一、介绍
DispatcherServlet是SpringMVC的核心，为接收到的请求找到对应的控制器方法处理。
二、初始化源码分析
（1）Spring的IOC容器初始化
initWebApplicationContext()方法，用于配置Spring的IOC容器和将其放入ServletContext域中。
public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
if (servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE) != null) {
throw new IllegalStateException("Cannot initialize context because there is already a root application context present - check whether you have multiple ContextLoader* definitions in your web.xml!");
} else {
servletContext.log("Initializing Spring root WebApplicationContext");
Log logger = LogFactory.getLog(ContextLoader.class);
if (logger.isInfoEnabled()) {
logger.info("Root WebApplicationContext: initialization started");
}
long startTime = System.currentTimeMillis();
try {
//context是创建监听器的构造函数传来的Spring的IOC容器
if (this.context == null) {
this.context = this.createWebApplicationContext(servletContext);
}
if (this.context instanceof ConfigurableWebApplicationContext) {
//context向下转型为可配置的IOC容器
ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext)this.context;
if (!cwac.isActive()) {
if (cwac.getParent() == null) {
ApplicationContext parent = this.loadParentContext(servletContext);
//Spring的IOC容器本身作为父容器，
//后面SpringMVC的IOC作为子容器
cwac.setParent(parent);
}
//配置完IOC容器然后进行刷新
this.configureAndRefreshWebApplicationContext(cwac, servletContext);
}
}
//将配置完的IOC容器放入ServletContext域中
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);
ClassLoader ccl = Thread.currentThread().getContextClassLoader();
if (ccl == ContextLoader.class.getClassLoader()) {
currentContext = this.context;
} else if (ccl != null) {
currentContextPerThread.put(ccl, this.context);
}
if (logger.isInfoEnabled()) {
long elapsedTime = System.currentTimeMillis() - startTime;
logger.info("Root WebApplicationContext initialized in " + elapsedTime + " ms");
}
//返回配置完的IOC容器
return this.context;
} catch (Error | RuntimeException var8) {
logger.error("Context initialization failed", var8);
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, var8);
throw var8;
}
}
}
（2）SpringMVC的IOC容器初始化
HttpServletBean使用FrameworkServlet重写过的initServletBean()方法，方法里面会调用initWebApplicationContext()方法
protected WebApplicationContext initWebApplicationContext() {
//从ServletContext中获取Spring的IOC容器
WebApplicationContext rootContext = WebApplicationContextUtils.getWebApplicationContext(this.getServletContext());
WebApplicationContext wac = null;
//webApplicationContext为SpringMVC的IOC容器，
//AbstractDispatcherServletInitializer类中的registerDispatcherServlet方法
//使得在大多数情况下有一个SpringMVC的IOC容器，在正常情况下不为空
if (this.webApplicationContext != null) {
wac = this.webApplicationContext;
if (wac instanceof ConfigurableWebApplicationContext) {
//wac向下转型为可配置的IOC容器（和上面相比代码风格不一样啊~）
ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext)wac;
if (!cwac.isActive()) {
if (cwac.getParent() == null) {
//将Spring的IOC容器作为父容器
cwac.setParent(rootContext);
}
//配置完IOC容器然后进行刷新
this.configureAndRefreshWebApplicationContext(cwac);
}
}
}
if (wac == null) {
wac = this.findWebApplicationContext();
}
if (wac == null) {
wac = this.createWebApplicationContext(rootContext);
}
if (!this.refreshEventReceived) {
synchronized(this.onRefreshMonitor) {
this.onRefresh(wac);
}
}
if (this.publishContext) {
//将配置完的IOC容器放入ServletContext域中
String attrName = this.getServletContextAttributeName();
this.getServletContext().setAttribute(attrName, wac);
}
//返回配置完的IOC容器
return wac;
}
（3）总结：以上两个IOC容器都是先经过创建，然后再进行配置，最后刷新IOC容器并将其存储在ServletContext域中。
三、执行源码分析
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
boolean multipartRequestParsed = false;
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
try {
try {
ModelAndView mv = null;
Object dispatchException = null;
try {
//处理文件类型请求
processedRequest = this.checkMultipart(request);
multipartRequestParsed = processedRequest != request;
//根据请求找到对应的控制器方法和拦截器链
mappedHandler = this.getHandler(processedRequest);
if (mappedHandler == null) {
this.noHandlerFound(processedRequest, response);
return;
}
//获取控制器方法
HandlerAdapter ha = this.getHandlerAdapter(mappedHandler.getHandler());
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;
}
}
//执行拦截器中的preHandle方法
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}
//执行控制器方法，返回值为ModelAndView（会对方法的参数和返回值进行处理）
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
this.applyDefaultViewName(processedRequest, mv);
//执行拦截器中的postHandle方法
mappedHandler.applyPostHandle(processedRequest, response, mv);
} catch (Exception var20) {
dispatchException = var20;
} catch (Throwable var21) {
dispatchException = new NestedServletException("Handler dispatch failed", var21);
}
//该方法用于对控制器返回结果进行处理，见下一个代码块
this.processDispatchResult(processedRequest, response, mappedHandler, mv, (Exception)dispatchException);
} catch (Exception var22) {
this.triggerAfterCompletion(processedRequest, response, mappedHandler, var22);
} catch (Throwable var23) {
this.triggerAfterCompletion(processedRequest, response, mappedHandler, new NestedServletException("Handler processing failed", var23));
}
} finally {
if (asyncManager.isConcurrentHandlingStarted()) {
if (mappedHandler != null) {
mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
}
} else if (multipartRequestParsed) {
this.cleanupMultipart(processedRequest);
}
}
}
private void processDispatchResult(HttpServletRequest request, HttpServletResponse response, @Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv, @Nullable Exception exception) throws Exception {
boolean errorView = false;
if (exception != null) {
//处理异常
if (exception instanceof ModelAndViewDefiningException) {
this.logger.debug("ModelAndViewDefiningException encountered", exception);
mv = ((ModelAndViewDefiningException)exception).getModelAndView();
} else {
Object handler = mappedHandler != null ? mappedHandler.getHandler() : null;
//利用异常处理器来处理异常，返回一个新的ModelAndView
mv = this.processHandlerException(request, response, handler, exception);
errorView = mv != null;
}
}
//处理ModelAndView
if (mv != null && !mv.wasCleared()) {
//利用三大域对象来渲染视图
this.render(mv, request, response);
if (errorView) {
WebUtils.clearErrorRequestAttributes(request);
}
} else if (this.logger.isTraceEnabled()) {
this.logger.trace("No view rendering, null ModelAndView returned.");
}
if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
if (mappedHandler != null) {
//执行拦截器中的triggerAfterCompletion方法
mappedHandler.triggerAfterCompletion(request, response, (Exception)null);
}
}
}
