Spring MVC之适配器的获取及执行(RequestMappingHandlerAdapter)

首先看下doDispatch()方法如何找到适合的适配器来执行方法的：

    protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
        Iterator i$ = this.handlerAdapters.iterator();

        HandlerAdapter ha;
        do {
            if (!i$.hasNext()) {
                throw new ServletException("No adapter for handler [" + handler + "]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
            }

            ha = (HandlerAdapter)i$.next();
            if (this.logger.isTraceEnabled()) {
                this.logger.trace("Testing handler adapter [" + ha + "]");
            }
        } while(!ha.supports(handler)); //遍历初始化时候保存好的适配器，通过执行每个适配器的supports方法，如果支持就是他了。

        return ha;
    }

注： 这块也有点责任链模式的意思...

下面看下RequestMappingHandlerAdapter是怎么实现support方法的，看之前先上类图。

实际上support方法是在AbstractHandlerMethodAdapter这个父类实现的，然后给自己留个钩子方法，让子类实现

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

    protected abstract boolean supportsInternal(HandlerMethod var1); //钩子方法

这里RequestMappingHandlerAdapter的supportInternal直接是返回的true，估计后续扩展其他子类可能会复杂些，这就是设计模式的好处。

这样就找到了合适的适配器，下面来看执行：

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

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

                try {
                    processedRequest = this.checkMultipart(request);
                    multipartRequestParsed = processedRequest != request;
                    mappedHandler = this.getHandler(processedRequest, false); //获取处理器
                    if (mappedHandler == null || mappedHandler.getHandler() == null) {
                        this.noHandlerFound(processedRequest, response);
                        return;
                    }

                    HandlerAdapter ha = this.getHandlerAdapter(mappedHandler.getHandler()); //获取适配器
                    String method = request.getMethod();
                    boolean isGet = "GET".equals(method);
                    if (isGet || "HEAD".equals(method)) {
                        long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
                        if (this.logger.isDebugEnabled()) {
                            String requestUri = urlPathHelper.getRequestUri(request);
                            this.logger.debug("Last-Modified value for [" + requestUri + "] is: " + lastModified);
                        }

                        if ((new ServletWebRequest(request, response)).checkNotModified(lastModified) && isGet) {
                            return;
                        }
                    } //这一块是处理重复请求？？？ 有大神知道请留言....

                    if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                        return;
                    } //执行拦截器的preHandle方法

                    try {
                        mv = ha.handle(processedRequest, response, mappedHandler.getHandler()); //执行真正的Controller方法，就是我们的方法
                    } finally {
                        if (asyncManager.isConcurrentHandlingStarted()) {
                            return;
                        }

                    }

                    this.applyDefaultViewName(request, mv); //设置视图名称
                    mappedHandler.applyPostHandle(processedRequest, response, mv); //执行拦截器的postHandle方法
                } catch (Exception var28) {
                    dispatchException = var28;
                }

                this.processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);//渲染视图
            } catch (Exception var29) {
                this.triggerAfterCompletion(processedRequest, response, mappedHandler, var29);
            } catch (Error var30) {
                this.triggerAfterCompletionWithError(processedRequest, response, mappedHandler, var30);
            }

        } finally {
            if (asyncManager.isConcurrentHandlingStarted()) {
                mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response); //执行拦截器的afterConcurrentHandlingStarted
                return;
            } else {
                if (multipartRequestParsed) {
                    this.cleanupMultipart(processedRequest);
                }

            }
        }
    }

拦截器这里就不在多说，这块就是返回false就不在往下执行。下面我们重点满ha.handle()方法，是如果映射参数，找到我们的方法，封装结果的。

类图前面已经展示了，实际上handle是在父类AbstractHandlerMethodAdapter实现的

    public final ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
        return this.handleInternal(request, response, (HandlerMethod)handler); //子类实现这个方法
    }

    protected abstract ModelAndView handleInternal(HttpServletRequest var1, HttpServletResponse var2, HandlerMethod var3) throws Exception;

    protected final ModelAndView handleInternal(HttpServletRequest request, HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {
        if (this.getSessionAttributesHandler(handlerMethod).hasSessionAttributes()) {
            this.checkAndPrepare(request, response, this.cacheSecondsForSessionAttributeHandlers, true);
        } else {
            this.checkAndPrepare(request, response, true);
        } //看代码应该是从session中获取一些信息，然后初始化header等信息，不知道准确不？请大家指正！
        //这块就是根据需要是否进行同步操作
        if (this.synchronizeOnSession) {
            HttpSession session = request.getSession(false);
            if (session != null) {
                Object mutex = WebUtils.getSessionMutex(session);
                synchronized(mutex) {
                    return this.invokeHandleMethod(request, response, handlerMethod);
                }
            }
        }
        //正式进入执行环节
        return this.invokeHandleMethod(request, response, handlerMethod);
    }

