SpringMVC之源码分析--请求过程

概述

根据Servlet规范，当用户请求到达应用时，由Servlet的service()方法进行处理，对于Spring MVC而言，处理用户请求的入口为DispatcherServlet，通过其父类FrameworkServlet的service()拦截处理。

FrameworkServlet

用户请求的入口方法为service(request,response)，源码如下：

/**
 * Override the parent class implementation in order to intercept PATCH requests.
 */
@Override
protected void service(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    HttpMethod httpMethod = HttpMethod.resolve(request.getMethod());
    // Spring3.0增加了PATCH方法拦截处理
    if (httpMethod == HttpMethod.PATCH || httpMethod == null) {
        processRequest(request, response);
    } else { // GET, HEAD, POST, PUT, DELETE, OPTIONS, TRACE
        super.service(request, response);
    }
}

该方法重写了HttpServlet的service()方法，并增加了PATCH请求的拦截处理，其他诸如GET、POST、PUT等请求则调用父类的service方法，HttpServlet类的service()方法就是根据请求类型进行不同的路由处理，如为GET请求，则路由至doGet()方法处理，以此类推，此处省略源码。

假如用户请求为GET，则路由到子类（FrameworkServlet）的doGet()方法进行处理，doGet()源码如下：

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

其实，所有类型的请求最终都由processRequest()方法进行处理，processRequest方法源码如下：

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

    long startTime = System.currentTimeMillis();
    Throwable failureCause = null;
    // 
    LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
    // 构建请求(request)的LocalContext上下文，提供基本的作为当前地区的主要语言环境
    LocaleContext localeContext = buildLocaleContext(request);

    RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
    // 构建请求(request)的ServletRequestAttributes对象，保存本次请求的request和response
    ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);
    // 异步请求处理的管理核心类
    WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
    asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());
    // 绑定request context上线到当前线程
    initContextHolders(request, localeContext, requestAttributes);

    try {
        // 模板方法，调用子类（DispatcherServlet）的doService方法进行处理
        doService(request, response);
    }
    catch (ServletException | IOException ex) {
        failureCause = ex;
        throw ex;
    }
    catch (Throwable ex) {
        failureCause = ex;
        throw new NestedServletException("Request processing failed", ex);
    }

    finally {
        // 接触请求线程与LocalContext和RequestAttributes的绑定
        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");
                }
            }
        }
        // 发布ApplicationEvent事件，可由ApplicationListener进行监听
        // 继承ApplicationListener接口，实现onApplicationEvent()接口，并注册到spring容器，即可捕获该事件
        publishRequestHandledEvent(request, response, startTime, failureCause);
    }
}

DispatcherServlet

@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) + "]");
    }

    // 如果是include请求，保存request attribute快照数据，并在finally中进行还原
    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));
            }
        }
    }

    // 把环境上下文设置到请求域中
    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 {
        // 调用请求处理方法
        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);
            }
        }
    }
}

// 请求处理关键方法
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) {
                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;
            }

            // Actually invoke the handler.
            mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

            if (asyncManager.isConcurrentHandlingStarted()) {
                return;
            }

            applyDefaultViewName(processedRequest, mv);
            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);
        }
        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()) {
            // Instead of postHandle and afterCompletion
            if (mappedHandler != null) {
                mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
            }
        }
        else {
            // Clean up any resources used by a multipart request.
            if (multipartRequestParsed) {
                cleanupMultipart(processedRequest);
            }
        }
    }
}

doDispatch()是Spring MVC中重要的方法，用户处理用户请求，其主要经过过程如下：

• 获取当前请求的Handler
• 获取当前请求的Handler Adapter
• 执行preHandle方法
• 执行Handle方法，即Controller中的方法
• 执行postHandle方法
• 处理返回结果

总结

本文简要概述了Spring MVC处理用户请求过程中所涉及的主要方法，对请求处理的流程做了概述，后续将对内部细节进行分析，以加深理解。

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