SpringMVC(四)：什么是HandlerAdapter

一、什么是HandlerAdapter

Note that a handler can be of type Object. This is to enable handlers from other frameworks to be integrated with this framework without custom coding.

boolean supports(Object handler)

Given a handler instance, return whether or not this HandlerAdapter can support it.

 HandlerAdapter 接口实现类：

1. HttpRequestHandlerAdapter ： 要求handler实现HttpRequestHandler接口

public class HttpRequestHandlerAdapter implements HandlerAdapter {

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

    @Override
    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;
    }

}

2. SimpleControllerHandlerAdapter：要求handler实现Controller接口，该接口的方法参数为ModelAndView 

public class SimpleControllerHandlerAdapter implements HandlerAdapter {

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

    @Override
    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;
    }

}

3. AbstracthandlerMethodAdapter的handle()使用模板方法，调用子类handleInternal()实现

    @Override
    public final boolean supports(Object handler) {
        return (handler instanceof HandlerMethod && supportsInternal((HandlerMethod) handler));
    }

    protected abstract boolean supportsInternal(HandlerMethod handlerMethod);


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

        return handleInternal(request, response, (HandlerMethod) handler);
    }


    protected abstract ModelAndView handleInternal(HttpServletRequest request,
            HttpServletResponse response, HandlerMethod handlerMethod) throws Exception;

AbstracthandlerMethodAdapter及其子类RequestMappingHandlerAdapter的supportsInternal（）方法

    /**
     * Always return {@code true} since any method argument and return value
     * type will be processed in some way. A method argument not recognized
     * by any HandlerMethodArgumentResolver is interpreted as a request parameter
     * if it is a simple type, or as a model attribute otherwise. A return value
     * not recognized by any HandlerMethodReturnValueHandler will be interpreted
     * as a model attribute.
     */
    @Override
    protected boolean supportsInternal(HandlerMethod handlerMethod) {
        return true;
    }

 二、什么是HandlerMethod：封装了请求处理方法的参数和返回值，及该方法上的注解

Encapsulates information about a handler method consisting of a method and a bean. Provides convenient access to method parameters, the method return value, method annotations, etc.

The class may be created with a bean instance or with a bean name (e.g. lazy-init bean, prototype bean).

Use createWithResolvedBean() to obtain a HandlerMethod instance with a bean instance resolved through the associated BeanFactory.

三、什么时候使用HandlerAdapter

1. DispatchServlet protected void doDispatch()方法会获取相应的HandlerAdapter

                // Determine handler adapter for the current request.
                HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
                mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

在以前的文章中，已经说过RequestMappingHandlerAdapter是处理输入输出的关键点，也是spring根据相应的路径查找对应的处理器的关键点

RequestMappingHandlerAdapter的handleInternal()会调用invokeHandlerMethod()

    protected ModelAndView invokeHandlerMethod(HttpServletRequest request,
            HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {

        ServletWebRequest webRequest = new ServletWebRequest(request, response);
        try {
            WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
            ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);
　　　　　　　　//创建ServletInvocableHandlerMethod
            ServletInvocableHandlerMethod invocableMethod = createInvocableHandlerMethod(handlerMethod);
            invocableMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
            invocableMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);
            invocableMethod.setDataBinderFactory(binderFactory);
            invocableMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer);

            ModelAndViewContainer mavContainer = new ModelAndViewContainer();
            mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));
            modelFactory.initModel(webRequest, mavContainer, invocableMethod);
            mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect);

            AsyncWebRequest asyncWebRequest = WebAsyncUtils.createAsyncWebRequest(request, response);
            asyncWebRequest.setTimeout(this.asyncRequestTimeout);

            WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
            asyncManager.setTaskExecutor(this.taskExecutor);
            asyncManager.setAsyncWebRequest(asyncWebRequest);
            asyncManager.registerCallableInterceptors(this.callableInterceptors);
            asyncManager.registerDeferredResultInterceptors(this.deferredResultInterceptors);

            if (asyncManager.hasConcurrentResult()) {
                Object result = asyncManager.getConcurrentResult();
                mavContainer = (ModelAndViewContainer) asyncManager.getConcurrentResultContext()[0];
                asyncManager.clearConcurrentResult();
                if (logger.isDebugEnabled()) {
                    logger.debug("Found concurrent result value [" + result + "]");
                }
                invocableMethod = invocableMethod.wrapConcurrentResult(result);
            }
            //调用
            invocableMethod.invokeAndHandle(webRequest, mavContainer);
            if (asyncManager.isConcurrentHandlingStarted()) {
                return null;
            }

            return getModelAndView(mavContainer, modelFactory, webRequest);
        }
        finally {
            webRequest.requestCompleted();
        }
    }

3.一个请求过来时，InvocableHandlerMethod的执行变量

从上面的分析看到，处理请求时，会调用invokeAndHandle()，最终是doInvoke执行 

4.  疑问：这些HandlerMethond是什么时候载入系统的

解答：系统启动的时候执行registerHandlerMethod()方法，创建HandlerMethod实例

三、HandlerMapping VS HandlerAdapter

The strategy interface HandlerAdapter takes the role of invoking handler methods selected by some HandlerMapping.