Spring入门进阶之DispatcherServlet源码分析

使用过HttpServlet的都应该用过其doGet和doPost方法，接下来看看DispatcherServlet对这两个方法的实现（源码在DispatcherServlet的父类FrameworkServlet中）：

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

processRequest(request, response);
}

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

processRequest(request, response);
}

方法里又将逻辑交由processRequest(request, response)方法处理，跟进源码：

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

// 记录当前时间，用于计算请求的处理时间
    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, response, startTime, failureCause);
    }
}

从源码可以看出在该方法中对请求进行处理，处理细节在doService方法中实现，同时在处理请求前后也做了准备及处理工作：

1. 提取LocaleContext及RequestAttributes两个属性保证可以在当前请求后还能恢复；

2. 根据当前的request创建对应的LocaleContext及RequestAttributes，并绑定到当前线程；

3. 委托给doService方法进一步处理；

4. 请求结束后恢复线程到原始状态；

5. 请求处理结束后发布事件通知。

跟进doService方法源码：

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));
            }
        }
    }