图片来源：https://www.jianshu.com/p/8a20c547e245

DispatcherServlet根据url定位到Controller和方法，依赖的是HandlerMapping接口的各个实现类，其中，RequestMappingHandlerMapping是专门用来处理注解方式的Controller的

下面，我们分RequestMappingHandlerMapping的加载以及RequestMappingHandlerMapping如何根据url定位Controller两部分来讲

RequestMappingHandlerMapping加载过程

1. RMHP在系统启动时会被注册成bean，见《springmvc源码笔记-HandlerMapping注入》

2. 因为RMHP实现了InitializingBean接口，在bean加载完成后会自动调用afterPropertiesSet方法，在此方法中调用了AbstractHandlerMethodMapping#initHandlerMethods()来实现初始化

   protected void initHandlerMethods() {
   // 遍历所有的bean
   for (String beanName : getCandidateBeanNames()) {
       if (!beanName.startsWith(SCOPED_TARGET_NAME_PREFIX)) {
           processCandidateBean(beanName);
       }
   }
   handlerMethodsInitialized(getHandlerMethods());
   }
   
   protected void processCandidateBean(String beanName) {
   Class<?> beanType = null;
   try {
       beanType = obtainApplicationContext().getType(beanName);
   }
   ......
   // isHandler判断类是否有Controller或者RequestMapping注解
   if (beanType != null && isHandler(beanType)) {
       detectHandlerMethods(beanName);
   }
   }
   
   protected void detectHandlerMethods(Object handler) {
   ......
   if (handlerType != null) {
       Class<?> userType = ClassUtils.getUserClass(handlerType);
       // 遍历类的所有方法
       Map<Method, T> methods = MethodIntrospector.selectMethods(userType,
               (MethodIntrospector.MetadataLookup<T>) method -> {
                   try {
                       // 获取方法上的映射（会读取方法上的RequestMapping注解获取url）
                       return getMappingForMethod(method, userType);
                   }
                   catch (Throwable ex) {
                       throw new IllegalStateException("Invalid mapping on handler class [" +
                               userType.getName() + "]: " + method, ex);
                   }
               });
       ......
       methods.forEach((method, mapping) -> {
           Method invocableMethod = AopUtils.selectInvocableMethod(method, userType);
           // 将映射关系放入AbstractHandlerMethodMapping.mappingRegistry中
           registerHandlerMethod(handler, invocableMethod, mapping);
       });
   }
   }
   

RequestMappingHandlerMapping解析过程

1. DispatcherServlet继承HttpServlet，所以执行链是FrameworkServlet#doGet→DispatcherServlet#doService→DispatcherServlet#doDispatch→DispatcherServlet#getHandler

   protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
   if (this.handlerMappings != null) {
       // handlerMappings已在onRefresh方法中初始化
       for (HandlerMapping mapping : this.handlerMappings) {
           // 第一个mapping就是RequestMappingHandlerMapping
           // 获取处理程序，也就是url对应的controller和method
           HandlerExecutionChain handler = mapping.getHandler(request);
           if (handler != null) {
               return handler;
           }
       }
   }
   return null;
   }
   
   // AbstractHandlerMapping#getHandler->RequestMappingHandlerMapping#getHandlerInternal->AbstractHandlerMapping#lookupHandlerMethod
   protected HandlerMethod lookupHandlerMethod(String lookupPath, HttpServletRequest request) throws Exception {
   // lookupPath就是请求路径
   List<Match> matches = new ArrayList<>();
   // 获取url路径匹配项
   List<T> directPathMatches = this.mappingRegistry.getMappingsByDirectPath(lookupPath);
   if (directPathMatches != null) {
       // 添加映射匹配集合
       addMatchingMappings(directPathMatches, matches, request);
   }
   if (matches.isEmpty()) {
       addMatchingMappings(this.mappingRegistry.getRegistrations().keySet(), matches, request);
   }
   if (!matches.isEmpty()) {
       ......
       // 至此，根据url获取到controller和method
       return bestMatch.getHandlerMethod();
   } else {
       return handleNoMatch(this.mappingRegistry.getRegistrations().keySet(), lookupPath, request);
   }
   }
   
   private void addMatchingMappings(Collection<T> mappings, List<Match> matches, HttpServletRequest request) {
   for (T mapping : mappings) {
       T match = getMatchingMapping(mapping, request);
       if (match != null) {
           // this.mappingRegistry.getRegistrations().get(mapping)就是获取HandlerMethodMapping
           // HandlerMethodMapping保存controller和method信息的类
           matches.add(new Match(match, this.mappingRegistry.getRegistrations().get(mapping)));
       }
   }
   }