内容参考https://my.oschina.net/lichhao/blog/104943

SpringMVC请求的大致处理流程图

 1、请求的入口

1.1、HttpServlet与FrameworkServlet

从设计上看，SpringMVC是依托Servlet容器来设计的。在SpringMVC初始化之后，同样通过service()方法来接受、处理、返回。

DispatcherServlet的service()方法是他的父类FrameworkServlet实现的。

protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { HttpMethod httpMethod = HttpMethod.resolve(request.getMethod()); if (httpMethod != HttpMethod.PATCH && httpMethod != null) { super.service(request, response); } else { // 最终会调用此方法 this.processRequest(request, response); } } protected final void processRequest(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { long startTime = System.currentTimeMillis(); Throwable failureCause = null; LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext(); LocaleContext localeContext = this.buildLocaleContext(request); RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes(); ServletRequestAttributes requestAttributes = this.buildRequestAttributes(request, response, previousAttributes); WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request); asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new FrameworkServlet.RequestBindingInterceptor()); this.initContextHolders(request, localeContext, requestAttributes); try { //doService（）是抽象方法，调用的就是DispatcherServlet的doService方法 this.doService(request, response); } catch (IOException | ServletException var16) { failureCause = var16; throw var16; } catch (Throwable var17) { failureCause = var17; throw new NestedServletException("Request processing failed", var17); } finally { this.resetContextHolders(request, previousLocaleContext, previousAttributes); if (requestAttributes != null) { requestAttributes.requestCompleted(); } this.logResult(request, response, (Throwable)failureCause, asyncManager); this.publishRequestHandledEvent(request, response, startTime, (Throwable)failureCause); } }

 1.2、DispatcherServlet的doService(HttpServletRequest request, HttpServletResponse response)方法

protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception { this.logRequest(request); Map<String, Object> attributesSnapshot = null; if (WebUtils.isIncludeRequest(request)) { attributesSnapshot = new HashMap(); Enumeration attrNames = request.getAttributeNames(); label95: while(true) { String attrName; do { if (!attrNames.hasMoreElements()) { break label95; } attrName = (String)attrNames.nextElement(); } while(!this.cleanupAfterInclude && !attrName.startsWith("org.springframework.web.servlet")); attributesSnapshot.put(attrName, request.getAttribute(attrName)); } } request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.getWebApplicationContext()); request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver); request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver); request.setAttribute(THEME_SOURCE_ATTRIBUTE, this.getThemeSource()); if (this.flashMapManager != null) { 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 { //为request设置各种属性，之后真正的业务请求处理在doDispatch中 this.doDispatch(request, response); } finally { if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted() && attributesSnapshot != null) { this.restoreAttributesAfterInclude(request, attributesSnapshot); } } }

在doDispatch()方法中，改出现的MVC设计终于尽收眼底。ModelAndView、HandlerExecutionChain、HandleAdapter、HandlerInterceptor等各司其职。这其中HandlerExecutionChain尤为重要，是带有主角光环的

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; Object dispatchException = null; try { processedRequest = this.checkMultipart(request); multipartRequestParsed = processedRequest != request; mappedHandler = this.getHandler(processedRequest); if (mappedHandler == null) { this.noHandlerFound(processedRequest, response); return; } HandlerAdapter ha = this.getHandlerAdapter(mappedHandler.getHandler()); 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; } } if (!mappedHandler.applyPreHandle(processedRequest, response)) { return; } mv = ha.handle(processedRequest, response, mappedHandler.getHandler()); if (asyncManager.isConcurrentHandlingStarted()) { return; } this.applyDefaultViewName(processedRequest, mv); mappedHandler.applyPostHandle(processedRequest, response, mv); } catch (Exception var20) { dispatchException = var20; } catch (Throwable var21) { dispatchException = new NestedServletException("Handler dispatch failed", var21); } this.processDispatchResult(processedRequest, response, mappedHandler, mv, (Exception)dispatchException); } catch (Exception var22) { this.triggerAfterCompletion(processedRequest, response, mappedHandler, var22); } catch (Throwable var23) { this.triggerAfterCompletion(processedRequest, response, mappedHandler, new NestedServletException("Handler processing failed", var23)); } finally { if (asyncManager.isConcurrentHandlingStarted()) { if (mappedHandler != null) { mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response); } } else if (multipartRequestParsed) { this.cleanupMultipart(processedRequest); } } }

