SpringMVC的核心就是DispatcherServlet，DispatcherServlet实质也是一个HttpServlet。DispatcherSevlet负责将请求分发，所有的请求都有经过它来统一分发。
大致看下SpringMVC请求处理的流程：
用户向服务器发送请求，请求会到DispatcherServlet，DispatcherServlet 对请求URL进行解析，得到请求资源标识符（URI）， 然后根据该URI，调用HandlerMapping获得该Handler（就是控制层）配置的所有相关的对象（包括一个Handler处理器对象、多个HandlerInterceptor拦截器对象），最后以HandlerExecutionChain对象的形式返回。
DispatcherServlet根据获得的Handler，选择一个合适的HandlerAdapter。提取Request中的模型数据，填充Handler入参，开始执行Handler（Controller)。 在填充Handler的入参过程中，根据你的配置，Spring将帮你做一些额外的工作：
Handler执行完成后，向DispatcherServlet 返回一个ModelAndView对象根据返回的ModelAndView，选择一个适合的ViewResolver返回给DispatcherServlet；ViewResolver 结合Model和View，来渲染视图，最后将渲染结果返回给客户端。
DispatcherSevlet中最重要的是doService方法：
//获取请求，设置一些request的参数，然后分发给doDispatch
@Override
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.
/* 设置web应用上下文**/
request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
/* 国际化本地**/
request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
/* 样式**/
request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
//设置样式资源
request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());
//请求刷新时保存属性
FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
if (inputFlashMap != null) {
request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
}
//Flash attributes 在对请求的重定向生效之前被临时存储（通常是在session)中，并且在重定向之后被立即移除
request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
//FlashMap 被用来管理 flash attributes 而 FlashMapManager 则被用来存储，获取和管理 FlashMap 实体.
request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);
try {
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);
}
}
}
}
doService方法对Request设置了一些全局属性，最终接下来的操作是在doDispatcher：
/**
*将Handler进行分发，handler会被handlerMapping有序的获得
*通过查询servlet安装的HandlerAdapters来获得HandlerAdapters来查找第一个支持handler的类
*所有的HTTP的方法都会被这个方法掌控。取决于HandlerAdapters 或者handlers 他们自己去决定哪些方法是可用
*@param request current HTTP request
*@param response current HTTP response
*/
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
/* 当前HTTP请求**/
HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
boolean multipartRequestParsed = false;
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
try {
ModelAndView mv = null;
Exception dispatchException = null;
try {
//判断是否有文件上传
processedRequest = checkMultipart(request);
multipartRequestParsed = (processedRequest != request);
// getHandler中遍历HandlerMapping，获取对应的HandlerExecutionChain
// HandlerExecutionChain，包含HandlerIntercrptor和HandlerMethod
mappedHandler = getHandler(processedRequest);
if (mappedHandler == null || mappedHandler.getHandler() == null) {
noHandlerFound(processedRequest, response);
return;
}
//获得HandlerAdapter
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
//获得HTTP请求方法
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;
}
}
//如果有拦截器的话，会执行拦截器的preHandler方法
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}
//返回ModelAndView
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
//当view为空时，，根据request设置默认view
applyDefaultViewName(processedRequest, mv);
//执行拦截器的postHandle
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 {
//判断是否是异步请求
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.
//删除上传资源
if (multipartRequestParsed) {
cleanupMultipart(processedRequest);
}
}
}
}
在doDispatcher方法中：
mappedHandler = getHandler(processedRequest);
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;
}
遍历HandlerMapping，获取对应的HandlerExecutionChain，HandlerExecutionChain中有3个变量:
1，private final Object handler; // HandlerMethod
2，private HandlerInterceptor[] interceptors; //所有的HandlerInterceptor的数组
3，private List<HandlerInterceptor> interceptorList; //所有的HandlerInterceptor的链表
在执行Controller的方法调用之前先执行：
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}
HandlerExecutionChain#applyPreHandle即调用HandlerInterceptor# preHandler方法。在Controller的方法调用之后：
mappedHandler.applyPostHandle(processedRequest, response, mv);
HandlerExecutionChain#applyPostHandle即调用HandlerInterceptor# postHandle方法。最后
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
方法会调用：
mappedHandler.triggerAfterCompletion(request, response, null);
调用HandlerInterceptor#afterCompletion方法。
在doDispatcher方法中会catch住Controller方法调用的异常，之后再处理：
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
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);
}
}
// …
}
如果有exception，调用processHandlerException：
protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response,
Object handler, Exception ex) throws Exception {
// Check registered HandlerExceptionResolvers...
