在面试的时候，面试官经常会问你对SpringMVC了解吗？大多数面试者多会说出SpringMVC的执行流程图，其实我觉得这也没有错，毕竟SpringMVC就是按照这一套流程执行的，下面就是该执行流程图：

今天我们就从源码的角度来看它到底是怎么执行的？下面这段代码它是DispatchServlet类中的doDispatch()方法，也就是我们上图的前端控制器

 1 protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
 2         HttpServletRequest processedRequest = request;
 3         HandlerExecutionChain mappedHandler = null;
 4         boolean multipartRequestParsed = false;
 5         WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
 6 
 7         try {
 8             try {
 9                 ModelAndView mv = null;
10                 Object dispatchException = null;
11 
12                 try {
13                     processedRequest = this.checkMultipart(request);
14                     multipartRequestParsed = processedRequest != request;
15                     mappedHandler = this.getHandler(processedRequest);
16                     if (mappedHandler == null) {
17                         this.noHandlerFound(processedRequest, response);
18                         return;
19                     }
20 
21                     HandlerAdapter ha = this.getHandlerAdapter(mappedHandler.getHandler());
22                     String method = request.getMethod();
23                     boolean isGet = "GET".equals(method);
24                     if (isGet || "HEAD".equals(method)) {
25                         long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
26                         if ((new ServletWebRequest(request, response)).checkNotModified(lastModified) && isGet) {
27                             return;
28                         }
29                     }
30 
31                     if (!mappedHandler.applyPreHandle(processedRequest, response)) {
32                         return;
33                     }
34 
35                     mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
36                     if (asyncManager.isConcurrentHandlingStarted()) {
37                         return;
38                     }
39 
40                     this.applyDefaultViewName(processedRequest, mv);
41                     mappedHandler.applyPostHandle(processedRequest, response, mv);
42                 } catch (Exception var20) {
43                     dispatchException = var20;
44                 } catch (Throwable var21) {
45                     dispatchException = new NestedServletException("Handler dispatch failed", var21);
46                 }
47 
48                 this.processDispatchResult(processedRequest, response, mappedHandler, mv, (Exception)dispatchException);
49             } catch (Exception var22) {
50                 this.triggerAfterCompletion(processedRequest, response, mappedHandler, var22);
51             } catch (Throwable var23) {
52                 this.triggerAfterCompletion(processedRequest, response, mappedHandler, new NestedServletException("Handler processing failed", var23));
53             }
54 
55         } finally {
56             if (asyncManager.isConcurrentHandlingStarted()) {
57                 if (mappedHandler != null) {
58                     mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
59                 }
60             } else if (multipartRequestParsed) {
61                 this.cleanupMultipart(processedRequest);
62             }
63 
64         }
65     }

下图就是该方法重要的第一步，获取执行链

执行链中包含两个重要的属性，一个是handler(上图所说的handler)，另一个是HandlerInterceptor(拦截器)，下面再说这两个的属性

进入getHandler()方法

四种HandlerMapping:

DefaultAnnotationHandlerMapping;

SimpleUrlHandlerMapping;

BeanNameUrlHandlerMapping;

ControllerClassNameHandlerMapping。