根据servlet规范，当一个请求到达时，首先会调用Servlet的service()方法，这里是DispatcherServlet#service。

org.springframework.web.servlet.FrameworkServlet#service

protected void service(HttpServletRequest request, HttpServletResponse response)  throws ServletException, IOException { 
           HttpMethod httpMethod = HttpMethod.resolve(request.getMethod());  if (httpMethod == HttpMethod.PATCH || httpMethod == null) { 
           // servlet不支持规范PATCH请求，spring mvc实现了这一点   processRequest(request, response);  }  else { 
           // 最后还是会调用processRequest()   super.service(request, response);  } } 

javax.servlet.http.HttpServlet#service(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse)

protected void service(HttpServletRequest req, HttpServletResponse resp)     throws ServletException, IOException { 
             String method = req.getMethod();     if (methodequals(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);
            }
        }
    } else if (method.equals(METHOD_HEAD)) { 
        
        long lastModified = getLastModified(req);
        maybeSetLastModified(resp, lastModified);
        doHead(req, resp);
    } else if (method.equals(METHOD_POST)) { 
        
        doPost(req, resp);
        
    } else if (method.equals(METHOD_PUT)) { 
        
        doPut(req, resp);
        
    } else if (method.equals(METHOD_DELETE)) { 
        
        doDelete(req, resp);
        
    } else if (method.equals(METHOD_OPTIONS)) { 
        
        doOptions(req,resp);
        
    } else if (method.equals(METHOD_TRACE)) { 
        
        doTrace(req,resp);
        
    } else { 
        
        //
        // Note that this means NO servlet supports whatever
        // method was requested, anywhere on this server.
        //
        String errMsg = lStrings.getString("http.method_not_implemented");
        Object[] errArgs = new Object[1];
        errArgs[0] = method;
        errMsg = MessageFormat.format(errMsg, errArgs);
        
        resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
    }
}

根据请求的method调用不同的doXxxx()方法，而FrameworkServlet实现了这些doXxxx()方法。

org.springframework.web.servlet.FrameworkServlet#doGet

@Override
protected final void doGet(HttpServletRequest request, HttpServletResponse response)
	throws ServletException, IOException { 
        

	processRequest(request, response);
}

@Override
protected final void doPost(HttpServletRequest request, HttpServletResponse response)
	throws ServletException, IOException { 
        

	processRequest(request, response);
}

最后所有的请求都会会调用processRequest()方法。

org.springframework.web.servlet.FrameworkServlet#processRequest

protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
	throws ServletException, IOException { 
        

	long startTime = System.currentTimeMillis();
	Throwable failureCause = null;

	// 国际化上下文
	LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
	LocaleContext localeContext = buildLocaleContext(request);

	RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
	// 将请求和响应封装为ServletRequestAttributes
	ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);

	WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
	asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());

	// 将LocaleContext和ServletRequestAttributes放入threadlocal中
	initContextHolders(request, localeContext, requestAttributes);

	try { 
        
		doService(request, response);
	}
	catch (ServletException | IOException ex) { 
        
		failureCause = ex;
		throw ex;
	}
	catch (Throwable ex) { 
        
		failureCause = ex;
		throw new NestedServletException("Request processing failed", ex);
	}

	finally { 
        
		// 将LocaleContext和ServletRequestAttributes从threadlocal中删除
		resetContextHolders(request, previousLocaleContext, previousAttributes);
		if (requestAttributes != null) { 
        
			requestAttributes.requestCompleted();
		}
		logResult(request, response, failureCause, asyncManager);
		publishRequestHandledEvent(request, response, startTime, failureCause);
	}
}

processRequest()主要是将request对象放入到了ThreadLocal中，这样我们可以在任意地方拿到当前的请求：

ServletRequestAttributes requestAttributes = (ServletRequestAttributes) RequestContextHolder.getRequestAttributes();
HttpServletRequest httpServletRequest = requestAttributes.getRequest();

org.springframework.web.servlet.DispatcherServlet#doService

protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception { 
        
	logRequest(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<>();
		Enumeration<?> attrNames = request.getAttributeNames();
		while (attrNames.hasMoreElements()) { 
        
