【深入浅出spring】Spring MVC 流程解析 -- HandlerAdapter

摘要：前言继上一篇深入浅出流程解析介绍了后，本文按照深入浅出流程解析的分析流程，继续往下分析，介绍下相关的内容。即适配类型为的处理器，对应。之前在问答社区发现很多的问题都集中再这块。中的就是通过适配的附录类图

继上一篇【深入浅出spring】Spring MVC 流程解析 -- HanndlerMapping介绍了handler mapping后，本文按照【深入浅出spring】Spring MVC 流程解析的分析流程，继续往下分析，介绍下HandlerAdapter相关的内容。

回顾下DispatcherServlet.doDispatch的代码：

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
        HttpServletRequest processedRequest = request;
        HandlerExecutionChain mappedHandler = null;
        boolean multipartRequestParsed = false;

        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

        try {
            ModelAndView mv = null;
            Exception dispatchException = null;

            try {
                processedRequest = checkMultipart(request);
                multipartRequestParsed = (processedRequest != request);

                // Determine handler for the current request.
                mappedHandler = getHandler(processedRequest);
                if (mappedHandler == null) {
                    noHandlerFound(processedRequest, response);
                    return;
                }

                // Determine handler adapter for the current request.
                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()) {
                        logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
                    }
                    if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
                        return;
                    }
                }

                if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                    return;
                }

                // Actually invoke the handler.
                mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

                if (asyncManager.isConcurrentHandlingStarted()) {
                    return;
                }

                applyDefaultViewName(processedRequest, mv);
                mappedHandler.applyPostHandle(processedRequest, response, mv);
            }
            catch (Exception ex) {
                dispatchException = ex;
            }
            catch (Throwable err) {
                // As of 4.3, we"re processing Errors thrown from handler methods as well,
                // making them available for @ExceptionHandler methods and other scenarios.
                dispatchException = new NestedServletException("Handler dispatch failed", err);
            }
            processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
        }
        catch (Exception ex) {
            triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
        }
        catch (Throwable err) {
            triggerAfterCompletion(processedRequest, response, mappedHandler,
                    new NestedServletException("Handler processing failed", err));
        }
        finally {
            if (asyncManager.isConcurrentHandlingStarted()) {
                // Instead of postHandle and afterCompletion
                if (mappedHandler != null) {
                    mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
                }
            }
            else {
                // Clean up any resources used by a multipart request.
                if (multipartRequestParsed) {
                    cleanupMultipart(processedRequest);
                }
            }
        }
    }

从源码可以看到，17行根据request拿到对象HandlerExecutionChain(包含一个处理器 handler 如HandlerMethod 对象、多个 HandlerInterceptor 拦截器对象)后，就是24行根据handler获取对应的adapter，并在44行调用适配器的handler方法（适配器设计模式可以自行google了解），返回ModelAndView。详细看下getHandlerAdapter这个方法：

    protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
        if (this.handlerAdapters != null) {
            for (HandlerAdapter ha : this.handlerAdapters) {
                if (logger.isTraceEnabled()) {
                    logger.trace("Testing handler adapter [" + ha + "]");
                }
                if (ha.supports(handler)) {
                    return ha;
                }
            }
        }
        throw new ServletException("No adapter for handler [" + handler +
                "]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
    }

和上文handler mapping的逻辑非常类似，遍历容器中的所有HandlerAdapter，然后判断是否支持适配此handler，这里的关键方法supports是接口HandlerAdapter中的方法，具体逻辑由其实现类决定。默认的HandlerAdapter的实现类有3种：

RequestMappingHandlerAdapter

HttpRequestHandlerAdapter

SimpleControllerHandlerAdapter

RequestMappingHandlerAdapter没有重写supports方法，即执行的是其父类AbstractHandlerMethodAdapter的方法，代码如下：

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

其中supportInternal由子类RequestMappingHandlerAdapter实现，直接返回常量true，故可以认为只要handler属于HandlerMethod类型，就由RequestMappingHandlerAdapter来适配。即RequestMappingHandlerAdapter适配类型为HandlerMethod的处理器，对应RequestMappingHandlerMapping。

RequestMappingHandlerAdapter的处理逻辑主要由handleInternal实现：

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

        ModelAndView mav;
        checkRequest(request);

        // Execute invokeHandlerMethod in synchronized block if required.
        if (this.synchronizeOnSession) {
            HttpSession session = request.getSession(false);
            if (session != null) {
                Object mutex = WebUtils.getSessionMutex(session);
                synchronized (mutex) {
                    mav = invokeHandlerMethod(request, response, handlerMethod);
                }
            }
            else {
                // No HttpSession available -> no mutex necessary
                mav = invokeHandlerMethod(request, response, handlerMethod);
            }
        }
        else {
            // No synchronization on session demanded at all...
            mav = invokeHandlerMethod(request, response, handlerMethod);
        }

        if (!response.containsHeader(HEADER_CACHE_CONTROL)) {
            if (getSessionAttributesHandler(handlerMethod).hasSessionAttributes()) {
                applyCacheSeconds(response, this.cacheSecondsForSessionAttributeHandlers);
            }
            else {
                prepareResponse(response);
            }
        }

        return mav;
    }

可以看到，核心处理逻辑由方法invokeHandlerMethod实现，这块处理逻辑比较复杂，涉及输入参数的解析，返回数据的处理，后面一篇文章【深入浅出spring】Spring MVC 流程解析 -- InvocableHandlerMethod会重点讲这块。之前在问答社区发现很多spring mvc的问题都集中再这块。

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

源码很简单，适配类型为HttpRequestHandler的处理器

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

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

处理逻辑也很简单，直接调用HttpRequestHandler.handleRequest方法，这里不是通过返回数据实现和前端交互，而是直接通过改写HttpServletResponse实现前后端交互

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

这里的Controller是一个接口，即所有实现Controller接口的类，SimpleControllerHandlerAdapter都适配

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

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

和HttpRequestHandlerAdapter类似，直接调用Controller.handleRequest，即具体实现类的handleRequest方法，然后支持直接返回数据来和前端交互。
handler_mapping_sample中的SimpleUrlController就是通过SimpleControllerHandlerAdapter适配的