@RestController
public class DemoController {
    @RequestMapping(value = "/api/v1/demo1/getdefault", method = RequestMethod.GET)
    public ResponseEntity<Demo1> getDefaultDemo1() {
        return new ResponseEntity<>(Demo1.defaultDemo1, HttpStatus.INTERNAL_SERVER_ERROR);
    }
    @RequestMapping(value = "/api/v1/demo2/getdefault", method = RequestMethod.GET)
    public Demo2 getDefaultDemo2() {
        return Demo2.defaultDemo2;
    }
}
public class Demo1 {
    private int id;
    private String name;
    public static Demo1 defaultDemo1 = new Demo1(1, "Admin");
    public Demo1() {}
    public Demo1(int id, String name) {		this.id = id;
		this.name = name;
    }
    // 省略getter和setter
}
public class Demo2 {
    private int id;
    private String desc;
    public static Demo2 defaultDemo2 = new Demo2(1, "Root");
    public Demo2() {}
    public Demo2(int id, String desc) {
		this.id = id;
        this.desc = desc;
    }
    // 省略getter和setter
}

public class ReturnResult<T> {
    private int statusCode;
    private T body;
    public ReturnResult() {}
    public ReturnResult(T body) {
        this.body = body;
    }
    // 省略getter和setter
}

@ControllerAdvice
public class ReturnResultAdvice implements ResponseBodyAdvice<Object> {
    @Override
    public boolean supports(@Nullable MethodParameter returnType, @Nullable Class converterType) {
        return true;
    }
    @Override
    public Object beforeBodyWrite(Object body, @Nullable MethodParameter returnType,
                                  @Nullable MediaType selectedContentType, @Nullable Class selectedConverterType,
                                  @Nullable ServerHttpRequest request, @Nullable ServerHttpResponse response) {
        if (body == null) {
            return null;
        }
        if (body instanceof ReturnResult) {
            return body;
        }
        return new ReturnResult<>(body);
    }
}

protected <T> void writeWithMessageConverters(@Nullable T value, MethodParameter returnType,
            ServletServerHttpRequest inputMessage, ServletServerHttpResponse outputMessage)
            throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException;

public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
        ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
        throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {
    ......
    writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);
}

public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
        ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {
    ......
    HttpEntity<?> responseEntity = (HttpEntity<?>) returnValue;
    ......
    writeWithMessageConverters(responseEntity.getBody(), returnType, inputMessage, outputMessage);
    ......
}

protected <T> void writeWithMessageConverters(@Nullable T value, MethodParameter returnType,
        ServletServerHttpRequest inputMessage, ServletServerHttpResponse outputMessage)
        throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {
    ......
    if (selectedMediaType != null) {
        selectedMediaType = selectedMediaType.removeQualityValue();
        for (HttpMessageConverter<?> converter : this.messageConverters) {
            GenericHttpMessageConverter genericConverter = (converter instanceof GenericHttpMessageConverter ?
                    (GenericHttpMessageConverter<?>) converter : null);
            if (genericConverter != null ?
                    ((GenericHttpMessageConverter) converter).canWrite(targetType, valueType, selectedMediaType) :
                    converter.canWrite(valueType, selectedMediaType)) {
                // ResponseBodyAdvice的調(diào)用發(fā)生在這里
                body = getAdvice().beforeBodyWrite(body, returnType, selectedMediaType,
                        (Class<? extends HttpMessageConverter<?>>) converter.getClass(),
                        inputMessage, outputMessage);
                if (body != null) {
                    Object theBody = body;
                    LogFormatUtils.traceDebug(logger, traceOn ->
                            "Writing [" + LogFormatUtils.formatValue(theBody, !traceOn) + "]");
                    addContentDispositionHeader(inputMessage, outputMessage);
                    if (genericConverter != null) {
                        genericConverter.write(body, targetType, selectedMediaType, outputMessage);
                    }
                    else {
                        ((HttpMessageConverter) converter).write(body, selectedMediaType, outputMessage);
                    }
                }
                else {
                    if (logger.isDebugEnabled()) {
                        logger.debug("Nothing to write: null body");
                    }
                }
                return;
            }
        }
    }
    ......
}

public Object beforeBodyWrite(@Nullable Object body, MethodParameter returnType, MediaType contentType,
        Class<? extends HttpMessageConverter<?>> converterType,
        ServerHttpRequest request, ServerHttpResponse response) {
    return processBody(body, returnType, contentType, converterType, request, response);
}
private <T> Object processBody(@Nullable Object body, MethodParameter returnType, MediaType contentType,
        Class<? extends HttpMessageConverter<?>> converterType,
        ServerHttpRequest request, ServerHttpResponse response) {
    // 從加載好的ResponseBodyAdvice中獲取適用于當(dāng)前handler的ResponseBodyAdvice
    for (ResponseBodyAdvice<?> advice : getMatchingAdvice(returnType, ResponseBodyAdvice.class)) {
        if (advice.supports(returnType, converterType)) {
            // 執(zhí)行ResponseBodyAdvice的beforeBodyWrite()方法以處理handler方法返回值
            body = ((ResponseBodyAdvice<T>) advice).beforeBodyWrite((T) body, returnType,
                    contentType, converterType, request, response);
        }
    }
    return body;
}
private <A> List<A> getMatchingAdvice(MethodParameter parameter, Class<? extends A> adviceType) {
    // 獲取ResponseBodyAdvice集合
    List<Object> availableAdvice = getAdvice(adviceType);
    if (CollectionUtils.isEmpty(availableAdvice)) {
        return Collections.emptyList();
    }
    List<A> result = new ArrayList<>(availableAdvice.size());
    for (Object advice : availableAdvice) {
        // 判斷ResponseBodyAdvice是否由@ControllerAdvice注解修飾
        if (advice instanceof ControllerAdviceBean) {
            ControllerAdviceBean adviceBean = (ControllerAdviceBean) advice;
            // 判斷ResponseBodyAdvice是否適用于當(dāng)前handler
            if (!adviceBean.isApplicableToBeanType(parameter.getContainingClass())) {
                continue;
            }
            advice = adviceBean.resolveBean();
        }
        if (adviceType.isAssignableFrom(advice.getClass())) {
            result.add((A) advice);
        }
    }
    return result;
}

