1.DispatcherServlet作用说明
DispatcherServlet提供Spring Web MVC的集中访问点，而且负责职责的分派，而且与Spring IoC容器无缝集成，从而可以获得Spring的所有好处。DispatcherServlet主要用作职责调度工作，本身主要用于控制流程，主要职责如下：
1、文件上传解析，如果请求类型是multipart将通过MultipartResolver进行文件上传解析；
2、通过HandlerMapping，将请求映射到处理器（返回一个HandlerExecutionChain，它包括一个处理器、多个HandlerInterceptor拦截器）；
3、通过HandlerAdapter支持多种类型的处理器(HandlerExecutionChain中的处理器)；
4、通过ViewResolver解析逻辑视图名到具体视图实现；
5、本地化解析；
6、渲染具体的视图等；
7、如果执行过程中遇到异常将交给HandlerExceptionResolver来解析。
2.具体实现说明
看着上面的总结肯定知道了大体上的处理过程，但是对于实际的逻辑实现肯定是一头雾水，DispatcherServlet的主要处理是在doDispatch（）方法中实现的。看看具体的代码实现：
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 {
processedRequest = checkMultipart(request);
multipartRequestParsed = (processedRequest != request);
// Determine handler for the current request.
mappedHandler = getHandler(processedRequest);
if (mappedHandler == null || mappedHandler.getHandler() == 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(request, mv);
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 {
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);
}
}
}
}
2.1请求类型处理
首先看代码：processedRequest = checkMultipart(request);就是为了处理文件类型的请求，如果请求类型是multipart将其转化通过MultipartResolver进行文件上传解析；如果不是就返回原来的请求。
protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {
if (this.multipartResolver != null && this.multipartResolver.isMultipart(request)) {
if (WebUtils.getNativeRequest(request, MultipartHttpServletRequest.class) != null) {
logger.debug("Request is already a MultipartHttpServletRequest - if not in a forward, " +
"this typically results from an additional MultipartFilter in web.xml");
}
else if (request.getAttribute(WebUtils.ERROR_EXCEPTION_ATTRIBUTE) instanceof MultipartException) {
logger.debug("Multipart resolution failed for current request before - " +
"skipping re-resolution for undisturbed error rendering");
}
else {
return this.multipartResolver.resolveMultipart(request);
}
}
// If not returned before: return original request.
return request;
}
首先判断是不是MultipartHttpServletRequest请求，如果是就不需要再处理了，如果是这继续判断是否含有文件解析失败异常，如果没有最后才会调用MultipartResolver接口实现类的resolveMultipart（）方法来具体处理这个请求并且最后返回MultipartHttpServletRequest。当然了如果不是一个文件类型的请求就会直接的返回原始请求。
2.2 根据请求获取能够处理该请求的HandlerExecutionChain
doDispatch（）方法中的mappedHandler = getHandler(processedRequest);完成了HandlerExecutionChain（处理器运行链，由处理器和处理器拦截器组成）的寻找定位工作。看下如何具体获取HandlerExecutionChain的过程：
protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
for (HandlerMapping hm : this.handlerMappings) {
if (logger.isTraceEnabled()) {
logger.trace(
"Testing handler map [" + hm + "] in DispatcherServlet with name ‘" + getServletName() + "‘");
}
HandlerExecutionChain handler = hm.getHandler(request);
if (handler != null) {
return handler;
}
}
return null;
}
这个过程就和SpringMVC处理化的过程有关了，SpringMVC初始化中通过initHandlerMappings(ApplicationContext context)方法初始化了List<HandlerMapping> handlerMappings;也就是通过将所有配置的请求映射放到了这个handlerMappings。这个方法的处理过程简单就是循环遍历这个handlerMappings，如果找到了就返回具体的HandlerMapping 中的HandlerExecutionChain，否则就返回null.如果返回的处理器为空，说明没有能够处理这个请求的 HandlerMapping，那么就会调用noHandlerFound(processedRequest, response)进行处理，看下noHandlerFound(processedRequest, response)具体的工作。
protected void noHandlerFound(HttpServletRequest request, HttpServletResponse response) throws Exception {
if (pageNotFoundLogger.isWarnEnabled()) {
pageNotFoundLogger.warn("No mapping found for HTTP request with URI [" + getRequestUri(request) +
"] in DispatcherServlet with name ‘" + getServletName() + "‘");
}
if (this.throwExceptionIfNoHandlerFound) {
ServletServerHttpRequest sshr = new ServletServerHttpRequest(request);
throw new NoHandlerFoundException(
sshr.getMethod().name(), sshr.getServletRequest().getRequestURI(), sshr.getHeaders());
}
else {
response.sendError(HttpServletResponse.SC_NOT_FOUND);
}
}
设置HTTP响应状态，并且抛出异常。
2.3 获取请求对应的处理器适配器
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());决定了获取请求对应的处理器适配器。看下具体的实现：
protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
for (HandlerAdapter ha : this.handlerAdapters) {
if (logger.isTraceEnabled()) {
logger.trace("Testing handler adapter [" + ha + "]");
}
if (ha.supports(handler)) {
return ha;
}
}
throw new ServletException("No adapter for handler [" + handler +
"]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
}
和获取请求的HandlerExecutionChain对象一样，遍历初始化就完成的handlerAdapters，有返回没有则抛出异常。
2.4 GET 和 HEAD请求处理
对于请求方式是GET或者HEAD的，会判断是否发生改变，如果没有改变就直接返回，否则就继续下去
2.5 调用具体的HandlerAdapter处理请求
方法ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception;是使用给定的处理器来处理这个请求，并且返回一个ModelAndView 。以SimpleServletHandlerAdapter为例，其实是调用一个Servlet的Service方法来进行相应的业务逻辑的处理的。
public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
throws Exception {
((Servlet) handler).service(request, response);
return null;
}
2.6返回视图的再处理
对上面处理后的视图或者异常转化成一个ModelAndView对象：
private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) throws Exception {
boolean errorView = false;
if (exception != null) {
if (exception instanceof ModelAndViewDefiningException) {
logger.debug("ModelAndViewDefiningException encountered", exception);
mv = ((ModelAndViewDefiningException) exception).getModelAndView();
}
else {
Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
mv = processHandlerException(request, response, handler, exception);
errorView = (mv != null);
}
}
// Did the handler return a view to render?