下面这个方法非常重要，将重点讲解：

    private ModelAndView invokeHandleMethod(HttpServletRequest request, HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {
        ServletWebRequest webRequest = new ServletWebRequest(request, response);
        WebDataBinderFactory binderFactory = this.getDataBinderFactory(handlerMethod); //创建@InitBinder注解的方法的工厂类，进行缓存
        ModelFactory modelFactory = this.getModelFactory(handlerMethod, binderFactory);//创建@ModelAttribute@ControllerAdvice注解方法工厂并缓存
        ServletInvocableHandlerMethod requestMappingMethod = this.createRequestMappingMethod(handlerMethod, binderFactory);
        ModelAndViewContainer mavContainer = new ModelAndViewContainer(); //创建结果容器并初始化一些参数，
        mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));
        modelFactory.initModel(webRequest, mavContainer, requestMappingMethod);//执行@ModelAttribute注解的方法，将结果放到结果容器中
        mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect);
        AsyncWebRequest asyncWebRequest = WebAsyncUtils.createAsyncWebRequest(request, response); //下面异步这一块不太明白，等后续在慢慢分析
        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 (this.logger.isDebugEnabled()) {
                this.logger.debug("Found concurrent result value [" + result + "]");
            }

            requestMappingMethod = requestMappingMethod.wrapConcurrentResult(result);
        }

        requestMappingMethod.invokeAndHandle(webRequest, mavContainer, new Object[0]); //继续执行方法
        return asyncManager.isConcurrentHandlingStarted() ? null : this.getModelAndView(mavContainer, modelFactory, webRequest); //返回值了，两种情况
    }

    public final void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception {
        Object returnValue = this.invokeForRequest(webRequest, mavContainer, providedArgs); //执行方法，获取返回值
        this.setResponseStatus(webRequest);
        if (returnValue == null) {
            if (this.isRequestNotModified(webRequest) || this.hasResponseStatus() || mavContainer.isRequestHandled()) {
                mavContainer.setRequestHandled(true);
                return;
            }
        } else if (StringUtils.hasText(this.responseReason)) {
            mavContainer.setRequestHandled(true);
            return;
        }

        mavContainer.setRequestHandled(false);

        try { //处理返回值 ，封装结果集
            this.returnValueHandlers.handleReturnValue(returnValue, this.getReturnValueType(returnValue), mavContainer, webRequest);
        } catch (Exception var6) {
            if (this.logger.isTraceEnabled()) {
                this.logger.trace(this.getReturnValueHandlingErrorMessage("Error handling return value", returnValue), var6);
            }

            throw var6;
        }
    }

    public final Object invokeForRequest(NativeWebRequest request, ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception {
        Object[] args = this.getMethodArgumentValues(request, mavContainer, providedArgs); //处理 参数
        if (this.logger.isTraceEnabled()) {
            StringBuilder builder = new StringBuilder("Invoking [");
            builder.append(this.getMethod().getName()).append("] method with arguments ");
            builder.append(Arrays.asList(args));
            this.logger.trace(builder.toString());
        }

        Object returnValue = this.invoke(args); //反射执行方法
        if (this.logger.isTraceEnabled()) {
            this.logger.trace("Method [" + this.getMethod().getName() + "] returned [" + returnValue + "]");
        }

        return returnValue;
    }

    private Object[] getMethodArgumentValues(NativeWebRequest request, ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception {
        MethodParameter[] parameters = this.getMethodParameters();
        Object[] args = new Object[parameters.length];

        for(int i = 0; i < parameters.length; ++i) { //遍历方法的所有参数
            MethodParameter parameter = parameters[i];
            parameter.initParameterNameDiscovery(this.parameterNameDiscoverer);
            GenericTypeResolver.resolveParameterType(parameter, this.getBean().getClass()); //获取设置参数类型
            args[i] = this.resolveProvidedArgument(parameter, providedArgs);
            if (args[i] == null) {
                if (this.argumentResolvers.supportsParameter(parameter)) { //这块是遍历预置的参数解析器，就是前面说的责任链模式，**composite负责查找和执行
                    try { //由找到的参数解析器，来解析参数
                        args[i] = this.argumentResolvers.resolveArgument(parameter, mavContainer, request, this.dataBinderFactory);
                    } catch (Exception var9) {
                        if (this.logger.isTraceEnabled()) {
                            this.logger.trace(this.getArgumentResolutionErrorMessage("Error resolving argument", i), var9);
                        }

