6、SpringMVC源码分析（1）：分析DispatcherServlet.doDispatch方法，了解总体流程

所有的http请求都会交给DispatcherServlet类的doDispatch方法进行处理，将DispatcherServlet.doDispatch函数的javadoc复制到下面：

/*
* Process the actual dispatching to the handler.
*
* The handler will be obtained by applying the servlet's HandlerMappings in
* order.The HandlerAdapter will be obtained by querying the servlet's
* installed HandlerAdapters to find the first that supports the handler
* class.
*
* All HTTP methods are handled by this method. It's up to HandlerAdapters
* or handlers themselves to decide which methods are acceptable.
*/
void org.springframework.web.servlet.DispatcherServlet.doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception;

下面分析doDispatch方法的流程，采用注释源码的方式：

protected void doDispatch(HttpServletRequest request,
HttpServletResponse response) throws Exception {

// processedRequest是经过checkMultipart方法处理过的request请求
HttpServletRequest processedRequest = request;
/**
* Handler execution chain, consisting of handler object and any handler
* interceptors. Returned by HandlerMapping's HandlerMapping.getHandler
* method. 看看HandlerExecutionChain类的属性就很清楚了：
*
public class HandlerExecutionChain {

private final Object handler; //这个就是和该请求对应的handler处理方法

//里面记录了所有的(any handler interceptors)和该请求相关的拦截器
private HandlerInterceptor[] interceptors;

private List<HandlerInterceptor> interceptorList;

private int interceptorIndex = -1;

//...
}
*
*/
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.Return a handler
// and any interceptors for this request.
/*
* 得到的mappedHandler包含一个请求的handler处理方法以及与该请求相关的所有拦截器
*
* DispatcherServlet.getHandler方法会在底层调用HandlerMapping.getHandler方法
* ，这个方法中会遍 历DispatcherServlet中的private List<HandlerMapping>
* handlerMappings链表，找到能够处理当前 request请求的第一个HandlerMapping实例并返回：
*
protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
for (HandlerMapping hm : this.handlerMappings) {
HandlerExecutionChain handler = hm.getHandler(request);
if (handler != null) {
return handler;
}
}
return null;
}
*
*/
mappedHandler = getHandler(processedRequest);
// 如果没有找到和该请求相对应的mappedHandler，那么就会直接返回，并应答noHandlerFound异常
if (mappedHandler == null || mappedHandler.getHandler() == null) {
noHandlerFound(processedRequest, response);
return;
}

// Determine handler adapter for the current request.
/*
* HandlerAdapter: 它是一个接口public interface HandlerAdapter
* 看看源码上的说明：The DispatcherServlet accesses all installed
* handlers through this interface, meaning that it does not
* contain code specific to any handler type.
*
* 从后面的源码看出，在使用@RequestMapping注解标注handler方法的时候，获取到的是HandlerAdapter的
* RequestMappingHandlerAdapter实现类的一个对象。
*
* 可以看看DispatcherServlet.getHandlerAdapter方法的定义，这个对理解上回很有帮助，我们会发现
* ，getHandlerAdapter 方法和上面提到的getHandler方法一样都是寻找第一个可用的作为返回结果：
*
*
protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
//this.handlerAdapters的定义是 private List<HandlerAdapter> handlerAdapters
for (HandlerAdapter ha : this.handlerAdapters) {
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");
}
*
*/
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;
}
}

// Apply preHandle methods of registered interceptors.
/*
* 会调用所有注册拦截器的preHandle方法，如果preHandle方法的返回结果为true，则会继续执行下面的程序，
* 否则会直接返回。
*
* 分析一下HandlerExecutionChain.applyPreHandle方法的源码 ：
boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {
//从上面的HandlerExecutionChain定义处可以看见有个interceptors，还有一个interceptorList。不知道有什么区别？？！
HandlerInterceptor[] interceptors = getInterceptors();
//如果已经注册有拦截器，则遍历拦截器
if (!ObjectUtils.isEmpty(interceptors)) {
for (int i = 0; i < interceptors.length; i++) {
HandlerInterceptor interceptor = interceptors[i];
//如果注册拦截器的preHandle方法返回一个false，则该applyPreHandle方法就会返回false，从而在doDispatcher中的代码就不会往下执行了
if (!interceptor.preHandle(request, response, this.handler)) {

