springMVC源码分析--DispatcherServlet请求获取及处理

在之前的博客 springMVC源码分析--容器初始化（二）DispatcherServlet中我们介绍过DispatcherServlet，是在容器初始化过程中出现的，我们之前也说过DispatcherServlet其实就是一个HttpServlet，其实他是HttpServlet的子类，所以它和普通的HttpServlet有同样的配置：
<servlet>
<servlet-name>springmvc</servlet-name>
<servlet-class>
org.springframework.web.servlet.DispatcherServlet
</servlet-class>
<init-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath:springmvc-config.xml</param-value>
</init-param>
<load-on-startup>1</load-on-startup>
</servlet>

<servlet-mapping>
<servlet-name>springmvc</servlet-name>
<url-pattern>*.action</url-pattern>
</servlet-mapping>仅仅把DispatcherServlet当做一个Servlet的话，上面配置的含义就是这个Servlet会被所有的*.action的请求所调用。
既然DispatcherServlet是一个HttpServlet那么它应该会实现HttpServlet提供的如下方法：

当然这些所有的方法的实现是DispatcherServlet的父类FrameworkServlet中实现的。

当然这些实现方法中的默认实现其实是如下的
FrameworkServlet类中
@Override
protected final void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

processRequest(request, response);
}

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

processRequest(request, response);
}

processRequest的实现是在FrameworkServlet中，此方法中最主要的操作就是调用doService方法

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 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);
}
}

doService方法的最终实现是在DispatcherServlet中，这样所有的Http请求（GET、POST、PUT和DELETE等）的最终操作就DispatcherServlet中实现了。
DispatcherServlet中doService的实现如下，对Request设置了一些全局属性，最终接下来的操作是在doDispatcher函数中实现了。
//获取请求，设置一些request的参数，然后分发给doDispatch
@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;
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));
}
}
}

// Make framework objects available to handlers and view objects.
/* 设置web应用上下文**/
request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
/* 国际化本地**/
request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
/* 样式**/
request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
//设置样式资源
request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());
//请求刷新时保存属性
FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
if (inputFlashMap != null) {
request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
}
//Flash attributes 在对请求的重定向生效之前被临时存储（通常是在session)中，并且在重定向之后被立即移除
request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
//FlashMap 被用来管理 flash attributes 而 FlashMapManager 则被用来存储，获取和管理 FlashMap 实体.
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);
}
}
}
}doDispatch函数中完成了对一个请求的所有操作，包含的内容还是比较多的，我们就不做详细分解，接下来我们会一步一步的分析一个请求调用Controller的完整过程。

/**
*将Handler进行分发，handler会被handlerMapping有序的获得
*通过查询servlet安装的HandlerAdapters来获得HandlerAdapters来查找第一个支持handler的类
*所有的HTTP的方法都会被这个方法掌控。取决于HandlerAdapters 或者handlers 他们自己去决定哪些方法是可用
*@param request current HTTP request
*@param response current HTTP response
*/
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
/* 当前HTTP请求**/
HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
boolean multipartRequestParsed = false;

WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

try {
ModelAndView mv = null;
Exception dispatchException = null;

try {
//判断是否有文件上传
processedRequest = checkMultipart(request);
multipartRequestParsed = (processedRequest != request);

// 获得HandlerExecutionChain，其包含HandlerIntercrptor和HandlerMethod
mappedHandler = getHandler(processedRequest);
if (mappedHandler == null || mappedHandler.getHandler() == null) {
noHandlerFound(processedRequest, response);
return;
}

//获得HandlerAdapter
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

//获得HTTP请求方法
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;
}
}
//如果有拦截器的话，会执行拦截器的preHandler方法
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}

//返回ModelAndView
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
//当view为空时，，根据request设置默认view
applyDefaultViewName(processedRequest, mv);
//执行拦截器的postHandle
mappedHandler.applyPostHandle(processedRequest, response, mv);
}
catch (Exception ex) {
dispatchException = ex;
}
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
}
catch (Exception ex) {
triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
}
catch (Error err) {
triggerAfterCompletionWithError(processedRequest, response, mappedHandler, err);
}
finally {
//判断是否是异步请求
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.
//删除上传资源
if (multipartRequestParsed) {
cleanupMultipart(processedRequest);
}
}
}
}调用完doDispatch之后就完成了一个请求的访问，其会将渲染后的页面或者数据返回给请求发起者。