首先看下面这种张图，这张图说明了spring mvc整体的流程。
本文讲的就是如何从DispatcherServlet中得到ModerAndView的过程。
首先看DispatherServlet这个类的doService方法，学过servlet的人都知道，它是web容器处理请求的入口。
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.
request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());
FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
if (inputFlashMap != null) {
request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
}
request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);
try {
doDispatch(request, response);
}
finally {
if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
return;
}
// Restore the original attribute snapshot, in case of an include.
if (attributesSnapshot != null) {
restoreAttributesAfterInclude(request, attributesSnapshot);
}
}
}
可以看到，doService做的事情一共就两件，一是保存request的Attribute并在框架处理完之后恢复回去，以防止attribute在框架处理的过程中*坏；第二件事就是在第36行调用doDispatch，进入真正的spring mvc流程中。我们来看doDispatch函数，这个函数是DispatchServlet控制整个spring mvc流程的核心，文章开始的那张图所描述的流程都是在这个函数中完成的。我们来看一下这个函数。
/**
* Process the actual dispatching to the handler.
* <p>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.
* <p>All HTTP methods are handled by this method. It's up to HandlerAdapters or handlers
* themselves to decide which methods are acceptable.
* @param request current HTTP request
* @param response current HTTP response
* @throws Exception in case of any kind of processing failure
*/
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;
} try {
// Actually invoke the handler.
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
}
finally {
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
mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
return;
}
// Clean up any resources used by a multipart request.
if (multipartRequestParsed) {
cleanupMultipart(processedRequest);
}
}
}
这个方法的核心工作内容包括这么几件：处理http请求得到模型和视图（存在一个ModelAndView对象中）、处理拦截器、渲染视图、卸载multipart内容，以及获取用于处理模型所需的处理器HanderExecutionChain和适配器HandlerAdapter。
http的请求具体会被怎么处理将取决于处理器执行器HanderExecutionChain和适配器HandlerAdapter，所以我们先看这两个对象是怎么得到的。方法注释的已经大致说明了这两个对象是如何得到的：HanderExecutionChain是通过应用servlet所拥有的HanderMapping生成的，HandlerAdapter是通过依次查询servlet所拥有的适配器中能够支持执行器的适配器得到的。
那么我们就来看一下HanderExecutionChain和HandlerAdapter到底是怎么得到的。
看28行的getHandler，HanderExecutionChain是从这里获取的，跟进去看：
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;
}
看第7行，真的是通过DistpacherServlet的HandlerMappings得到的,跟进去看怎么得到的：
发现这个servlet拥有的一个HM是AbstractHanderMethondMapping对象，跟进他的方法，发现它在第3行获取了一个HandlerMethod（名为handler的object对象）。
@Override
public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
Object handler = getHandlerInternal(request);
if (handler == null) {
handler = getDefaultHandler();
}
if (handler == null) {
return null;
}
// Bean name or resolved handler?
if (handler instanceof String) {
String handlerName = (String) handler;
handler = getApplicationContext().getBean(handlerName);
}
return getHandlerExecutionChain(handler, request);
}
这个HandlerMethod对象存的是什么呢？我们看看：
原来是它封装了我们映射到的控制器，包括bean，以及map到的方法。有了这个封装对象，剩下的事情就好办了。
继续往下看便到了第15行，跟进去getHandlerExecutionChain方法：
protected HandlerExecutionChain getHandlerExecutionChain(Object handler, HttpServletRequest request) {
HandlerExecutionChain chain = (handler instanceof HandlerExecutionChain ?
(HandlerExecutionChain) handler : new HandlerExecutionChain(handler));
chain.addInterceptors(getAdaptedInterceptors());
String lookupPath = this.urlPathHelper.getLookupPathForRequest(request);
for (MappedInterceptor mappedInterceptor : this.mappedInterceptors) {
if (mappedInterceptor.matches(lookupPath, this.pathMatcher)) {
chain.addInterceptor(mappedInterceptor.getInterceptor());
}
}
return chain;
}
可以看到，这个方法new了HandlerExecutionChain对象，并且把HM的拦截器和用户的拦截去都加进去了。至此HandlerExecutionChain的获取过程就讲完了。
接下来看适配器HandlerAdapter是怎么来的。
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");
}
真的就是看servlet有那些适配器，然后一个个查询是否支持，最后返回。
HanderExecutionChain和HandlerAdapter都有了，那么接下来就要看它们是怎么获取到模型和视图了。
跟进去dispatch方法的56行，一路下去，来到了 ModelAndView invokeHandleMethod(HttpServletRequest request, HttpServletResponse response, HandlerMethod handlerMethod) 方法，这个方法是HandlerAdapter适配器的方法啊， 来看看这个方法：