HandlerExecutionChain.java

public class HandlerExecutionChain { private static final Log logger = LogFactory.getLog(HandlerExecutionChain.class); //handler对象 private final Object handler; //拦截器链，可以在handler执行前后调用。类似AOP环绕通知一样 @Nullable private HandlerInterceptor[] interceptors; @Nullable private List<HandlerInterceptor> interceptorList; private int interceptorIndex; public HandlerExecutionChain(Object handler) { this(handler, (HandlerInterceptor[])null); } public HandlerExecutionChain(Object handler, @Nullable HandlerInterceptor... interceptors) { this.interceptorIndex = -1; if (handler instanceof HandlerExecutionChain) { HandlerExecutionChain originalChain = (HandlerExecutionChain)handler; this.handler = originalChain.getHandler(); this.interceptorList = new ArrayList(); CollectionUtils.mergeArrayIntoCollection(originalChain.getInterceptors(), this.interceptorList); CollectionUtils.mergeArrayIntoCollection(interceptors, this.interceptorList); } else { this.handler = handler; this.interceptors = interceptors; } } public Object getHandler() { return this.handler; } public void addInterceptor(HandlerInterceptor interceptor) { this.initInterceptorList().add(interceptor); } public void addInterceptor(int index, HandlerInterceptor interceptor) { this.initInterceptorList().add(index, interceptor); } public void addInterceptors(HandlerInterceptor... interceptors) { if (!ObjectUtils.isEmpty(interceptors)) { CollectionUtils.mergeArrayIntoCollection(interceptors, this.initInterceptorList()); } } private List<HandlerInterceptor> initInterceptorList() { if (this.interceptorList == null) { this.interceptorList = new ArrayList(); if (this.interceptors != null) { CollectionUtils.mergeArrayIntoCollection(this.interceptors, this.interceptorList); } } this.interceptors = null; return this.interceptorList; } @Nullable public HandlerInterceptor[] getInterceptors() { if (this.interceptors == null && this.interceptorList != null) { this.interceptors = (HandlerInterceptor[])this.interceptorList.toArray(new HandlerInterceptor[0]); } return this.interceptors; } boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception { HandlerInterceptor[] interceptors = this.getInterceptors(); if (!ObjectUtils.isEmpty(interceptors)) { for(int i = 0; i < interceptors.length; this.interceptorIndex = i++) { HandlerInterceptor interceptor = interceptors[i]; if (!interceptor.preHandle(request, response, this.handler)) { this.triggerAfterCompletion(request, response, (Exception)null); return false; } } } return true; } void applyPostHandle(HttpServletRequest request, HttpServletResponse response, @Nullable ModelAndView mv) throws Exception { HandlerInterceptor[] interceptors = this.getInterceptors(); if (!ObjectUtils.isEmpty(interceptors)) { for(int i = interceptors.length - 1; i >= 0; --i) { HandlerInterceptor interceptor = interceptors[i]; interceptor.postHandle(request, response, this.handler, mv); } } } void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response, @Nullable Exception ex) throws Exception { HandlerInterceptor[] interceptors = this.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 var8) { logger.error("HandlerInterceptor.afterCompletion threw exception", var8); } } } } void applyAfterConcurrentHandlingStarted(HttpServletRequest request, HttpServletResponse response) { HandlerInterceptor[] interceptors = this.getInterceptors(); if (!ObjectUtils.isEmpty(interceptors)) { for(int i = interceptors.length - 1; i >= 0; --i) { if (interceptors[i] instanceof AsyncHandlerInterceptor) { try { AsyncHandlerInterceptor asyncInterceptor = (AsyncHandlerInterceptor)interceptors[i]; asyncInterceptor.afterConcurrentHandlingStarted(request, response, this.handler); } catch (Throwable var6) { logger.error("Interceptor [" + interceptors[i] + "] failed in afterConcurrentHandlingStarted", var6); } } } } } }