Spring MVC 源码分析之 DispatcherServlet

本文主要是介绍Spring MVC 源码分析之 DispatcherServlet，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

一、概述

二、类图

三、初始化过程

四、请求过程

五、总结

一、概述

MVC大家比较熟悉

M即model，是业务处理层，与我们开发中的（service、dao、model）等对应起来；
V即view，是视图层，以前jsp、freemarker、velocity等，现在都是前后端分离了。使用@ResponseBody注解把Controller方法返回的对象通过转换器转换成指定的格式（如json/xml/protobuf）后，再写入到Response对象的body区，不再走视图解析器，把渲染到工作交给前端去做。
C，即controller，控制器，可以分为前端控制器（负责请求的分发，即DispatcherServlet）、映射处理器（uri与处理方法的映射，即HandlerMapping）、业务控制器（即我们的controller层）、视图解析器（即ViewResolver）。

二、类图

如上图类的继承关系可知，DispatcherServlet就是一个Servlet.

三、初始化过程

了解Servlet的都知道在Servlet中主要的方法有：

init 初始化方法
service 用于处理请求的方法
destroy servlet的销毁方法

3.1、HttpServletBean类初始化方法 init

public final void init() throws ServletException {// Set bean properties from init parameters.PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);if (!pvs.isEmpty()) {try {//将Servlet中配置的参数封装到pvs变量中，requiredProperties为必须参数，如果没//配置将报异常BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));//模板方法，可以在子类中实现，做一些初始化工作，initBeanWrapper(bw);bw.setPropertyValues(pvs, true);}catch (BeansException ex) {if (logger.isErrorEnabled()) {logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);}throw ex;}}// Let subclasses do whatever initialization they like.// 模板方法，子类初始化入口方法，在子类中可以做一些具体的实现方法initServletBean();}

3.2、在 FrameworkServlet 中实现了 initServletBean 方法

@Overrideprotected final void initServletBean() throws ServletException {// 略 .....try {//主要方法，主要实现了 初始化 WebApplicationContext this.webApplicationContext = initWebApplicationContext();initFrameworkServlet();}catch (ServletException | RuntimeException ex) {logger.error("Context initialization failed", ex);throw ex;}// 略 .....}}

3.3、initWebApplicationContext 方法

protected WebApplicationContext initWebApplicationContext() {//  略...if (!this.refreshEventReceived) {// Either the context is not a ConfigurableApplicationContext with refresh// support or the context injected at construction time had already been// refreshed -> trigger initial onRefresh manually here.synchronized (this.onRefreshMonitor) {onRefresh(wac);  //重点方法}}if (this.publishContext) {// Publish the context as a servlet context attribute.String attrName = getServletContextAttributeName();getServletContext().setAttribute(attrName, wac);}return wac;}

initWebApplicationContext方法做了三件事：

获取spring的根容器rootContext。
设置webApplicationContext并根据情况调用onRefresh方法。
将webApplicationContext设置到ServletContext中。

3.4、DispatcherServlet中的 onRefresh 方法

@Overrideprotected void onRefresh(ApplicationContext context) {initStrategies(context);}/*** Initialize the strategy objects that this servlet uses.* <p>May be overridden in subclasses in order to initialize further strategy objects.*/protected void initStrategies(ApplicationContext context) {//初始化多媒体解析器，这个不分析initMultipartResolver(context);//初始化国际化解析器，这个不进行分析initLocaleResolver(context);//初始化主题解析器，基本不用，不进行分析initThemeResolver(context);//初始化映射器initHandlerMappings(context);//初始化适配器initHandlerAdapters(context);//初始化异常解析器initHandlerExceptionResolvers(context);//初始化视图名转换器，不进行分析initRequestToViewNameTranslator(context);//初始化视图解析器，前端分离后，用得少了，也分析下initViewResolvers(context);//初始化FlashMapManager，只知道是重定向时用来保存数据用的，没有使用过，这里不进行分析initFlashMapManager(context);}

下面贴的代码的几个初始化代码的逻辑其实是一样的，分为三步，以initHandlerMappings进行说明

判断是否寻找所有容器中实现HandlerMapping接口的，是则寻找所有容器中实现了HandlerMapping接口的对象，设置到handlerMappings对象中并排序（平常开发中走的都是这个分支）
否的话，只找寻到前容器实现了HandlerMapping接口的对象
如果上面查找后为空的话，则加载配置文件中的类并实例化，再设置到handlerMappings中。注意的是：这个配置文件位置是:org/springframework/web/servlet/DispatcherServlet.properties

