【深刻淺出spring】Spring MVC 流程解析 -- HandlerAdapter

時間 2019-11-08

標籤深刻淺出spring spring mvc 流程解析 handleradapter 欄目 Spring 简体版

原文原文鏈接

前言

繼上一篇【深刻淺出spring】Spring MVC 流程解析 -- HanndlerMapping介紹了handler mapping後，本文按照【深刻淺出spring】Spring MVC 流程解析的分析流程，繼續往下分析，介紹下HandlerAdapter相關的內容。前端

總流程

回顧下DispatcherServlet.doDispatch的代碼：ios

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);
                }
            }
        }
    }

從源碼能夠看到，17行根據request拿到對象HandlerExecutionChain(包含一個處理器 handler 如HandlerMethod 對象、多個 HandlerInterceptor 攔截器對象)後，就是24行根據handler獲取對應的adapter，並在44行調用適配器的handler方法（適配器設計模式能夠自行google瞭解），返回ModelAndView。詳細看下getHandlerAdapter這個方法：git

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的邏輯很是相似，遍歷容器中的全部HandlerAdapter，而後判斷是否支持適配此handler，這裏的關鍵方法supports是接口HandlerAdapter中的方法，具體邏輯由其實現類決定。默認的HandlerAdapter的實現類有3種：spring

RequestMappingHandlerAdapter
HttpRequestHandlerAdapter
SimpleControllerHandlerAdapter

RequestMappingHandlerAdapter

適配哪類處理器

RequestMappingHandlerAdapter沒有重寫supports方法，即執行的是其父類AbstractHandlerMethodAdapter的方法，代碼以下：segmentfault

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

其中supportInternal由子類RequestMappingHandlerAdapter實現，直接返回常量true，故能夠認爲只要handler屬於HandlerMethod類型，就由RequestMappingHandlerAdapter來適配。即RequestMappingHandlerAdapter適配類型爲HandlerMethod的處理器，對應RequestMappingHandlerMapping。後端

處理邏輯

RequestMappingHandlerAdapter的處理邏輯主要由handleInternal實現：設計模式

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;
    }

能夠看到，核心處理邏輯由方法invokeHandlerMethod實現，這塊處理邏輯比較複雜，涉及輸入參數的解析，返回數據的處理，後面一篇文章【深刻淺出spring】Spring MVC 流程解析 -- InvocableHandlerMethod會重點講這塊。以前在問答社區發現不少spring mvc的問題都集中再這塊。session

HttpRequestHandlerAdapter

適配哪類處理器

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

源碼很簡單，適配類型爲HttpRequestHandler的處理器mvc

處理邏輯

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

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

處理邏輯也很簡單，直接調用HttpRequestHandler.handleRequest方法，這裏不是經過返回數據實現和前端交互，而是直接經過改寫HttpServletResponse實現先後端交互app

SimpleControllerHandlerAdapter

適配哪類處理器

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

這裏的Controller是一個接口，即全部實現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相似，直接調用Controller.handleRequest，即具體實現類的handleRequest方法，而後支持直接返回數據來和前端交互。
handler_mapping_sample中的SimpleUrlController就是經過SimpleControllerHandlerAdapter適配的