ModelAndView exMv = null;
for (HandlerExceptionResolver handlerExceptionResolver : this.handlerExceptionResolvers) {
exMv = handlerExceptionResolver.resolveException(request, response, handler, ex);
if (exMv != null) {
break;
}
}
if (exMv != null) {
if (exMv.isEmpty()) {
request.setAttribute(EXCEPTION_ATTRIBUTE, ex);
return null;
}
// We might still need view name translation for a plain error model...
if (!exMv.hasView()) {
exMv.setViewName(getDefaultViewName(request));
}
if (logger.isDebugEnabled()) {
logger.debug("Handler execution resulted in exception - forwarding to resolved error view: " + exMv, ex);
}
WebUtils.exposeErrorRequestAttributes(request, ex, getServletName());
return exMv;
}
throw ex;
}
SpringMVC启动加载时会将所有的异常处理HandlerExceptionResolver实现类存放到handlerExceptionResolvers中，当默认执行时，只要返回的exMv不为空就中止执行其他的异常处理实现类。
可以实现多个HandlerExceptionResolver的异常处理类，执行顺序由其order属性决定, order值越小，越是优先执行, 在执行到第一个返回不是null的ModelAndView的Resolver时，不再执行后续的尚未执行的Resolver的异常处理方法。
HandlerMethodArgumentResolver是用来为处理器解析参数的，主要用在HandlerMethod中，每个Resolver对应一种类型的参数，其实现类特别的多。HandlerMethodArgumentResolver的接口定义如下：
参数的处理是在InvocableHandlerMethod类中，当执行具体的请求处理时执行invokeForRequest：
public Object invokeForRequest(NativeWebRequest request, ModelAndViewContainer mavContainer,
Object... providedArgs) throws Exception {
//获取执行Controller的函数的参数对象
Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);
if (logger.isTraceEnabled()) {
logger.trace("Invoking '" + ClassUtils.getQualifiedMethodName(getMethod(), getBeanType()) +
"' with arguments " + Arrays.toString(args));
}
Object returnValue = doInvoke(args);
if (logger.isTraceEnabled()) {
logger.trace("Method [" + ClassUtils.getQualifiedMethodName(getMethod(), getBeanType()) +
"] returned [" + returnValue + "]");
}
return returnValue;
}
具体的参数解析器的选择和使用参数解析器去解析参数的实现都在getMethodArgumentValues中。SpringMVC在启动时会将所有的参数解析器放到HandlerMethodArgumentResolverComposite，然后从HandlerMethodArgumentResolverComposite参数解析器集合中选择一个支持对parameter解析的参数解析器，之后就使用支持参数解析的解析器进行参数解析。
private Object[] getMethodArgumentValues(NativeWebRequest request, ModelAndViewContainer mavContainer,
Object... providedArgs) throws Exception {
//获取执行的具体函数的参数
MethodParameter[] parameters = getMethodParameters();
Object[] args = new Object[parameters.length];
for (int i = 0; i < parameters.length; i++) {
MethodParameter parameter = parameters[i];
parameter.initParameterNameDiscovery(this.parameterNameDiscoverer);
GenericTypeResolver.resolveParameterType(parameter, getBean().getClass());
//获取参数值对象
args[i] = resolveProvidedArgument(parameter, providedArgs);
if (args[i] != null) {
continue;
}
//首先判断是否有参数解析器支持参数parameter，采用职责链的设计模式
if (this.argumentResolvers.supportsParameter(parameter)) {
try {
//如果参数解析器支持解析参数parameter，那么解析参数成Controller的函数需要的格式
args[i] = this.argumentResolvers.resolveArgument(
parameter, mavContainer, request, this.dataBinderFactory);
continue;
}
catch (Exception ex) {
if (logger.isDebugEnabled()) {
logger.debug(getArgumentResolutionErrorMessage("Error resolving argument", i), ex);
}
throw ex;
}
}
if (args[i] == null) {
String msg = getArgumentResolutionErrorMessage("No suitable resolver for argument", i);
throw new IllegalStateException(msg);
}
}
//返回参数值
return args;
}
以上代码会将URL链接提交的参数值转换为对象值。
作用：HandlerMethodReturnValueHandler是用于对Controller中函数执行的返回值进行处理操作的。接口HandlerMethodReturnValueHandler定义的抽象方法如下：
执行过程是在ServletInvocableHandlerMethod的invokeAndHandle中：
public void invokeAndHandle(ServletWebRequest webRequest,
ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception {
// 获取返回值
Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
.......
//对返回值进行处理操作
this.returnValueHandlers.handleReturnValue(
returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
.........
}
具体实现是在HandlerMethodReturnValueHandlerComposite中，HandlerMethodReturnValueHandlerComposite中包含了所有springMVC提供的返回值处理器。
public void handleReturnValue(Object returnValue, MethodParameter returnType,
ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {
//选择可以处理返回值的处理器
HandlerMethodReturnValueHandler handler = selectHandler(returnValue, returnType);
if (handler == null) {
throw new IllegalArgumentException("Unknown return value type: " + returnType.getParameterType().getName());
}
//处理返回值
handler.handleReturnValue(returnValue, returnType, mavContainer, webRequest);
}
private HandlerMethodReturnValueHandler selectHandler(Object value, MethodParameter returnType) {
boolean isAsyncValue = isAsyncReturnValue(value, returnType);
for (HandlerMethodReturnValueHandler handler : this.returnValueHandlers) {
if (isAsyncValue && !(handler instanceof AsyncHandlerMethodReturnValueHandler)) {
continue;
}
if (handler.supportsReturnType(returnType)) {
return handler;
}
}
return null;
}
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