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;
    }

}