRequestParsed = false;
    // 異步請求屬性解析（重定向）
    WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
    try {
        ModelAndView mv = null;
        Exception dispatchException = null;
        try {
            // 如果是Multipart上傳檔案請求，則調用multipartResolver.resolveMultipart（上傳檔案解析器進行解析）
            processedRequest = checkMultipart(request);
            multipartRequestParsed = (processedRequest != request);

            // 确定Handler處理該請求（HandlerExecutionChain調用鍊，責任鍊模式）
            mappedHandler = getHandler(processedRequest);
            if (mappedHandler == null) {
                noHandlerFound(processedRequest, response);
                return;
            }

            // 根據初始化時加載的擴充卡挨個比對是否能适配該調用鍊
            HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

            // 對請求頭中的last-modified進行處理
            String method = request.getMethod();
            boolean isGet = "GET".equals(method);
            if (isGet || "HEAD".equals(method)) {
                long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
                if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
                    return;
                }
            }
            // 對HandlerExecutionChain中的所有HandlerInterceptor調用preHandle方法
            if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                return;
            }

            // 真正的請求調用
            mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

            if (asyncManager.isConcurrentHandlingStarted()) {
                return;
            }
            // 判斷Controller傳回的ModelAndView中是否有View，
            // 沒有則使用viewNameTranslator執行沒有試圖的解析規則
            applyDefaultViewName(processedRequest, mv);
            // 對HandlerExecutionChain中的所有HandlerInterceptor調用postHandle方法
            mappedHandler.applyPostHandle(processedRequest, response, mv);
        } catch (Exception ex) {
            dispatchException = ex;
        } catch (Throwable err) {
            // As of 4.3, we're processing Errors thrown from handler methods as well,
            // making them available for @ExceptionHandler methods and other scenarios.
            dispatchException = new NestedServletException("Handler dispatch failed", err);
        }
        // 處理結果解析（ModelAndView或者Exception），保證最終會執行是以Interceptor的triggerAfterCompletion
        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 {
        if (asyncManager.isConcurrentHandlingStarted()) {
            // 不知道什麼情況下回進入相當于執行是以Interceptor的postHandle和afterCompletion方法
            if (mappedHandler != null) {
                mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
            }
        }
        else {
            // 清除目前檔案上傳請求的資源
            if (multipartRequestParsed) {
                cleanupMultipart(processedRequest);
            }
        }
    }
}

這裡是整個SpringMVC的核心，每個流程都可能會比較複雜，後續單獨分析。流程如下（主要是标紅的步驟）：

1、異步請求屬性解析（重定向）

2、如果是Multipart上傳檔案請求，則調用multipartResolver.resolveMultipart（上傳檔案解析器進行解析）

4、确定Handler處理該請求（HandlerExecutionChain調用鍊，責任鍊模式）

5、根據初始化時加載的擴充卡挨個比對是否能适配該調用鍊

6、對請求頭中的last-modified進行處理

7、對HandlerExecutionChain中的所有HandlerInterceptor調用preHandle方法

8、真正的請求調用

9、沒有試圖的解析傳回

10、對HandlerExecutionChain中的所有HandlerInterceptor調用postHandle方法

11、處理結果解析（ModelAndView或者Exception），保證最終會執行是以Interceptor的triggerAfterCompletion

12、清除目前檔案上傳請求的資源

HandlerInterceptor方法調用

當我們需要使用Spring的攔截器時，會集實作HandlerInterceptor的preHandle、postHandle、triggerAfterCompletion方法。當第4步完成後我們擷取到了HandlerExecutionChain調用鍊，其中包括需要執行的攔截器和真正調用的方法（後面專門分析專門擷取的）。但是在上面的步驟看到了對攔截器的三個方法的調用時機，其實每個調用的方法都差不多，就看preHandle方法即可，調用HandlerExecutionChain的applyPreHandle方法如下：

boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {
    HandlerInterceptor[] interceptors = getInterceptors();
    if (!ObjectUtils.isEmpty(interceptors)) {
        for (int i = 0; i < interceptors.length; i++) {
            HandlerInterceptor interceptor = interceptors[i];
            if (!interceptor.preHandle(request, response, this.handler)) {
                triggerAfterCompletion(request, response, null);
                return false;
            }
            this.interceptorIndex = i;
        }
    }
    return true;
}

由于preHandle方法允許傳回false則不執行真實的Controller方法調用，是以需要每次判斷。但是在執行preHandle和postHandle方法時，都允許最後調用一次triggerAfterCompletion方法。都是在拿到調用鍊中的是以有序的攔截器，輪訓調用其對應的方法即可。

SpringMVC源碼（三）- DispatcherServlet請求分析

一、service請求（servlet請求轉換為Http請求）

二、doService請求（request中添加SpringMVC初始化的九大件資訊）

三、doDispatch

HandlerInterceptor方法調用

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