private ModelAndView invokeHandleMethod(HttpServletRequest request,
HttpServletResponse response, HandlerMethod handlerMethod) throws Exception { ServletWebRequest webRequest = new ServletWebRequest(request, response); WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);
ServletInvocableHandlerMethod requestMappingMethod = createRequestMappingMethod(handlerMethod, binderFactory); ModelAndViewContainer mavContainer = new ModelAndViewContainer();
mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));
modelFactory.initModel(webRequest, mavContainer, requestMappingMethod);
mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect); AsyncWebRequest asyncWebRequest = WebAsyncUtils.createAsyncWebRequest(request, response);
asyncWebRequest.setTimeout(this.asyncRequestTimeout); final WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
asyncManager.setTaskExecutor(this.taskExecutor);
asyncManager.setAsyncWebRequest(asyncWebRequest);
asyncManager.registerCallableInterceptors(this.callableInterceptors);
asyncManager.registerDeferredResultInterceptors(this.deferredResultInterceptors); if (asyncManager.hasConcurrentResult()) {
Object result = asyncManager.getConcurrentResult();
mavContainer = (ModelAndViewContainer) asyncManager.getConcurrentResultContext()[0];
asyncManager.clearConcurrentResult(); if (logger.isDebugEnabled()) {
logger.debug("Found concurrent result value [" + result + "]");
}
requestMappingMethod = requestMappingMethod.wrapConcurrentResult(result);
} requestMappingMethod.invokeAndHandle(webRequest, mavContainer); if (asyncManager.isConcurrentHandlingStarted()) {
return null;
} return getModelAndView(mavContainer, modelFactory, webRequest);
}
终于知道为什么需要适配器了，原来这个适配器的方法中，就适配了spring mvc和servlet，还有几个适配对象，其中最重要的是ServletWebRequest webRequest ，它适配了request。另外还有ModelAndViewContainer mavContainer，通过它持有模型和视图。
另外看这三行，这三行采用了工厂模式。
第一行获取了数据绑定的工厂，最重要的是它有handlerMethod。然后它会传给第三行，得到ServletInvocableHandlerMethod requestMappingMethod 。这个 requestMappingMethod 就是我们进入控制器调用的关键。
第二行获取了模型工厂，有了它，就可以创建模型了。
值得注意的是，这三个对象，每个里面都有handlerMethod的信息。
WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);
ServletInvocableHandlerMethod requestMappingMethod = createRequestMappingMethod(handlerMethod, binderFactory);
看第35行，requestMappingMethod.invokeAndHandle(webRequest, mavContainer)，这里开始真正的填充模型了，怎么填充呢？其实猜都能猜到，首先它自己有handlermethod，又传入了webRequest和mavContainer，很容易想到，它肯定是根据请求，通过反射获取控制器的requestMap方法，将参数传入并调用方法，最后得到控制器处理后的模型，并得到控制器指定的视图。至于传入的参数是如何得到的，我在另一篇文章李有比较详细的描述，请看spring mvc中的控制器方法中的参数从哪里传进来这篇文章。
看起来貌似已经把整个流程讲完了，但是等等，有个很重要的问题，生成处理器执行器HandlerExecutionChain的时候需要从HM里get到一个真正的处理器传入给HandlerExecutionChain的构造器。这个处理器怎么来的？这个带着这个疑问，我们继续来看他们是怎么得到的。
前面已经说了，这个handler是在HandlerExecutionChain getHandler(HttpServletRequest request) 的第三行调用HM的Object handler = getHandlerInternal(request)得到的，我们看一看getHandlerInternal这个函数：
protected HandlerMethod getHandlerInternal(HttpServletRequest request) throws Exception {
String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);
if (logger.isDebugEnabled()) {
logger.debug("Looking up handler method for path " + lookupPath);
}
HandlerMethod handlerMethod = lookupHandlerMethod(lookupPath, request);
if (logger.isDebugEnabled()) {
if (handlerMethod != null) {
logger.debug("Returning handler method [" + handlerMethod + "]");
}
else {
logger.debug("Did not find handler method for [" + lookupPath + "]");
}
}
return (handlerMethod != null ? handlerMethod.createWithResolvedBean() : null);
}
可以看到，getHandlerInternal是通过请求的路径来查询得到那个关键的处理器（在这里是一个HandlerMethod，它含有控制器的requestMap方法签名），再看一下第6行，它是怎么通过路径查询到处理器的，跟进去lookupHandlerMethod函数：
protected HandlerMethod lookupHandlerMethod(String lookupPath, HttpServletRequest request) throws Exception {
List<Match> matches = new ArrayList<Match>();
List<T> directPathMatches = this.urlMap.get(lookupPath);
if (directPathMatches != null) {
addMatchingMappings(directPathMatches, matches, request);
}
if (matches.isEmpty()) {
// No choice but to go through all mappings...
