Spring MVC 源码-运行调用阶段

这一步步是由请求触发的，所以入口为DispatcherServlet 的核心方法为doService()，doService()中的核心逻辑由doDispatch()实现，源代码如下：

/** 中央控制器,控制请求的转发 **/
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 {// 1.检查是否是文件上传的请求processedRequest = checkMultipart(request);multipartRequestParsed = (processedRequest != request);// Determine handler for the current request.// 2.取得处理当前请求的controller,这里也称为hanlder,处理器,//    第一个步骤的意义就在这里体现了.这里并不是直接返回controller,//  而是返回的HandlerExecutionChain请求处理器链对象,//   该对象封装了handler和interceptors.mappedHandler = getHandler(processedRequest);// 如果handler为空,则返回404if (mappedHandler == null) {noHandlerFound(processedRequest, response);return;}// Determine handler adapter for the current request.//3. 获取处理request的处理器适配器handler adapterHandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());// Process last-modified header, if supported by the handler.// 处理 last-modified 请求头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.// 4.实际的处理器处理请求,返回结果视图对象mv = ha.handle(processedRequest, response, mappedHandler.getHandler());if (asyncManager.isConcurrentHandlingStarted()) {return;}// 结果视图对象的处理applyDefaultViewName(processedRequest, mv);mappedHandler.applyPostHandle(processedRequest, response, mv);}catch (Exception ex) {dispatchException = ex;}catch (Throwable err) {// As of 4.3, we're processing Errors thrown from handler methods as well,// making them available for @ExceptionHandler methods and other scenarios.dispatchException = new NestedServletException("Handler dispatch failed", err);}processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);}catch (Exception ex) {triggerAfterCompletion(processedRequest, response, mappedHandler, ex);}catch (Throwable err) {triggerAfterCompletion(processedRequest, response, mappedHandler,new NestedServletException("Handler processing failed", err));}finally {if (asyncManager.isConcurrentHandlingStarted()) {// Instead of postHandle and afterCompletionif (mappedHandler != null) {// 请求成功响应之后的方法mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);}}else {// Clean up any resources used by a multipart request.if (multipartRequestParsed) {cleanupMultipart(processedRequest);}}}
}

getHandler(processedRequest)方法实际上就是从HandlerMapping 中找到url 和Controller 的对应关系。也就是Map<url,Controller>。我们知道，最终处理Request的是Controller 中的方法，我们现在只是知道了Controller，我们如何确认Controller中处理Request 的方法呢？继续往下看。

从Map<urls,beanName>中取得Controller 后，经过拦截器的预处理方法，再通过反射获取该方法上的注解和参数，解析方法和参数上的注解，然后反射调用方法获取ModelAndView 结果视图。最后，调用的就是RequestMappingHandlerAdapter 的handle()中的核心逻辑由handleInternal(request, response, handler)实现。

@Override
protected ModelAndView handleInternal(HttpServletRequest request,HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {ModelAndView mav;checkRequest(request);// Execute invokeHandlerMethod in synchronized block if required.if (this.synchronizeOnSession) {HttpSession session = request.getSession(false);if (session != null) {Object mutex = WebUtils.getSessionMutex(session);synchronized (mutex) {mav = invokeHandlerMethod(request, response, handlerMethod);}}else {// No HttpSession available -> no mutex necessarymav = invokeHandlerMethod(request, response, handlerMethod);}}else {// No synchronization on session demanded at all...mav = invokeHandlerMethod(request, response, handlerMethod);}if (!response.containsHeader(HEADER_CACHE_CONTROL)) {if (getSessionAttributesHandler(handlerMethod).hasSessionAttributes()) {applyCacheSeconds(response, this.cacheSecondsForSessionAttributeHandlers);}else {prepareResponse(response);}}return mav;
}

