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。。。。

mappedHandler = getHandler(processedRequest);

if (mappedHandler == null) {

        noHandlerFound(processedRequest, response);

        return;

}

// Determine handler adapter for the current request.

HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

。。。。

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protected ModelAndView handleInternal(HttpServletRequest request,

                HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {

。。。

                // No synchronization on session demanded at all...

                mav = invokeHandlerMethod(request, response, handlerMethod);

        }

。。。

}

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protected ModelAndView invokeHandlerMethod(HttpServletRequest request,

                HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {

...

                ServletInvocableHandlerMethod invocableMethod = createInvocableHandlerMethod(handlerMethod);

                if (this.argumentResolvers != null) {

                        invocableMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);

                }

                if (this.returnValueHandlers != null) {

                        invocableMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);

                }

...

}

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public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,

                Object... providedArgs) throws Exception {

        Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);

        setResponseStatus(webRequest);

        if (returnValue == null) {

                if (isRequestNotModified(webRequest) || getResponseStatus() != null || mavContainer.isRequestHandled()) {

                        mavContainer.setRequestHandled(true);

                        return;

                }

        }

        else if (StringUtils.hasText(getResponseStatusReason())) {

                mavContainer.setRequestHandled(true);

                return;

        }

        mavContainer.setRequestHandled(false);

        Assert.state(this.returnValueHandlers != null, "No return value handlers");

        try {

                this.returnValueHandlers.handleReturnValue(

                                returnValue, getReturnValueType(returnValue), mavContainer, webRequest);

        }

        catch (Exception ex) {

                if (logger.isTraceEnabled()) {

                        logger.trace(formatErrorForReturnValue(returnValue), ex);

                }

                throw ex;

        }

}

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public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,

                ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {

        HandlerMethodReturnValueHandler handler = selectHandler(returnValue, returnType);

        if (handler == null) {

                throw new IllegalArgumentException("Unknown return value type: " + returnType.getParameterType().getName());

        }

        handler.handleReturnValue(returnValue, returnType, mavContainer, webRequest);

}

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public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,

                ModelAndViewContainer mavContainer, NativeWebRequest webRequest)

                throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {

        mavContainer.setRequestHandled(true);

        ServletServerHttpRequest inputMessage = createInputMessage(webRequest);

        ServletServerHttpResponse outputMessage = createOutputMessage(webRequest);

        // Try even with null return value. ResponseBodyAdvice could get involved.

        writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);

}

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protected <T> void writeWithMessageConverters(@Nullable T value, MethodParameter returnType,

                ServletServerHttpRequest inputMessage, ServletServerHttpResponse outputMessage)

                throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {

1... select mediaType   "application/json"

        if (selectedMediaType != null) {

                2 . ....  convert

                                        if (genericConverter != null) {

                                                genericConverter.write(body, targetType, selectedMediaType, outputMessage);

                                        }

                                        else {

                                                ((HttpMessageConverter) converter).write(body, selectedMediaType, outputMessage);

                                        }

                                }

                ...

}