spring mvc(5) HandlerAdapter

　前面我们讲到了通过HandlerMapping可以获得不同类型的处理器，可以是Controller、HttpRequestHandler、Servlet、HandlerMethod甚至是我们自定义的处理器。针对不同的处理器，需要调用不同的执行流程。HandlerAdapter的作用相当于将这个执行流程与DispatcherServlet进行了解耦。

　　DispatcherServlet在初始化是会加载所有容器中的HandlerAdapter，通过order属性进行排序。然后在对处理器调用时循环找到可以支持该处理器的HandlerAdapter，然后执行调用。如果容器中没有HandlerAdapter，会读取DispatcherServlet.properties配置，默认配置HttpRequestHandlerAdapter、SimpleControllerHandlerAdapter、AnnotationMethodHandlerAdapter。

DispatcherServlet初始化HandlerAdapter。

 private void initHandlerAdapters(ApplicationContext context) {
         this.handlerAdapters = null;

         if (this.detectAllHandlerAdapters) {
             // Find all HandlerAdapters in the ApplicationContext, including ancestor contexts.
             Map<String, HandlerAdapter> matchingBeans =
                     BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerAdapter.class, true, false);
             if (!matchingBeans.isEmpty()) {
                 this.handlerAdapters = new ArrayList<>(matchingBeans.values());
                 // We keep HandlerAdapters in sorted order.
                 AnnotationAwareOrderComparator.sort(this.handlerAdapters);
             }
         }
         else {
             //如果detectAllHandlerAdapters为false，则使用名为handlerAdapter的HandlerAdapter
             try {
                 HandlerAdapter ha = context.getBean(HANDLER_ADAPTER_BEAN_NAME, HandlerAdapter.class);
                 this.handlerAdapters = Collections.singletonList(ha);
             }
             catch (NoSuchBeanDefinitionException ex) {
                 // Ignore, we'll add a default HandlerAdapter later.
             }
         }

         if (this.handlerAdapters == null) {
             this.handlerAdapters = getDefaultStrategies(context, HandlerAdapter.class);
             if (logger.isDebugEnabled()) {
                 logger.debug("No HandlerAdapters found in servlet '" + getServletName() + "': using default");
             }
         }
     }

spring mvc目前有一下的HandlerAdapter：

　　SimpleControllerHandlerAdapter执行Controller的调用。

 public class SimpleControllerHandlerAdapter implements HandlerAdapter {

     @Override
     public boolean supports(Object handler) {
         return (handler instanceof Controller);
     }

     @Override
     @Nullable
     public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
             throws Exception {

         return ((Controller) handler).handleRequest(request, response);
     }

     @Override
     public long getLastModified(HttpServletRequest request, Object handler) {
         if (handler instanceof LastModified) {
             return ((LastModified) handler).getLastModified(request);
         }
         return -1L;
     }

 }

　　HttpRequestHandlerAdapter执行HttpRequestHandler调用。

 public class HttpRequestHandlerAdapter implements HandlerAdapter {

     @Override
     public boolean supports(Object handler) {
         return (handler instanceof HttpRequestHandler);
     }

     @Override
     @Nullable
     public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
             throws Exception {

         ((HttpRequestHandler) handler).handleRequest(request, response);
         return null;
     }

     @Override
     public long getLastModified(HttpServletRequest request, Object handler) {
         if (handler instanceof LastModified) {
             return ((LastModified) handler).getLastModified(request);
         }
         return -1L;
     }

 }

　　SimpleServletHandlerAdapter执行Servlet的调用。

 public class SimpleServletHandlerAdapter implements HandlerAdapter {

     @Override
     public boolean supports(Object handler) {
         return (handler instanceof Servlet);
     }

     @Override
     @Nullable
     public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
             throws Exception {

         ((Servlet) handler).service(request, response);
         return null;
     }

     @Override
     public long getLastModified(HttpServletRequest request, Object handler) {
         return -1;
     }

 }