public void afterPropertiesSet() {
    // 加載ControllerAdviceBean相關(guān)內(nèi)容（同時(shí)就會(huì)將由@ControllerAdvice注解修飾的ResponseBodyAdvice接口加載）
    initControllerAdviceCache();
    if (this.argumentResolvers == null) {
        List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();
        this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
    }
    if (this.initBinderArgumentResolvers == null) {
        List<HandlerMethodArgumentResolver> resolvers = getDefaultInitBinderArgumentResolvers();
        this.initBinderArgumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
    }
    if (this.returnValueHandlers == null) {
        // 獲取返回值處理器，在這里就會(huì)完成RequestResponseBodyMethodProcessor和HttpEntityMethodProcessor的初始化，初始化的同時(shí)就會(huì)完成ResponseBodyAdvice接口的加載
        List<HandlerMethodReturnValueHandler> handlers = getDefaultReturnValueHandlers();
        this.returnValueHandlers = new HandlerMethodReturnValueHandlerComposite().addHandlers(handlers);
    }
}

private void initControllerAdviceCache() {
    if (getApplicationContext() == null) {
        return;
    }
    // 獲取由@ControllerAdvice注解修飾的bean
    List<ControllerAdviceBean> adviceBeans = ControllerAdviceBean.findAnnotatedBeans(getApplicationContext());
    List<Object> requestResponseBodyAdviceBeans = new ArrayList<>();
    for (ControllerAdviceBean adviceBean : adviceBeans) {
        Class<?> beanType = adviceBean.getBeanType();
        if (beanType == null) {
            throw new IllegalStateException("Unresolvable type for ControllerAdviceBean: " + adviceBean);
        }
        Set<Method> attrMethods = MethodIntrospector.selectMethods(beanType, MODEL_ATTRIBUTE_METHODS);
        if (!attrMethods.isEmpty()) {
            this.modelAttributeAdviceCache.put(adviceBean, attrMethods);
        }
        Set<Method> binderMethods = MethodIntrospector.selectMethods(beanType, INIT_BINDER_METHODS);
        if (!binderMethods.isEmpty()) {
            this.initBinderAdviceCache.put(adviceBean, binderMethods);
        }
        // 如果ControllerAdviceBean實(shí)現(xiàn)了ResponseBodyAdvice接口，那么這個(gè)ControllerAdviceBean需要加載到requestResponseBodyAdvice中
        if (RequestBodyAdvice.class.isAssignableFrom(beanType) || ResponseBodyAdvice.class.isAssignableFrom(beanType)) {
            requestResponseBodyAdviceBeans.add(adviceBean);
        }
    }
    if (!requestResponseBodyAdviceBeans.isEmpty()) {
        this.requestResponseBodyAdvice.addAll(0, requestResponseBodyAdviceBeans);
    }
    ......
}

private List<HandlerMethodReturnValueHandler> getDefaultReturnValueHandlers() {
    List<HandlerMethodReturnValueHandler> handlers = new ArrayList<>(20);
    ......
    // 創(chuàng)建并加載HttpEntityMethodProcessor
    handlers.add(new HttpEntityMethodProcessor(getMessageConverters(),
            this.contentNegotiationManager, this.requestResponseBodyAdvice));
    ...
    // 創(chuàng)建并加載RequestResponseBodyMethodProcessor
    handlers.add(new RequestResponseBodyMethodProcessor(getMessageConverters(),
            this.contentNegotiationManager, this.requestResponseBodyAdvice));
    ......
    return handlers;
}

public HttpEntityMethodProcessor(List<HttpMessageConverter<?>> converters,
        @Nullable ContentNegotiationManager manager, List<Object> requestResponseBodyAdvice) {
    super(converters, manager, requestResponseBodyAdvice);
}

protected AbstractMessageConverterMethodProcessor(List<HttpMessageConverter<?>> converters,
        @Nullable ContentNegotiationManager manager, @Nullable List<Object> requestResponseBodyAdvice) {
    super(converters, requestResponseBodyAdvice);
    this.contentNegotiationManager = (manager != null ? manager : new ContentNegotiationManager());
    this.safeExtensions.addAll(this.contentNegotiationManager.getAllFileExtensions());
    this.safeExtensions.addAll(SAFE_EXTENSIONS);
}

public AbstractMessageConverterMethodArgumentResolver(List<HttpMessageConverter<?>> converters,
        @Nullable List<Object> requestResponseBodyAdvice) {
    Assert.notEmpty(converters, "'messageConverters' must not be empty");
    this.messageConverters = converters;
    this.allSupportedMediaTypes = getAllSupportedMediaTypes(converters);
    this.advice = new RequestResponseBodyAdviceChain(requestResponseBodyAdvice);
}