這次的議題是返回json和返回普通view經過的路線差異。

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org.springframework.web.servlet.DispatcherServlet#doDispatch

    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
 
 || mappedHandler.getHandler() == 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(request, mv);
                mappedHandler.applyPostHandle(processedRequest, response, mv);
            }
            catch (Exception ex) {
                dispatchException = ex;
            }
            processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
        }
        catch (Exception ex) {
            triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
        }
        catch (Error err) {
            triggerAfterCompletionWithError(processedRequest, response, mappedHandler, 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);
                }
            }
        }
    }

1 view的設定

org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter#invokeHandleMethod

    private ModelAndView invokeHandleMethod(HttpServletRequest request,
            HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {

        ServletWebRequest webRequest = new ServletWebRequest(request, response);

        WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
        ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);
        ServletInvocableHandlerMethod requestMappingMethod = createRequestMappingMethod(handlerMethod, binderFactory);

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

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

        final 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 + "]");
            }
            requestMappingMethod = requestMappingMethod.wrapConcurrentResult(result);
        }

        requestMappingMethod.invokeAndHandle(webRequest, mavContainer); // 請求並設定mavContainer

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

        return getModelAndView(mavContainer, modelFactory, webRequest);
    }

org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite#getReturnValueHandler

    private HandlerMethodReturnValueHandler getReturnValueHandler(MethodParameter returnType) {
        for (HandlerMethodReturnValueHandler returnValueHandler : returnValueHandlers) {
            if (logger.isTraceEnabled()) {
                logger.trace("Testing if return value handler [" + returnValueHandler + "] supports [" +
                        returnType.getGenericParameterType() + "]");
            }
            if (returnValueHandler.supportsReturnType(returnType)) { // 遍歷獲取合適的returnValueHandler
                return returnValueHandler;
            }
        }
        return null;
    }

1.1 根據viewName來解析的Handler

org.springframework.web.servlet.mvc.method.annotation.ViewNameMethodReturnValueHandler#supportsReturnType

    public boolean supportsReturnType(MethodParameter returnType) {
        Class<?> paramType = returnType.getParameterType();
        return (void.class.equals(paramType) || String.class.equals(paramType));
    }

org.springframework.web.servlet.mvc.method.annotation.ViewNameMethodReturnValueHandler#handleReturnValue

    public void handleReturnValue(
            Object returnValue, MethodParameter returnType,
            ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
            throws Exception {

        if (returnValue == null) {
            return;
        }
        else if (returnValue instanceof String) {
            String viewName = (String) returnValue;
            mavContainer.setViewName(viewName);
            if (isRedirectViewName(viewName)) {
                mavContainer.setRedirectModelScenario(true);
            }
        }
        else {
            // should not happen
            throw new UnsupportedOperationException("Unexpected return type: " +
                    returnType.getParameterType().getName() + " in method: " + returnType.getMethod());
        }
    }

1.2 支援ResponseBody的Handler

org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor#supportsReturnType

    public boolean supportsReturnType(MethodParameter returnType) {
        return ((AnnotationUtils.findAnnotation(returnType.getContainingClass(), ResponseBody.class) != null) ||
                (returnType.getMethodAnnotation(ResponseBody.class) != null));
    }

org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor#handleReturnValue

    public void handleReturnValue(Object returnValue, MethodParameter returnType,
            ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
            throws IOException, HttpMediaTypeNotAcceptableException {

        mavContainer.setRequestHandled(true);
        if (returnValue != null) {
            writeWithMessageConverters(returnValue, returnType, webRequest);
        }
    }

org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor#writeWithMessageConverters(T, org.springframework.core.MethodParameter, org.springframework.http.server.ServletServerHttpRequest, org.springframework.http.server.ServletServerHttpResponse)

    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 = getAcceptableMediaTypes(servletRequest);
        List<MediaType> producibleMediaTypes = getProducibleMediaTypes(servletRequest, returnValueClass);

        Set<MediaType> compatibleMediaTypes = new LinkedHashSet<MediaType>();
        for (MediaType requestedType : requestedMediaTypes) {
            for (MediaType producibleType : producibleMediaTypes) {
                if (requestedType.isCompatibleWith(producibleType)) {
                    compatibleMediaTypes.add(getMostSpecificMediaType(requestedType, producibleType));
                }
            }
        }
        if (compatibleMediaTypes.isEmpty()) {
            throw new HttpMediaTypeNotAcceptableException(producibleMediaTypes);
        }

        List<MediaType> mediaTypes = new ArrayList<MediaType>(compatibleMediaTypes);
        MediaType.sortBySpecificityAndQuality(mediaTypes);

        MediaType selectedMediaType = null;
        for (MediaType mediaType : mediaTypes) {
            if (mediaType.isConcrete()) {
                selectedMediaType = mediaType;
                break;
            }
            else if (mediaType.equals(MediaType.ALL) || mediaType.equals(MEDIA_TYPE_APPLICATION)) {
                selectedMediaType = MediaType.APPLICATION_OCTET_STREAM;
                break;
            }
        }

        if (selectedMediaType != null) {
            selectedMediaType = selectedMediaType.removeQualityValue();
            for (HttpMessageConverter<?> messageConverter : this.messageConverters) { // 遍歷獲取合適的messageConverter（mediaType）
                if (messageConverter.canWrite(returnValueClass, selectedMediaType)) {
                    returnValue = this.adviceChain.invoke(returnValue, returnType, selectedMediaType,
                            (Class<HttpMessageConverter<?>>) messageConverter.getClass(), inputMessage, outputMessage);
                    ((HttpMessageConverter<T>) messageConverter).write(returnValue, selectedMediaType, outputMessage);
                    if (logger.isDebugEnabled()) {
                        logger.debug("Written [" + returnValue + "] as \"" + selectedMediaType + "\" using [" +
                                messageConverter + "]");
                    }
                    return;
                }
            }
        }
        throw new HttpMediaTypeNotAcceptableException(this.allSupportedMediaTypes);
    }

2.設定了viewName的view解析

org.springframework.web.servlet.DispatcherServlet#processDispatchResult

    private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
            HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) throws Exception {

        boolean errorView = false;

        if (exception != null) {
            if (exception instanceof ModelAndViewDefiningException) {
                logger.debug("ModelAndViewDefiningException encountered", exception);
                mv = ((ModelAndViewDefiningException) exception).getModelAndView();
            }
            else {
                Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
                mv = processHandlerException(request, response, handler, exception);
                errorView = (mv != null);
            }
        }

        // Did the handler return a view to render?
        if (mv != null && !mv.wasCleared()) { // 如果沒有設定view則不會進入viewResolver，返回json物件就不進入viewResolver
            render(mv, request, response);
            if (errorView) {
                WebUtils.clearErrorRequestAttributes(request);
            }
        }
        else {
            if (logger.isDebugEnabled()) {
                logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + getServletName() +
                        "': assuming HandlerAdapter completed request handling");
            }
        }

        if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
            // Concurrent handling started during a forward
            return;
        }

        if (mappedHandler != null) {
            mappedHandler.triggerAfterCompletion(request, response, null);
        }
    }

org.springframework.web.servlet.DispatcherServlet#resolveViewName

    protected View resolveViewName(String viewName, Map<String, Object> model, Locale locale,
            HttpServletRequest request) throws Exception {

        for (ViewResolver viewResolver : this.viewResolvers) { // 遍歷獲取合適的viewResolver
            View view = viewResolver.resolveViewName(viewName, locale);
            if (view != null) {
                return view;
            }
        }
        return null;
    }