addMatchingMappings(this.handlerMethods.keySet(), matches, request);
}
if (!matches.isEmpty()) {
Comparator<Match> comparator = new MatchComparator(getMappingComparator(request));
Collections.sort(matches, comparator);
if (logger.isTraceEnabled()) {
logger.trace("Found " + matches.size() + " matching mapping(s) for [" + lookupPath + "] : " + matches);
}
Match bestMatch = matches.get(0);
if (matches.size() > 1) {
Match secondBestMatch = matches.get(1);
if (comparator.compare(bestMatch, secondBestMatch) == 0) {
Method m1 = bestMatch.handlerMethod.getMethod();
Method m2 = secondBestMatch.handlerMethod.getMethod();
throw new IllegalStateException(
"Ambiguous handler methods mapped for HTTP path '" + request.getRequestURL() + "': {" +
m1 + ", " + m2 + "}");
}
}
handleMatch(bestMatch.mapping, lookupPath, request);
return bestMatch.handlerMethod;
}
else {
return handleNoMatch(handlerMethods.keySet(), lookupPath, request);
}
}
lookupHandlerMethod函数把获取的处理器包装成一个个Match，并对Match进行了排序（因为一个path可能不仅仅对应一个处理器，比如存在两个requestMap路径相同的函数，那么一个path就会对应两个处理器）之后取最匹配的那个。在这个过程中甚至还做了重复性检测，如果有两个一模一样的处理器存在，那说明我们的控制器写的有歧义了，直接抛出异常。我们看到第5行 addMatchingMappings(directPathMatches, matches, request); ，这里是真正获取HandlerMethod的地方，跟进去看：
private void addMatchingMappings(Collection<T> mappings, List<Match> matches, HttpServletRequest request) {
for (T mapping : mappings) {
T match = getMatchingMapping(mapping, request);
if (match != null) {
matches.add(new Match(match, this.handlerMethods.get(mapping)));
}
}
}
直接看到第5行 matches.add(new Match(match, this.handlerMethods.get(mapping))) ，这里传入了一个重要的this.handlerMethods.get(mapping)，this.handlerMethods 是一个LinkedHashMap，存放了此HM所有的处理器，根据mapping对象进行索引（mapping对象标识了唯一的请求映射），看一看这个handlerMethods 里有啥：
果然我们写的控制器方法对应的handler，都在这里头保存着。
接下来我们看看，这些handler是怎么保存到HM中的。我们在HM中找到了一个initHandlerMethods方法，从名字中就可以看出来这个方法是初始化methods用的，我们看看这个方法的内容：
protected void initHandlerMethods() {
if (logger.isDebugEnabled()) {
logger.debug("Looking for request mappings in application context: " + getApplicationContext());
}
String[] beanNames = (this.detectHandlerMethodsInAncestorContexts ?
BeanFactoryUtils.beanNamesForTypeIncludingAncestors(getApplicationContext(), Object.class) :
getApplicationContext().getBeanNamesForType(Object.class));
for (String beanName : beanNames) {
if (!beanName.startsWith(SCOPED_TARGET_NAME_PREFIX) &&
isHandler(getApplicationContext().getType(beanName))){
detectHandlerMethods(beanName);
}
}
handlerMethodsInitialized(getHandlerMethods());
}
它从ApplicationContext中获取了bean的名字，然后根据名字，在第13行中通过 detectHandlerMethods(beanName) 得到真正的bean：
protected void detectHandlerMethods(final Object handler) {
Class<?> handlerType =
(handler instanceof String ? getApplicationContext().getType((String) handler) : handler.getClass());
// Avoid repeated calls to getMappingForMethod which would rebuild RequestMappingInfo instances
final Map<Method, T> mappings = new IdentityHashMap<Method, T>();
final Class<?> userType = ClassUtils.getUserClass(handlerType);
Set<Method> methods = HandlerMethodSelector.selectMethods(userType, new MethodFilter() {
@Override
public boolean matches(Method method) {
T mapping = getMappingForMethod(method, userType);
if (mapping != null) {
mappings.put(method, mapping);
return true;
}
else {
return false;
}
}
});
for (Method method : methods) {
registerHandlerMethod(handler, method, mappings.get(method));
}
}
看第24行，在这里注册HM所持的HandlerMethod，跟进去：
protected void registerHandlerMethod(Object handler, Method method, T mapping) {
HandlerMethod newHandlerMethod = createHandlerMethod(handler, method);
HandlerMethod oldHandlerMethod = this.handlerMethods.get(mapping);
if (oldHandlerMethod != null && !oldHandlerMethod.equals(newHandlerMethod)) {
throw new IllegalStateException("Ambiguous mapping found. Cannot map '" + newHandlerMethod.getBean() +
"' bean method \n" + newHandlerMethod + "\nto " + mapping + ": There is already '" +
oldHandlerMethod.getBean() + "' bean method\n" + oldHandlerMethod + " mapped.");
}
this.handlerMethods.put(mapping, newHandlerMethod);
if (logger.isInfoEnabled()) {
logger.info("Mapped \"" + mapping + "\" onto " + newHandlerMethod);
}
Set<String> patterns = getMappingPathPatterns(mapping);
for (String pattern : patterns) {
if (!getPathMatcher().isPattern(pattern)) {
this.urlMap.add(pattern, mapping);
}
}
}
看第10行 this.handlerMethods.put(mapping, newHandlerMethod) ，一个个Put 进去了，这样，就完成了HandlerMethod的注册。在第18行 this.urlMap.add(pattern, mapping) ，完成了路径到mapping的映射。
最后只要框架在启动是调用initMethod方法，就可以完成处理器的注册了。
自此，DispatcherServlet如何得到ModelAndView的过程就讲完了。