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。

 1 private void initHandlerAdapters(ApplicationContext context) {
 2         this.handlerAdapters = null;
 3         
 4         if (this.detectAllHandlerAdapters) {
 5             // Find all HandlerAdapters in the ApplicationContext, including ancestor contexts.
 6             Map<String, HandlerAdapter> matchingBeans =
 7                     BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerAdapter.class, true, false);
 8             if (!matchingBeans.isEmpty()) {
 9                 this.handlerAdapters = new ArrayList<>(matchingBeans.values());
10                 // We keep HandlerAdapters in sorted order.
11                 AnnotationAwareOrderComparator.sort(this.handlerAdapters);
12             }
13         }
14         else {
15             //如果detectAllHandlerAdapters为false，则使用名为handlerAdapter的HandlerAdapter
16             try {
17                 HandlerAdapter ha = context.getBean(HANDLER_ADAPTER_BEAN_NAME, HandlerAdapter.class);
18                 this.handlerAdapters = Collections.singletonList(ha);
19             }
20             catch (NoSuchBeanDefinitionException ex) {
21                 // Ignore, we'll add a default HandlerAdapter later.
22             }
23         }
24 
25         if (this.handlerAdapters == null) {
26             this.handlerAdapters = getDefaultStrategies(context, HandlerAdapter.class);
27             if (logger.isDebugEnabled()) {
28                 logger.debug("No HandlerAdapters found in servlet '" + getServletName() + "': using default");
29             }
30         }
31     }

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spring mvc目前有一下的HandlerAdapter：

　　SimpleControllerHandlerAdapter执行Controller的调用。

 1 public class SimpleControllerHandlerAdapter implements HandlerAdapter {
 2 
 3     @Override
 4     public boolean supports(Object handler) {
 5         return (handler instanceof Controller);
 6     }
 7 
 8     @Override
 9     @Nullable
10     public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
11             throws Exception {
12 
13         return ((Controller) handler).handleRequest(request, response);
14     }
15 
16     @Override
17     public long getLastModified(HttpServletRequest request, Object handler) {
18         if (handler instanceof LastModified) {
19             return ((LastModified) handler).getLastModified(request);
20         }
21         return -1L;
22     }
23 
24 }

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　　HttpRequestHandlerAdapter执行HttpRequestHandler调用。

 1 public class HttpRequestHandlerAdapter implements HandlerAdapter {
 2 
 3     @Override
 4     public boolean supports(Object handler) {
 5         return (handler instanceof HttpRequestHandler);
 6     }
 7 
 8     @Override
 9     @Nullable
10     public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
11             throws Exception {
12 
13         ((HttpRequestHandler) handler).handleRequest(request, response);
14         return null;
15     }
16 
17     @Override
18     public long getLastModified(HttpServletRequest request, Object handler) {
19         if (handler instanceof LastModified) {
20             return ((LastModified) handler).getLastModified(request);
21         }
22         return -1L;
23     }
24 
25 }

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　　SimpleServletHandlerAdapter执行Servlet的调用。

 1 public class SimpleServletHandlerAdapter implements HandlerAdapter {
 2 
 3     @Override
 4     public boolean supports(Object handler) {
 5         return (handler instanceof Servlet);
 6     }
 7 
 8     @Override
 9     @Nullable
10     public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
11             throws Exception {
12 
13         ((Servlet) handler).service(request, response);
14         return null;
15     }
16 
17     @Override
18     public long getLastModified(HttpServletRequest request, Object handler) {
19         return -1;
20     }
21 
22 }

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