至此，DispatcherServlet的初始化工作已经完成。

四、请求过程

上文介绍过 Servlet中处理请求的为 service 方法。下面先看service方法的处理逻辑

4.1、HttpServlet中的 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 logicdoGet(req, resp);} else {long ifModifiedSince;try {ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);} catch (IllegalArgumentException iae) {// Invalid date header - proceed as if none was setifModifiedSince = -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 lessmaybeSetLastModified(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 {//// Note that this means NO servlet supports whatever// method was requested, anywhere on this server.//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);}}

从源码中可得知 HttpServlet中 service 方法根据请求方法，把请求具体交给了 doGet、doPost、doPut等方法来处理

4.2、FrameworkServlet 中的 service 方法

在FrameworkServlet中重写了 service 方法同时也重写了 doGet、doPost，doPut等方法，源码如下所示

@Overrideprotected void service(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {HttpMethod httpMethod = HttpMethod.resolve(request.getMethod());if (httpMethod == HttpMethod.PATCH || httpMethod == null) {processRequest(request, response);}else {super.service(request, response);}}@Overrideprotected final void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {// 主要通过此方法实现请求的分发processRequest(request, response);}

有源码可知，在doGet 方法中有调用了 processRequest方法，其他doXX中同样调用了此方法，接下来看看此方法。

protected final void processRequest(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {long startTime = System.currentTimeMillis();Throwable failureCause = null;// 获取LocaleContextHolder 中保存的LocalContextLocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();//获取当前请求的LocaleContextLocaleContext localeContext = buildLocaleContext(request);//获取RequestContextHolder中保存的RequestAttributesRequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();//获取当前请求的ServletRequestAttributesServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);// 略 .....//将当前请求的LocaleContext和ServletRequestAttributes保存到//LocaleConextHolder和RequestContextHolder中initContextHolders(request, localeContext, requestAttributes);try {//模板方法，在子类中实现doService(request, response);}catch (ServletException | IOException ex) {//略 ....}catch (Throwable ex) {//略 ....}finally {//恢复之前的LocaleContext和ServletRequestAttributes到//LocaleConextHolder和RequestContextHolder中resetContextHolders(request, previousLocaleContext, previousAttributes);if (requestAttributes != null) {requestAttributes.requestCompleted();}//发布ServletRequestHandledEvent消息publishRequestHandledEvent(request, response, startTime, failureCause);}}

说明：由于在请求时把LocaleContext和ServletRequestAttributes保存到了LocaleContextHolder和RequestContextHolder中，这两个类都是在ThreadLocal中所有在后续的方法中可以根据通过这两个类获取LocalContext和ServletRequestAttributes，从而获取HttpServletRequest、HttpServletResponse和HttpSession。

4.3、DispatcherServlet中实现了 doService方法，

protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {//省略 部分代码 .....try {//真正处理 请求转发的方法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);}}}}

有源码可知 doService方法中主要是调用了 doDispatch方法，接下来看看此方法的实现

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 {//检测是否为上传请求processedRequest = checkMultipart(request);multipartRequestParsed = (processedRequest != request);// Determine handler for the current request.//根据 request 查找对应的 Handler mappedHandler = getHandler(processedRequest);if (mappedHandler == null) {noHandlerFound(processedRequest, response);return;}// Determine handler adapter for the current request.//根据 Handler查找执行此Handler的 handler适配器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 (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {return;}}//执行拦截器的前置方法if (!mappedHandler.applyPreHandle(processedRequest, response)) {return;}// Actually invoke the handler.//实际调用 Handlermv = ha.handle(processedRequest, response, mappedHandler.getHandler());if (asyncManager.isConcurrentHandlingStarted()) {return;}applyDefaultViewName(processedRequest, mv);//执行拦截器中的 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);}//处理返回结果。包括处理异常、渲染页面、发出完成通知调用拦截器的 afterComletion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 {if (asyncManager.isConcurrentHandlingStarted()) {// Instead of postHandle and afterCompletionif (mappedHandler != null) {mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);}}else {// Clean up any resources used by a multipart request.if (multipartRequestParsed) {cleanupMultipart(processedRequest);}}}}