                        throw var9;
                    }
                } else if (args[i] == null) {
                    String msg = this.getArgumentResolutionErrorMessage("No suitable resolver for argument", i);
                    throw new IllegalStateException(msg);
                }
            }
        }

        return args;
    }

这块以，没有任何注解，参数为javaBean的解析器为例：ModelAttributeMethodProcessor

    public boolean supportsParameter(MethodParameter parameter) {
        if (parameter.hasParameterAnnotation(ModelAttribute.class)) {
            return true;
        } else if (this.annotationNotRequired) {
            return !BeanUtils.isSimpleProperty(parameter.getParameterType());
        } else {
            return false;
        }
    }

    public final Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer, NativeWebRequest request, WebDataBinderFactory binderFactory) throws Exception {
        String name = ModelFactory.getNameForParameter(parameter); //如果当前参数用@ModelAttribute修饰了，返回value值或者参数类型第一个字母小写
        // 获取需要绑定的表单对象，看参数容器包含name为key的对象不，有的话就用它，没有创建个新的。 
　　　　　　Object attribute = mavContainer.containsAttribute(name) ? mavContainer.getModel().get(name) : this.createAttribute(name, parameter, binderFactory, request);
        WebDataBinder binder = binderFactory.createBinder(request, attribute, name);
        if (binder.getTarget() != null) {
            this.bindRequestParameters(binder, request);
            this.validateIfApplicable(binder, parameter);
            if (binder.getBindingResult().hasErrors() && this.isBindExceptionRequired(binder, parameter)) {
                throw new BindException(binder.getBindingResult());
            }
        }
　　　　 //以上就是参数绑定， 这块领开一篇文章详细说
        Map<String, Object> bindingResultModel = binder.getBindingResult().getModel();
        mavContainer.removeAttributes(bindingResultModel);
        mavContainer.addAllAttributes(bindingResultModel);
        return binder.getTarget();
    }

参数就这样遍历处理，然后就开始通过反射 invoke执行了。接下来我们看是怎么封装换回结果的

        try {
            this.returnValueHandlers.handleReturnValue(returnValue, this.getReturnValueType(returnValue), mavContainer, webRequest);
        } catch (Exception var6) {
            if (this.logger.isTraceEnabled()) {
                this.logger.trace(this.getReturnValueHandlingErrorMessage("Error handling return value", returnValue), var6);
            }

            throw var6;
        }

this.returnValuehandlers. 就是那个返回结果的包装类，初始化的结果解析器就保存这里，处理思路和参数解析器一样的，

    public void handleReturnValue(Object returnValue, MethodParameter returnType, ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {
        HandlerMethodReturnValueHandler handler = this.getReturnValueHandler(returnType);
        Assert.notNull(handler, "Unknown return value type [" + returnType.getParameterType().getName() + "]");
        handler.handleReturnValue(returnValue, returnType, mavContainer, webRequest);
    }

    private HandlerMethodReturnValueHandler getReturnValueHandler(MethodParameter returnType) {
        Iterator i$ = this.returnValueHandlers.iterator();

        HandlerMethodReturnValueHandler returnValueHandler;
        do {
            if (!i$.hasNext()) {
                return null;
            }

            returnValueHandler = (HandlerMethodReturnValueHandler)i$.next();
            if (this.logger.isTraceEnabled()) {
                this.logger.trace("Testing if return value handler [" + returnValueHandler + "] supports [" + returnType.getGenericParameterType() + "]");
            }
        } while(!returnValueHandler.supportsReturnType(returnType));

        return returnValueHandler;
    }

遍历预置的所有结果解析器，结果解析器统一实现HandlerMethodReturnValueHandler 接口，实现supportReturnType方法：

这里我们距离用@ResponseBody注解的结果解析器RequestResponseBodyMethodProcessor 前面说了，参数和结果集他都实现了

    public boolean supportsReturnType(MethodParameter returnType) {
        return returnType.getMethodAnnotation(ResponseBody.class) != null; //判断是否有@ResponseBody注解
    }
    public void handleReturnValue(Object returnValue, MethodParameter returnType, ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws IOException, HttpMediaTypeNotAcceptableException {
        mavContainer.setRequestHandled(true);
        if (returnValue != null) {
            this.writeWithMessageConverters(returnValue, returnType, webRequest); //用内置的消息转换器来转换结果集
        }

