HandlerMapping接口

HandlerMapping是用来处理请求与handler对象的对应关系的。

HandlerMapping主要包含getHandler这个方法。

HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception;

DispatcherServlet就是通过调用HandlerMapping的getHandler来进行找到request对应的handler的。

AbstractHandlerMapping类

AbstractHandlerMapping是实现HandlerMapping接口的抽象基类。

首先我们来看其中的getHandler方法：

如果不考虑类中的跨域请求的处理。那么我们可以只关注下面两个方法，getHandlerExecutionChain和getHandler。因为getHandler中调用了getHandlerExecutionChain，所以我们先来看一下getHandlerExecutionChain方法。

protected HandlerExecutionChain getHandlerExecutionChain(Object handler, HttpServletRequest request) {
   HandlerExecutionChain chain = (handler instanceof HandlerExecutionChain ?
         (HandlerExecutionChain) handler : new HandlerExecutionChain(handler));

   String lookupPath = this.urlPathHelper.getLookupPathForRequest(request);
   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;
}

这段代码的意思就是 找到该handler的所有对应MappedInterceptor并将其与handler组合成HandlerExecutionChain，当然还包括所有的非MappedInterceptor类型的HandlerInterceptor。

简言之，就是返回handler所对应的HandlerExecutionChain。

然后我们就来阅读getHandler部分的代码

public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
   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);
   }

   HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request);
   if (CorsUtils.isCorsRequest(request)) {
      CorsConfiguration globalConfig = this.globalCorsConfigSource.getCorsConfiguration(request);
      CorsConfiguration handlerConfig = getCorsConfiguration(handler, request);
      CorsConfiguration config = (globalConfig != null ? globalConfig.combine(handlerConfig) : handlerConfig);
      executionChain = getCorsHandlerExecutionChain(request, executionChain, config);
   }
   return executionChain;
}

这段代码的意思就是从request中抽离出handler并调用getHandlerExecutionChain方法组装。（其中getHandlerInternal是一种抽象方法）。

总结

我们可以从上面的代码中总结出整个HandlerMapping的设计思路。

首先[request]由[AbstractHandlerMapping的实现类]处理得到其对应的[handler]，接着[handler]由[AbstractHandlerMapping]找到其对应的所有拦截器并包装成[HandlerExecutionChain]返回。