整个处理过程中最核心的逻辑其实就是拼接Controller 的url 和方法的url，与Request的url 进行匹配，找到匹配的方法。

protected HandlerMethod getHandlerInternal(HttpServletRequest request) throws Exception {String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);if (logger.isDebugEnabled()) {logger.debug("Looking up handler method for path " + lookupPath);}this.mappingRegistry.acquireReadLock();try {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);}finally {this.mappingRegistry.releaseReadLock();}
}

通过上面的代码分析，已经可以找到处理Request 的Controller 中的方法了，现在看如何解析该方法上的参数，并反射调用该方法。

/** 获取处理请求的方法,执行并返回结果视图 **/
@Nullable
protected ModelAndView invokeHandlerMethod(HttpServletRequest request,HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {ServletWebRequest webRequest = new ServletWebRequest(request, response);try {WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);ServletInvocableHandlerMethod invocableMethod = createInvocableHandlerMethod(handlerMethod);if (this.argumentResolvers != null) {invocableMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);}if (this.returnValueHandlers != null) {invocableMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);}invocableMethod.setDataBinderFactory(binderFactory);invocableMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer);ModelAndViewContainer mavContainer = new ModelAndViewContainer();mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));modelFactory.initModel(webRequest, mavContainer, invocableMethod);mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect);AsyncWebRequest asyncWebRequest = WebAsyncUtils.createAsyncWebRequest(request, response);asyncWebRequest.setTimeout(this.asyncRequestTimeout);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 + "]");}invocableMethod = invocableMethod.wrapConcurrentResult(result);}invocableMethod.invokeAndHandle(webRequest, mavContainer);if (asyncManager.isConcurrentHandlingStarted()) {return null;}return getModelAndView(mavContainer, modelFactory, webRequest);}finally {webRequest.requestCompleted();}
}

invocableMethod.invokeAndHandle()最终要实现的目的就是：完成Request 中的参数和方法参数上数据的绑定。Spring MVC 中提供两种Request 参数到方法中参数的绑定方式：

1、通过注解进行绑定，@RequestParam。

2、通过参数名称进行绑定。

使用注解进行绑定，我们只要在方法参数前面声明@RequestParam("name")，就可以将request 中参数name 的值绑定到方法的该参数上。使用参数名称进行绑定的前提是必须要获取方法中参数的名称，Java 反射只提供了获取方法的参数的类型，并没有提供获取参数名称的方法。SpringMVC 解决这个问题的方法是用asm 框架读取字节码文件，来获取方法的参数名称。asm 框架是一个字节码操作框架，关于asm 更多介绍可以参考其官网。个人建议，使用注解来完成参数绑定，这样就可以省去asm 框架的读取字节码的操作。

public Object invokeForRequest(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,Object... providedArgs) throws Exception {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;
}

private Object[] getMethodArgumentValues(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,Object... providedArgs) throws Exception {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);args[i] = resolveProvidedArgument(parameter, providedArgs);if (args[i] != null) {continue;}if (this.argumentResolvers.supportsParameter(parameter)) {try {args[i] = this.argumentResolvers.resolveArgument(parameter, mavContainer, request, this.dataBinderFactory);continue;}catch (Exception ex) {if (logger.isDebugEnabled()) {logger.debug(getArgumentResolutionErrorMessage("Failed to resolve", i), ex);}throw ex;}}if (args[i] == null) {throw new IllegalStateException("Could not resolve method parameter at index " +parameter.getParameterIndex() + " in " + parameter.getExecutable().toGenericString() +": " + getArgumentResolutionErrorMessage("No suitable resolver for", i));}}return args;
}

关于asm 框架获取方法参数的部分,这里就不再进行分析了。感兴趣的小伙伴可以继续深入了解这个处理过程。

到这里,方法的参数值列表也获取到了,就可以直接进行方法的调用了。整个请求过程中最复杂的一步就是在这里了。到这里整个请求处理过程的关键步骤都已了解。理解了Spring MVC 中的请求处理流程,整个代码还是比较清晰的。最后我们再来梳理一下Spring MVC 核心组件的关联关系（如下图）：

最后来一张时序图：

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