引言
之前我们都是从一些官方文档或者是博客中了解了springMVC的整个处理流程，并且在前面博客我们也自己动手模拟了一个简单的springMVC容器，那么今天我们就对照流程图从源码分析一下S pringMVC的核心处理流程。
一、SpringMVC处理流程图
这张图已经非常清楚的画除了整个SpringMVC的处理流程，里面还写出了每个处理环节当中的类，当然如果没有 分析过源码，那么上面的图看起来会非常的乱。尤其是里面的一些核心类，所以如果想很愉快的欣赏上面的 流程图，就需要我们看过几遍源码。
二、DispatcherServlet 核心处理流程
DispatcherServlet 其实就是一个普通的servlet，所以当一个请求来到servlet的时候，肯定首先会进入service()方法，所以我们的入口方法肯定是service()方法：
进入DispatcherServlet方法：
public class DispatcherServlet extends FrameworkServlet
我们在DispatcherServlet方法中我们没有发现service方法，所以肯定在父类的FrameworkServlet类中
@Override protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { HttpMethod httpMethod = HttpMethod.resolve(request.getMethod()); if (httpMethod == HttpMethod.PATCH || httpMethod == null) { processRequest(request, response); } else { super.service(request, response); } }
从上面代码中我们会发现它继续调用了父类的service方法，所以我们继续进入到HttpServlet类中，在这个类中会见到我们非常熟悉的代码，也就是处理我们常见的几种处理方法：GET HEAD DELETE POST PUT等。
protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { String method = req.getMethod(); if (method.equals(METHOD_GET)) { long lastModified = getLastModified(req); if (lastModified == -1) { // servlet doesn't support if-modified-since, no reason // to go through further expensive logic doGet(req, resp); } else { long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE); if (ifModifiedSince < lastModified) { // If the servlet mod time is later, call doGet() // Round down to the nearest second for a proper compare // A ifModifiedSince of -1 will always be less maybeSetLastModified(resp, lastModified); doGet(req, resp); } else { resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED); } } } 。。。。。。。。。。。 }
这里我们分析Get请求，当我们进入到源码以后会发现，get方法比其他的方法复杂一点，这是因为在get 和 head 方法里面处理一个缓存，也就是这个 lastModified，这在我们前后端分离之前会用到，也就是提高get请求的性能，类似时间戳的作用。这个东西在现在的前后端分离的架构中不是很重要了。
下面我们就开始进入找到get请求的主要实现里面：
这样我们进入到一个processRequest方法，在doservice() 方法之前都是完成一些属性赋值。doservice()就是我们的核心方法
protected final void processRequest(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { long startTime = System.currentTimeMillis(); Throwable failureCause = null; LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext(); LocaleContext localeContext = buildLocaleContext(request); //赋值一些属性 RequestAttributes 非常熟悉哈 RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes(); ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes); WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request); asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor()); initContextHolders(request, localeContext, requestAttributes); try { //核心方法 doService(request, response); } catch (ServletException | IOException ex) { failureCause = ex; throw ex; } catch (Throwable ex) { failureCause = ex; throw new NestedServletException("Request processing failed", ex); } finally { resetContextHolders(request, previousLocaleContext, previousAttributes); if (requestAttributes != null) { requestAttributes.requestCompleted(); } logResult(request, response, failureCause, asyncManager); publishRequestHandledEvent(request, response, startTime, failureCause); } }
进入doService()核心方法：
@Override protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception { logRequest(request); 。。。。。。。。。。 //省略的代码主要是往request对象中赋值属性 try { //核心代码 doDispatch(request, response); } }
三、doDispatch()核心方法
核心执行流程图：
开始分析doDispatch()核心方法，这里面完成核心流程，先来总体分析一下里面的几个核心方法，我都已经添加了注释，其实就是我们自己手写框架的时候实现的几个步骤。
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 { //检查文件上传 processedRequest = checkMultipart(request); multipartRequestParsed = (processedRequest != request); // Determine handler for the current request. //获得处理器HandlerMapping，这里返回的不在单个的HandlerMapping，而是HandlerExecutionChain 处理器+拦截器链 mappedHandler = getHandler(processedRequest); if (mappedHandler == null) { noHandlerFound(processedRequest, response); return; } // Determine handler adapter for the current request. //获得对应的 适配器 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 (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) { return; } } //执行拦截器前 if (!mappedHandler.applyPreHandle(processedRequest, response)) { return; } // Actually invoke the handler. //执行具体的处理器逻辑 mv = ha.handle(processedRequest, response, mappedHandler.getHandler()); if (asyncManager.isConcurrentHandlingStarted()) { return; } applyDefaultViewName(processedRequest, mv); //执行拦截器中方法 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); } // 处理返回结果和视图 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 afterCompletion if (mappedHandler != null) { mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response); } } else { // Clean up any resources used by a multipart request. if (multipartRequestParsed) { cleanupMultipart(processedRequest); } } } }