    }

这里可能有人会问，消息转换器什么时候加载的？是在RequestMappingHandlerAdapter这个bean实例化的时候加载的，同时加载参数和结果解析器时候注入到解析器当中的

    public RequestMappingHandlerAdapter() { //无参构造函数中初始化
        StringHttpMessageConverter stringHttpMessageConverter = new StringHttpMessageConverter();
        stringHttpMessageConverter.setWriteAcceptCharset(false);
        this.messageConverters = new ArrayList();
        this.messageConverters.add(new ByteArrayHttpMessageConverter());
        this.messageConverters.add(stringHttpMessageConverter);
        this.messageConverters.add(new SourceHttpMessageConverter());
        this.messageConverters.add(new AllEncompassingFormHttpMessageConverter());
    }
    private List<HandlerMethodReturnValueHandler> getDefaultReturnValueHandlers() { //构造参数解析器时候，注入进去
        List<HandlerMethodReturnValueHandler> handlers = new ArrayList();
        handlers.add(new ModelAndViewMethodReturnValueHandler());
        handlers.add(new ModelMethodProcessor());
        handlers.add(new ViewMethodReturnValueHandler());
        handlers.add(new HttpEntityMethodProcessor(this.getMessageConverters(), this.contentNegotiationManager));
        handlers.add(new CallableMethodReturnValueHandler());
        handlers.add(new DeferredResultMethodReturnValueHandler());
        handlers.add(new AsyncTaskMethodReturnValueHandler(this.beanFactory));
        handlers.add(new ModelAttributeMethodProcessor(false));
        handlers.add(new RequestResponseBodyMethodProcessor(this.getMessageConverters(), this.contentNegotiationManager));
        handlers.add(new ViewNameMethodReturnValueHandler());
        handlers.add(new MapMethodProcessor());
        if (this.getCustomReturnValueHandlers() != null) {
            handlers.addAll(this.getCustomReturnValueHandlers());
        }

        if (!CollectionUtils.isEmpty(this.getModelAndViewResolvers())) {
            handlers.add(new ModelAndViewResolverMethodReturnValueHandler(this.getModelAndViewResolvers()));
        } else {
            handlers.add(new ModelAttributeMethodProcessor(true));
        }

        return handlers;
    }

下面来看是怎么寻找可以合适的消息转换器的

    protected <T> void writeWithMessageConverters(T returnValue, MethodParameter returnType, NativeWebRequest webRequest) throws IOException, HttpMediaTypeNotAcceptableException {
        ServletServerHttpRequest inputMessage = this.createInputMessage(webRequest);
        ServletServerHttpResponse outputMessage = this.createOutputMessage(webRequest);
        this.writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);
    }

    protected <T> void writeWithMessageConverters(T returnValue, MethodParameter returnType, ServletServerHttpRequest inputMessage, ServletServerHttpResponse outputMessage) throws IOException, HttpMediaTypeNotAcceptableException {
        Class<?> returnValueClass = returnValue.getClass();
        HttpServletRequest servletRequest = inputMessage.getServletRequest();
        List<MediaType> requestedMediaTypes = this.getAcceptableMediaTypes(servletRequest); //获取请求的MediaType,eg："application/json"
        List<MediaType> producibleMediaTypes = this.getProducibleMediaTypes(servletRequest, returnValueClass); //寻找支持这个返回类型的转换器支持的MediaTyoe
        Set<MediaType> compatibleMediaTypes = new LinkedHashSet();
        Iterator i$ = requestedMediaTypes.iterator();
　　　　 //双循环两个list，进行匹配，把复核条件的MediaType放到compatibleMediaTypes中 //TODO有些不懂得是为啥这块要过滤一遍，  后面实现了 父类也做了判断每个字类是否支持MediaType了？？
        MediaType selectedMediaType;
        Iterator i$;
        MediaType mediaType;
        while(i$.hasNext()) {
            selectedMediaType = (MediaType)i$.next();
            i$ = producibleMediaTypes.iterator();

            while(i$.hasNext()) {
                mediaType = (MediaType)i$.next();
                if (selectedMediaType.isCompatibleWith(mediaType)) {
                    compatibleMediaTypes.add(this.getMostSpecificMediaType(selectedMediaType, mediaType));
                }
            }
        }