//这个方法要注意，它会调用所有已经成功执行的拦截器的afterCompletion方法，而且是反序调用的过程，可以分析triggerAfterCompletion
//的源代码，主要是利用interceptorIndex反减的方式实现的。下面是源码的英文注释：
//Trigger afterCompletion callbacks on the mapped HandlerInterceptors.
//Will just invoke afterCompletion for all interceptors whose preHandle invocation
//has successfully completed and returned true.
triggerAfterCompletion(request, response, null);
return false;
}
//没成功执行一个拦截器的preHandle方法，其interceptorIndex就会增加1；原始值为-1。
this.interceptorIndex = i;
}
}
return true;
}
*
*
*  顺带看看triggerAfterCompletion的源代码，很容易理解为什么拦截器的afterCompletion方法是反序执行的：
*    void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response, Exception ex)
throws Exception {

HandlerInterceptor[] interceptors = getInterceptors();
if (!ObjectUtils.isEmpty(interceptors)) {
for (int i = this.interceptorIndex; i >= 0; i--) {
HandlerInterceptor interceptor = interceptors[i];
try {
interceptor.afterCompletion(request, response, this.handler, ex);
}
catch (Throwable ex2) {
logger.error("HandlerInterceptor.afterCompletion threw exception", ex2);
}
}
}
}
*
*
*/
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}

// Actually invoke the handler.
/*
* 在这个函数里面会真正的执行request请求相对于的handler方法，可以想象：在真正调用方法之前还会有很多的
* 先前处理。在这里仅仅是分析出大概的代码执行流程，其细节的部分在后面的单独模块源码分析的时候做详细的讲解。
* 上面讲解到HandlerAdapter是一个接口：public interface HandlerAdapter，那么必然会有很多
* 中实现类，在采用注解@RequstMapping的方式标注handler的情况下，ha.handle方法会在底层调用具体的
* HandlerAdapter类实现方法RequestMappingHandlerAdapter.handleInternal
*
* 分析一下RequestMappingHandlerAdapter.handleInternal的源代码：
protected ModelAndView handleInternal(HttpServletRequest request,
HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {
//好像是看control的类定义处是否使用了@SessionAttributes注解，checkAndPrepare方法有什么作用？？？
if (getSessionAttributesHandler(handlerMethod).hasSessionAttributes()) {
// Always prevent caching in case of session attribute management.
checkAndPrepare(request, response, this.cacheSecondsForSessionAttributeHandlers, true);
}
else {
// Uses configured default cacheSeconds setting.
checkAndPrepare(request, response, true);
}

// Execute invokeHandlerMethod in synchronized block if required.
// 这里是个值得注意的地方，synchronizeOnSession的值默认为false，如果通过某个方法使得其为true，那么request对应的handler
// 将会被放在同步快中进行处理。在什么时机下，使用什么方法才能将其设置为true呢？？？
if (this.synchronizeOnSession) {
HttpSession session = request.getSession(false);
if (session != null) {
Object mutex = WebUtils.getSessionMutex(session);
// 将handler放在同步块中处理
synchronized (mutex) {
return invokeHandleMethod(request, response, handlerMethod);
}
}
}
//在invokeHandleMethod中会①将所有标注有@ModelAttrib的方法都执行一遍，②调用invokeAndHandle(webRequest, mavContainer)
//方法，在这里面调用handler方法，③最后调用getModelAndView(mavContainer, modelFactory, webRequest)方法的到ModelAndView。
//invokeHandleMethod这个方法还有很多东西要分析，留在后面。
//从上面的③我们可以看出，无论handler采用哪种模型化处理方式，最后都是将结果转化为ModelAndView
return invokeHandleMethod(request, response, handlerMethod);
}
*/
mv = ha.handle(processedRequest, response,
mappedHandler.getHandler());

if (asyncManager.isConcurrentHandlingStarted()) {
return;
}

applyDefaultViewName(request, mv);
/*
* 调用request相关的拦截器的postHandle方法，注意，这个也是反序调用的。看看源代码：
*
void applyPostHandle(HttpServletRequest request, HttpServletResponse response, ModelAndView mv) throws Exception {
HandlerInterceptor[] interceptors = getInterceptors();
if (!ObjectUtils.isEmpty(interceptors)) {
//注意，这里也是反序执行，而且是所有成功执行了的postHandle拦截器
for (int i = interceptors.length - 1; i >= 0; i--) {
HandlerInterceptor interceptor = interceptors[i];
//这里传入的参数中有mv，也就是说，我们是有办法在拦截器的postHandle方法中修改已经返回的mv
interceptor.postHandle(request, response, this.handler, 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);
}
}
}
}

看完源代码就可以总结出doDispath方法中处理http请求的流程了：