if (mv != null && !mv.wasCleared()) {
render(mv, request, response);
if (errorView) {
WebUtils.clearErrorRequestAttributes(request);
}
}
else {
if (logger.isDebugEnabled()) {
logger.debug("Null ModelAndView returned to DispatcherServlet with name ‘" + getServletName() +
"‘: assuming HandlerAdapter completed request handling");
}
}
if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
// Concurrent handling started during a forward
return;
}
if (mappedHandler != null) {
mappedHandler.triggerAfterCompletion(request, response, null);
}
}
如果有异常则将异常转化成ModelAndView，如果视图不为空，则调用render(mv, request, response);进行视图的解析渲染。
protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
// Determine locale for request and apply it to the response.
Locale locale = this.localeResolver.resolveLocale(request);
response.setLocale(locale);
View view;
if (mv.isReference()) {
// We need to resolve the view name.
view = resolveViewName(mv.getViewName(), mv.getModelInternal(), locale, request);
if (view == null) {
throw new ServletException("Could not resolve view with name ‘" + mv.getViewName() +
"‘ in servlet with name ‘" + getServletName() + "‘");
}
}
else {
// No need to lookup: the ModelAndView object contains the actual View object.
view = mv.getView();
if (view == null) {
throw new ServletException("ModelAndView [" + mv + "] neither contains a view name nor a " +
"View object in servlet with name ‘" + getServletName() + "‘");
}
}
// Delegate to the View object for rendering.
if (logger.isDebugEnabled()) {
logger.debug("Rendering view [" + view + "] in DispatcherServlet with name ‘" + getServletName() + "‘");
}
try {
view.render(mv.getModelInternal(), request, response);
}
catch (Exception ex) {
if (logger.isDebugEnabled()) {
logger.debug("Error rendering view [" + view + "] in DispatcherServlet with name ‘" +
getServletName() + "‘", ex);
}
throw ex;
}
}
首先获取请求的本地化处理并且将其设置到response中，如果指定了视图名字那么DispatcherServlet就会调用相应的视图解析器来进行解析，返回View对象
protected View resolveViewName(String viewName, Map<String, Object> model, Locale locale,
HttpServletRequest request) throws Exception {
for (ViewResolver viewResolver : this.viewResolvers) {
View view = viewResolver.resolveViewName(viewName, locale);
if (view != null) {
return view;
}
}
return null;
}
如果ModelAndView 没有视图引用说明，它已经含有了一个真实的View对象直接过去即可。如果获取的View对象为空则需要爆出异常。最后也是最重要的就是调用View对象的render方法（ view.render(mv.getModelInternal(), request, response)）进行视图的渲染了。看具体实现是在View的实现抽象类中AbstractView中。
public void render(Map<String, ?> model, HttpServletRequest request, HttpServletResponse response) throws Exception {
if (logger.isTraceEnabled()) {
logger.trace("Rendering view with name ‘" + this.beanName + "‘ with model " + model +
" and static attributes " + this.staticAttributes);
}
Map<String, Object> mergedModel = createMergedOutputModel(model, request, response);
prepareResponse(request, response);
renderMergedOutputModel(mergedModel, getRequestToExpose(request), response);
}
简单的说就是获取数据并且填充，打完收工。
3.总结
上面便是按照DispatcherServlet从接受到一个请求到请求完全处理完成所经历的一个过程，很是清晰明确。设计也很简单。从设计到实现，可以察觉设计模式的几大原则和具体的设计都有涉及，从中也学到了很多东西，接着学习，加油。