        if (compatibleMediaTypes.isEmpty()) {
            throw new HttpMediaTypeNotAcceptableException(this.allSupportedMediaTypes);
        } else {
            List<MediaType> mediaTypes = new ArrayList(compatibleMediaTypes);
            MediaType.sortBySpecificityAndQuality(mediaTypes);
            selectedMediaType = null;
            i$ = mediaTypes.iterator();
            //排序之后，选择适合的MediaType
            while(i$.hasNext()) {
                mediaType = (MediaType)i$.next();
                if (mediaType.isConcrete()) {
                    selectedMediaType = mediaType;
                    break;
                }

                if (mediaType.equals(MediaType.ALL) || mediaType.equals(MEDIA_TYPE_APPLICATION)) {
                    selectedMediaType = MediaType.APPLICATION_OCTET_STREAM;
                    break;
                }
            }

            if (selectedMediaType != null) {
                selectedMediaType = selectedMediaType.removeQualityValue();
                i$ = this.messageConverters.iterator();
                //遍历所有消息转换器，canWrite是接口方法，相当于前面的support等，模式都是一个。然后满足的进行write。输出结果。
                while(i$.hasNext()) {
                    HttpMessageConverter<?> messageConverter = (HttpMessageConverter)i$.next();
                    if (messageConverter.canWrite(returnValueClass, selectedMediaType)) {
                        messageConverter.write(returnValue, selectedMediaType, outputMessage);
                        if (this.logger.isDebugEnabled()) {
                            this.logger.debug("Written [" + returnValue + "] as "" + selectedMediaType + "" using [" + messageConverter + "]");
                        }

                        return;
                    }
                }
            }

            throw new HttpMediaTypeNotAcceptableException(this.allSupportedMediaTypes);
        }
    }

下面介绍下，@ResponseBode标签用的消息转换器是MappingJacksonHttpMessageConverter；先看下类图吧

MappingJacksonHttpMessageConverter重写了父类的write方法：

    public boolean canWrite(Class<?> clazz, MediaType mediaType) {
        return this.objectMapper.canSerialize(clazz) && this.canWrite(mediaType); //canWrite(mediaType)是父类实现的
    }

    protected boolean canWrite(MediaType mediaType) {
        if (mediaType != null && !MediaType.ALL.equals(mediaType)) {
            Iterator i$ = this.getSupportedMediaTypes().iterator(); //获取子类解析器支持的MediaType，看下是否支持

            MediaType supportedMediaType;
            do {
                if (!i$.hasNext()) {
                    return false;
                }

                supportedMediaType = (MediaType)i$.next();
            } while(!supportedMediaType.isCompatibleWith(mediaType));

            return true;
        } else {
            return true;
        }
    }

write方法 父类也帮着实现了，父类具体做了如输出，拼凑输出流头等信息

    public final void write(T t, MediaType contentType, HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException {
        HttpHeaders headers = outputMessage.getHeaders();
        if (headers.getContentType() == null) {
            if (contentType == null || contentType.isWildcardType() || contentType.isWildcardSubtype()) {
                contentType = this.getDefaultContentType(t);
            }

            if (contentType != null) {
                headers.setContentType(contentType);
            }
        }

        if (headers.getContentLength() == -1L) {
            Long contentLength = this.getContentLength(t, headers.getContentType());
            if (contentLength != null) {
                headers.setContentLength(contentLength);
            }
        }

        this.writeInternal(t, outputMessage); //钩子方法，让子类去实现
        outputMessage.getBody().flush();
    }

    protected void writeInternal(Object object, HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException {
        JsonEncoding encoding = this.getJsonEncoding(outputMessage.getHeaders().getContentType());
        JsonGenerator jsonGenerator = this.objectMapper.getJsonFactory().createJsonGenerator(outputMessage.getBody(), encoding);
        if (this.objectMapper.getSerializationConfig().isEnabled(Feature.INDENT_OUTPUT)) {
            jsonGenerator.useDefaultPrettyPrinter();
        }
        //这块就是默认用Jackson进行翻译结果集了
        try {
            if (this.prefixJson) {
                jsonGenerator.writeRaw("{} && ");
            }

            this.objectMapper.writeValue(jsonGenerator, object);
        } catch (JsonProcessingException var6) {
            throw new HttpMessageNotWritableException("Could not write JSON: " + var6.getMessage(), var6);
        }
    }

因为用@ResponseBody不需要返回视图，所以视图那块就返回Null,不需要渲染视图了