			String attrName = (String) attrNames.nextElement();
			if (this.cleanupAfterInclude || attrName.startsWith(DEFAULT_STRATEGIES_PREFIX)) { 
        
				attributesSnapshot.put(attrName, request.getAttribute(attrName));
			}
		}
	}

	// Make framework objects available to handlers and view objects.
	// 往请求头中塞入了一堆对象，后面会使用到
	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());

	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 { 
        
		// 重点
		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);
			}
		}
	}
}

org.springframework.web.servlet.DispatcherServlet#doDispatch

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 { 
        
			// 判断请求头中是否有文件，如果有会将请求包装为MultipartHttpServletRequest
			processedRequest = checkMultipart(request);
			multipartRequestParsed = (processedRequest != request);

			// Determine handler for the current request.
			// HandlerExecutionChain，获取Handler
			mappedHandler = getHandler(processedRequest);
			if (mappedHandler == null) { 
        
				noHandlerFound(processedRequest, response);
				return;
			}

			// Determine handler adapter for the current request.
			// 获取HandlerAdapter
			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;
				}
			}

			// 调用拦截器的HandlerInterceptor.preHandle()
			if (!mappedHandler.applyPreHandle(processedRequest, response)) { 
        
				return;
			}

			// Actually invoke the handler.
			mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

			if (asyncManager.isConcurrentHandlingStarted()) { 
        
				return;
			}

			applyDefaultViewName(processedRequest, mv);
			// 调用拦截器的HandlerInterceptor.postHandle()
			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);
		}
		// 进行异常处理
		// 视图渲染
		// 调用拦截器的HandlerInterceptor.afterCompletion()
		processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
	}
	catch (Exception ex) { 
        
		// 调用拦截器的HandlerInterceptor.afterCompletion()
		triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
	}
	catch (Throwable err) { 
        
		// 调用拦截器的HandlerInterceptor.afterCompletion()
		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);
			}
		}
	}
}

doDispatch()是对整个请求的核心处理逻辑，大致包括文件处理、获取Handler、获取HandlerAdapter、拦截器的执行、Handler的调用、异常处理、视图渲染等。

org.springframework.web.servlet.DispatcherServlet#processDispatchResult

private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
								   @Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
								   @Nullable 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);
			// 异常处理
			mv = processHandlerException(request, response, handler, exception);
			errorView = (mv != null);
		}
	}

	// Did the handler return a view to render?
	if (mv != null && !mv.wasCleared()) { 
        
		// 视图渲染
		render(mv, request, response);
		if (errorView) { 
        
			WebUtils.clearErrorRequestAttributes(request);
		}
	}
	else { 
        
		if (logger.isTraceEnabled()) { 
        
			logger.trace("No view rendering, null ModelAndView returned.");
		}
	}

	if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) { 
        
		// Concurrent handling started during a forward
		return;
	}

	if (mappedHandler != null) { 
        
		// Exception (if any) is already handled..
		// 调用拦截器的HandlerInterceptor.afterCompletion()
		mappedHandler.triggerAfterCompletion(request, response, null);
	}
}

HandlerExecutionChain主要封装了拦截器和Handle，由它来执行拦截器中的一系列方法。

public class HandlerExecutionChain { 
        

    private final Object handler;

    @Nullable
    private HandlerInterceptor[] interceptors;

    @Nullable
    private List<HandlerInterceptor> interceptorList;

    private int interceptorIndex = -1;
... ...

HandlerMapping表示的是一个URL与一个Handler的对应关系，主要包括以下三类：

• RequestMappingHandlerMapping：处理@RequestMapping注解
• BeanNameUrlHandlerMapping：处理bean的name直接是url
• SimpleUrlHandlerMapping：处理ViewController、静态资源映射、默认的servlet处理器等

HandlerAdapter处理器适配器用来执行Controller中的目标方法，由于上面有三种HandlerMapping，对应这里三种HandlerAdapter：

• SimpleServletHandlerAdapter：处理servlet请求
• RequestMappingHandlerAdapter：对应RequestMappingHandlerMapping
• HttpRequestHandlerAdapter：对应SimpleUrlHandlerMapping
• SimpleControlle