【深入淺出spring】Spring MVC 流程解析 -- HandlerAdapter

villainhr 發(fā)布于2019-08-15 15:12 / 1672人閱讀

摘要：前言繼上一篇深入淺出流程解析介紹了后，本文按照深入淺出流程解析的分析流程，繼續(xù)往下分析，介紹下相關(guān)的內(nèi)容。即適配類型為的處理器，對(duì)應(yīng)。之前在問(wèn)答社區(qū)發(fā)現(xiàn)很多的問(wèn)題都集中再這塊。中的就是通過(guò)適配的附錄類圖

前言

繼上一篇【深入淺出spring】Spring MVC 流程解析 -- HanndlerMapping介紹了handler mapping后，本文按照【深入淺出spring】Spring MVC 流程解析的分析流程，繼續(xù)往下分析，介紹下HandlerAdapter相關(guān)的內(nèi)容。

總流程

回顧下DispatcherServlet.doDispatch的代碼：

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);
                }
            }
        }
    }

從源碼可以看到，17行根據(jù)request拿到對(duì)象HandlerExecutionChain(包含一個(gè)處理器 handler 如HandlerMethod 對(duì)象、多個(gè) HandlerInterceptor 攔截器對(duì)象)后，就是24行根據(jù)handler獲取對(duì)應(yīng)的adapter，并在44行調(diào)用適配器的handler方法（適配器設(shè)計(jì)模式可以自行g(shù)oogle了解），返回ModelAndView。詳細(xì)看下getHandlerAdapter這個(gè)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的邏輯非常類似，遍歷容器中的所有HandlerAdapter，然后判斷是否支持適配此handler，這里的關(guān)鍵方法supports是接口HandlerAdapter中的方法，具體邏輯由其實(shí)現(xiàn)類決定。默認(rèn)的HandlerAdapter的實(shí)現(xiàn)類有3種：

RequestMappingHandlerAdapter

HttpRequestHandlerAdapter

SimpleControllerHandlerAdapter

RequestMappingHandlerAdapter 適配哪類處理器

RequestMappingHandlerAdapter沒(méi)有重寫supports方法，即執(zhí)行的是其父類AbstractHandlerMethodAdapter的方法，代碼如下：

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

其中supportInternal由子類RequestMappingHandlerAdapter實(shí)現(xiàn)，直接返回常量true，故可以認(rèn)為只要handler屬于HandlerMethod類型，就由RequestMappingHandlerAdapter來(lái)適配。即RequestMappingHandlerAdapter適配類型為HandlerMethod的處理器，對(duì)應(yīng)RequestMappingHandlerMapping。

處理邏輯

RequestMappingHandlerAdapter的處理邏輯主要由handleInternal實(shí)現(xiàn)：

    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，核心處理邏輯由方法invokeHandlerMethod實(shí)現(xiàn)，這塊處理邏輯比較復(fù)雜，涉及輸入?yún)?shù)的解析，返回?cái)?shù)據(jù)的處理，后面一篇文章【深入淺出spring】Spring MVC 流程解析 -- InvocableHandlerMethod會(huì)重點(diǎn)講這塊。之前在問(wèn)答社區(qū)發(fā)現(xiàn)很多spring mvc的問(wèn)題都集中再這塊。

HttpRequestHandlerAdapter 適配哪類處理器

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

源碼很簡(jiǎn)單，適配類型為HttpRequestHandler的處理器

處理邏輯

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

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

處理邏輯也很簡(jiǎn)單，直接調(diào)用HttpRequestHandler.handleRequest方法，這里不是通過(guò)返回?cái)?shù)據(jù)實(shí)現(xiàn)和前端交互，而是直接通過(guò)改寫HttpServletResponse實(shí)現(xiàn)前后端交互

SimpleControllerHandlerAdapter 適配哪類處理器

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

這里的Controller是一個(gè)接口，即所有實(shí)現(xiàn)Controller接口的類，SimpleControllerHandlerAdapter都適配

處理邏輯

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

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

和HttpRequestHandlerAdapter類似，直接調(diào)用Controller.handleRequest，即具體實(shí)現(xiàn)類的handleRequest方法，然后支持直接返回?cái)?shù)據(jù)來(lái)和前端交互。
handler_mapping_sample中的SimpleUrlController就是通過(guò)SimpleControllerHandlerAdapter適配的

附錄 RequestMappingHandlerAdapter 類圖