[Java知识库]5 SpringBoot请求是如何返回各种值的

在SpringBoot中, 我们在请求方法标注 @ResponseBody, 便会给浏览器返回 json 数据

@GetMapping("/test/getPerson")
@ResponseBody
public Person getPerson() {
    return new Person(1, "Mahone");
}

那返回的原理是什么呢?

上篇说道, 在 invokeHandlerMethod 执行真正目标方法时, 会得到 returnValueHandlers(返回值处理器)

执行完请求逻辑后, 会获取目标方法的返回值

public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception {

    // returnValue - 目标方法返回值
    Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
    // 设置响应状态
    setResponseStatus(webRequest);

    if (returnValue == null) {
        if (isRequestNotModified(webRequest) || getResponseStatus() != null || mavContainer.isRequestHandled()) {
            disableContentCachingIfNecessary(webRequest);
            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;
    }
}

拿到返回值后, 判断是不是为空

不为空则判断是否为响应状态原因

不是的话, 则执行一下逻辑

this.returnValueHandlers.handleReturnValue(returnValue, getReturnValueType(returnValue), mavContainer, webRequest);

处理返回值:

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

    // 寻找哪个返回值处理器能处理此返回值, 寻找方式还是跟getHandler一样, 遍历匹配返回值处理器
    HandlerMethodReturnValueHandler handler = selecstHandler(returnValue, returnType);
    // 如果没有找到则抛出异常
    if (handler == null) {
        throw new IllegalArgumentException("Unknown return value type: " + returnType.getParameterType().getName());
    }
    handler.handleReturnValue(returnValue, returnType, mavContainer, webRequest);
}

selecstHandler:

@Nullable
private HandlerMethodReturnValueHandler selectHandler(@Nullable Object value, MethodParameter returnType) {
    // 判断是否为异步返回值
    boolean isAsyncValue = isAsyncReturnValue(value, returnType);
    for (HandlerMethodReturnValueHandler handler : this.returnValueHandlers) {
        if (isAsyncValue && !(handler instanceof AsyncHandlerMethodReturnValueHandler)) {
            continue;
        }
        
        // 当前返回值处理器是否支持返回类型
        if (handler.supportsReturnType(returnType)) {
            return handler;
        }
    }
    return null;
}

当获取到对应的返回值处理器后, 就会对返回值做对应的操作

那标注了 @ResponseBody 注解, 是如何给浏览器返回 Json 数据的呢?

@ResponseBody , 对应的返回值处理器就是 RequestResponseBodyMethodProcessor

来看看 RequestResponseBodyMethodProcessor 的 处理方法(handleReturnValue)

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

    // 进行初始化操作
    mavContainer.setRequestHandled(true);
    ServletServerHttpRequest inputMessage = createInputMessage(webRequest);
    ServletServerHttpResponse outputMessage = createOutputMessage(webRequest);

    // Try even with null return value. ResponseBodyAdvice could get involved.
    // 用消息转换器将返回值写出
    writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);
}

怎么写出的? 看看 writeWithMessageConverters 源码

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

    Object body;
    Class<?> valueType;
    Type targetType;

    if (value instanceof CharSequence) {
        body = value.toString();
        valueType = String.class;
        targetType = String.class;
    }
    else {
        body = value;
        valueType = getReturnValueType(body, returnType);
        targetType = GenericTypeResolver.resolveType(getGenericType(returnType), returnType.getContainingClass());
    }

    // 省略... 判断是不是IO流

    // 开始判断内容类型
    MediaType selectedMediaType = null;
    // 获取头信息的内容类型
    MediaType contentType = outputMessage.getHeaders().getContentType();
    boolean isContentTypePreset = contentType != null && contentType.isConcrete();
    if (isContentTypePreset) {
        if (logger.isDebugEnabled()) {
            logger.debug("Found 'Content-Type:" + contentType + "' in response");
        }
        selectedMediaType = contentType;
    }
    else {
        // 如果获取到null, 则进行获取浏览器允许发送的内容类型
        HttpServletRequest request = inputMessage.getServletRequest();
        // 获取到了浏览器允许发送的类型
        List<MediaType> acceptableTypes = getAcceptableMediaTypes(request);
        // 获取当前服务器能响应的类型, 图见下文
        List<MediaType> producibleTypes = getProducibleMediaTypes(request, valueType, targetType);

        if (body != null && producibleTypes.isEmpty()) {
            throw new HttpMessageNotWritableException(
                "No converter found for return value of type: " + valueType);
        }
        List<MediaType> mediaTypesToUse = new ArrayList<>();
        // 服务器响应类型跟浏览器接收类型进行匹配
        for (MediaType requestedType : acceptableTypes) {
            for (MediaType producibleType : producibleTypes) {
                if (requestedType.isCompatibleWith(producibleType)) {
                    mediaTypesToUse.add(getMostSpecificMediaType(requestedType, producibleType));
                }
            }
        }
        
        if (mediaTypesToUse.isEmpty()) {
            if (body != null) {
                throw new HttpMediaTypeNotAcceptableException(producibleTypes);
            }
            if (logger.isDebugEnabled()) {
                logger.debug("No match for " + acceptableTypes + ", supported: " + producibleTypes);
            }
            return;
        }

        MediaType.sortBySpecificityAndQuality(mediaTypesToUse);

        // 得到内容类型
        for (MediaType mediaType : mediaTypesToUse) {
            if (mediaType.isConcrete()) {
                selectedMediaType = mediaType;
                break;
            }
            else if (mediaType.isPresentIn(ALL_APPLICATION_MEDIA_TYPES)) {
                selectedMediaType = MediaType.APPLICATION_OCTET_STREAM;
                break;
            }
        }

        if (logger.isDebugEnabled()) {
            logger.debug("Using '" + selectedMediaType + "', given " +
                         acceptableTypes + " and supported " + producibleTypes);
        }
    }

    if (selectedMediaType != null) {
        selectedMediaType = selectedMediaType.removeQualityValue();
        
        // 开始遍历所有的消息转换器
        // 判断是否支持将 此 Class对象 转换为 内容类型对象
        for (HttpMessageConverter<?> converter : this.messageConverters) {
            GenericHttpMessageConverter genericConverter = (converter instanceof GenericHttpMessageConverter ?
                                                            (GenericHttpMessageConverter<?>) converter : null);
            
            // 此时能支持返回 Json 类型的消息转换器是 MappingJackson2CborHttpMessageConverter
            // 当之前的消息转换器都不支持的情况下, MappingJackson2CborHttpMessageConverter会返回true
            // 无论是任何类型, 都可以转化为 Json
            if (genericConverter != null ?
                ((GenericHttpMessageConverter) converter).canWrite(targetType, valueType, selectedMediaType) :
                converter.canWrite(valueType, selectedMediaType)) {
                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) {
                        // 将数据写出至浏览器
                        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;
            }
        }
    }

    if (body != null) {
        Set<MediaType> producibleMediaTypes =
            (Set<MediaType>) inputMessage.getServletRequest()
            .getAttribute(HandlerMapping.PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE);

        if (isContentTypePreset || !CollectionUtils.isEmpty(producibleMediaTypes)) {
            throw new HttpMessageNotWritableException(
                "No converter for [" + valueType + "] with preset Content-Type '" + contentType + "'");
        }
        throw new HttpMediaTypeNotAcceptableException(this.allSupportedMediaTypes);
    }
}

浏览器允许发送的内容类型

服务器能响应的内容类型

默认的消息转换器

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