1） 首先我们进入到 mappedHandler = getHandler(processedRequest); 这个方法，也就是完成根据URI获得处理器映射器的流程：
//这个代码是不是非常的熟悉，和我们手写框架中的代码几乎一样 @Nullable protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception { if (this.handlerMappings != null) { for (HandlerMapping mapping : this.handlerMappings) { //获得对应的处理器+拦截器链 HandlerExecutionChain handler = mapping.getHandler(request); if (handler != null) { return handler; } } } return null; }
2、mapping.getHandler(request)；具体实现逻辑在模板类AbstractHandlerMapping 实现
@Override @Nullable public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception { //获得框架内置的处理器和拦截器链 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 = obtainApplicationContext().getBean(handlerName); } //获得完整的链 核心方法 HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request); if (logger.isTraceEnabled()) { logger.trace("Mapped to " + handler); } else if (logger.isDebugEnabled() && !request.getDispatcherType().equals(DispatcherType.ASYNC)) { logger.debug("Mapped to " + executionChain.getHandler()); } if (hasCorsConfigurationSource(handler) || CorsUtils.isPreFlightRequest(request)) { CorsConfiguration config = (this.corsConfigurationSource != null ? this.corsConfigurationSource.getCorsConfiguration(request) : null); CorsConfiguration handlerConfig = getCorsConfiguration(handler, request); config = (config != null ? config.combine(handlerConfig) : handlerConfig); executionChain = getCorsHandlerExecutionChain(request, executionChain, config); } return executionChain; }
3、getHandlerExecutionChain(handler, request);
protected HandlerExecutionChain getHandlerExecutionChain(Object handler, HttpServletRequest request) { //获得HandlerExecutionChain对象，HandlerExecutionChain构造器方法合并interceptors到interceptorList HandlerExecutionChain chain = (handler instanceof HandlerExecutionChain ? (HandlerExecutionChain) handler : new HandlerExecutionChain(handler)); String lookupPath = this.urlPathHelper.getLookupPathForRequest(request, LOOKUP_PATH); // 将adaptedInterceptors添加到chain中，并存入到initInterceptorList中 for (HandlerInterceptor interceptor : this.adaptedInterceptors) { if (interceptor instanceof MappedInterceptor) { MappedInterceptor mappedInterceptor = (MappedInterceptor) interceptor; if (mappedInterceptor.matches(lookupPath, this.pathMatcher)) { chain.addInterceptor(mappedInterceptor.getInterceptor()); } } else { chain.addInterceptor(interceptor); } } return chain; }
上面几个核心方法我们获得了处理器，下面我们需要根据 处理器获得对应的适配器。
4、getHandlerAdapter(mappedHandler.getHandler());获得适配器
protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException { if (this.handlerAdapters != null) { for (HandlerAdapter adapter : this.handlerAdapters) { if (adapter.supports(handler)) { return adapter; } } } throw new ServletException("No adapter for handler [" + handler + "]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler"); }
上面的方法我们就会非常的熟悉，获得适配器的代码和我们自己手写的框架中的代码是一样的。
5、mappedHandler.applyPreHandle(processedRequest, response)；执行拦截器前方法，在执行具体的处理逻辑之前执行
boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception { //获得拦截器 HandlerInterceptor[] interceptors = getInterceptors(); if (!ObjectUtils.isEmpty(interceptors)) { for (int i = 0; i < interceptors.length; i++) { HandlerInterceptor interceptor = interceptors[i]; //执行拦截器前方法， if (!interceptor.preHandle(request, response, this.handler)) { //执行拦截器后方法，规范上有规定，拦截器后的方法必须执行一次 triggerAfterCompletion(request, response, null); return false; } this.interceptorIndex = i; } } return true; }
6、mv = ha.handle(processedRequest, response, mappedHandler.getHandler()); 执行具体业务逻辑
7、处理以后逻辑
//渲染视图 applyDefaultViewName(processedRequest, mv); //执行拦截中的方法 mappedHandler.applyPostHandle(processedRequest, response, mv); //处理返回值逻辑 rocessDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
四、小结
本文从源码分析了springmvc的执行流程，通过这次分析，对于整个执行流程有了更深入的了解， 重要的是培养我们阅读源码的习惯。
