HandlerAdapter接口类的简介说明

下文笔者将对HandlerAdapter接口类进行简介说明，如下所示:

HandlerAdapter接口实现类的作用:
    对用户请求过来的方法进行相应的处理，
	在SpringMVC中通过使用HandlerAdapter中的处理函数进行相关的方法运行

HandlerAdapter类继承

HandlerAdapter(接口)
    AbstractHandlerMethodAapter
	   RequestMappingHandlerAdapter
	AnnotationMethodHandlerAdapter
	HttpRequestHandlerAdapter
	SimpleControllerHandlerAdapter
	SimpleServletHandlerAdapter

HandlerAdapter接口定义

public interface HandlerAdapter {  
    //检测是否支持传入的Handler
    boolean supports(Object handler);  
    //使用Handler处理请求
    ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception;  
    //获取Last-Modified值
    long getLastModified(HttpServletRequest request, Object handler);  
}

DispatcherServlet中注册HandlerAdapter流程

DispatcherServlet中使用initHandlerAdapters方法
当没有对HandlerAdapter对象进行默认设置时，此时DispatcherServlet将加载三个默认的HandlerAdapter对象
HttpRequestHandlerAdapter，SimpleControllerHandlerAdapter和 AnnotationMethodHandlerAdapter
例：

private void initHandlerAdapters(ApplicationContext context) {
    this.handlerAdapters = null;
    if (this.detectAllHandlerAdapters) {
        // Find all HandlerAdapters in the ApplicationContext, including ancestor contexts.
        Map matchingBeans =
                BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerAdapter.class, true, false);
        if (!matchingBeans.isEmpty()) {
            this.handlerAdapters = new Arraylist(matchingBeans.values());
            // We keep HandlerAdapters in sorted order.
            OrderComparator.sort(this.handlerAdapters);
        }
    }
    else {
        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.
        }
    }

    // Ensure we have at least some HandlerAdapters, by registering
    // default HandlerAdapters if no other adapters are found.
    if (this.handlerAdapters == null) {
        this.handlerAdapters = getDefaultStrategies(context, HandlerAdapter.class);
        if (logger.isDebugEnabled()) {
            logger.debug("No HandlerAdapters found in servlet '" + getServletName() + "': using default");
        }
    }
}

HandlerMapping匹配Handler对象的方式

使用handlerMapping传过来的Handler对象同DispatcherServlet集合属性handlerAdapter中的 HandlerAdapter一一匹配
当存在支持Handler对象的HandlerAdapter
则HandlerAdapter就会调用自己的 handle方法处理请求

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
    HttpServletRequest processedRequest = request;
    HandlerExecutionChain mappedHandler = null;
    int interceptorIndex = -1;

    try {
        ModelAndView mv;
        boolean errorView = false;
        try {
            processedRequest = checkMultipart(request);
            // Determine handler for the current request.
            mappedHandler = getHandler(processedRequest, false);
            if (mappedHandler == null || mappedHandler.getHandler() == null) {
                noHandlerFound(processedRequest, response);
                return;
            }
            //获取合适的HandlerAdapter实现类  
            HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

            // Process last-modified header, if supported by the handler.
            String method = request.getMethod();
            boolean isGet = "GET".equals(method);
            if (isGet || "HEAD".equals(method)) {
                long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
                if (logger.isDebugEnabled()) {
                    String requestUri = urlPathHelper.getRequestUri(request);
                    logger.debug("Last-Modified value for [" + requestUri + "] is: " + lastModified);
                }
                if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
                    return;
                }
            }
            // Apply preHandle methods of registered interceptors.
            HandlerInterceptor[] interceptors = mappedHandler.getInterceptors();
            if (interceptors != null) {
                for (int i = 0; i < interceptors.length; i++) { HandlerInterceptor interceptor = interceptors[i]; if (!interceptor.preHandle(processedRequest, response, mappedHandler.getHandler())) { triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, null); return; } interceptorIndex = i; } } // 执行真正的请求操作 mv = ha.handle(processedRequest, response, mappedHandler.getHandler()); // Do we need view name translation? if (mv != null && !mv.hasView()) { mv.setViewName(getDefaultViewName(request)); } // Apply postHandle methods of registered interceptors. if (interceptors != null) { for (int i = interceptors.length - 1; i >= 0; i--) {
                    HandlerInterceptor interceptor = interceptors[i];
                    interceptor.postHandle(processedRequest, response, mappedHandler.getHandler(), mv);
                }
            }
        }
        catch (ModelAndViewDefiningException ex) {
            logger.debug("ModelAndViewDefiningException encountered", ex);
            mv = ex.getModelAndView();
        }
        catch (Exception ex) {
            Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
            mv = processHandlerException(processedRequest, response, handler, ex);
            errorView = (mv != null);
        }

        // Did the handler return a view to render?
        if (mv != null && !mv.wasCleared()) {
            render(mv, processedRequest, response);
            if (errorView) {
                WebUtils.clearErrorRequestAttributes(request);
            }
        }
        else {
            if (logger.isDebugEnabled()) {
                logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + getServletName() +
                        "': assuming HandlerAdapter completed request handling");
            }
        }

        // Trigger after-completion for successful outcome.
        triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, null);
    }

    catch (Exception ex) {
        // Trigger after-completion for thrown exception.
        triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, ex);
        throw ex;
    }
    catch (Error err) {
        ServletException ex = new NestedServletException("Handler processing failed", err);
        // Trigger after-completion for thrown exception.
        triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, ex);
        throw ex;
    }

    finally {
        // Clean up any resources used by a multipart request.
        if (processedRequest != request) {
            cleanupMultipart(processedRequest);
        }
    }
}