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@ResponseBody注解原理 SpringMVC注解之@ResponseBody注解原理

永远做一个善良的人 人气:0
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一、介绍

二、作用范围

通过 @RestController 注解实现,此时所有的方法都将会被添加 @ResponseBody 注解

三、源码分析

具体为何调用了以下方法可以看我的另一篇文章。SpringMVC 执行流程解析

ServletInvocableHandlerMethod # invokeAndHandle

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()) {
			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 {
		// 处理返回值
		this.returnValueHandlers.handleReturnValue(
				returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
	}
	catch (Exception ex) {
		if (logger.isTraceEnabled()) {
			logger.trace(formatErrorForReturnValue(returnValue), ex);
		}
		throw ex;
	}
}

该方法中调用了 handleReturnValue() 方法去处理返回值。SpringMVC 中使用 RequestResponseBodyMethodProcessor 类来处理 @ResponseBody 标记的方法

RequestResponseBodyMethodProcessor # handleReturnValue

public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
			ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
			throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {
	// 设置请求已经被完全处理了,则后面不再做视图解析
	// 后面我还会提到的
	mavContainer.setRequestHandled(true);
	ServletServerHttpRequest inputMessage = createInputMessage(webRequest);
	ServletServerHttpResponse outputMessage = createOutputMessage(webRequest);

	writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);
}

该方法中通过 mavContainer.setRequestHandled(true); 设置请求已经被完全处理了,则后面不再做视图解析。然后调用了 writeWithMessageConverters() 方法。

protected <T> void writeWithMessageConverters(@Nullable T value, MethodParameter returnType,
			ServletServerHttpRequest inputMessage, ServletServerHttpResponse outputMessage)
			throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {
	// 存储响应体的信息
	Object body;
	// 返回值类型
	Class<?> valueType;
	// 目标类型
	Type targetType;

	// 返回值类型是否是 CharSequence 
	// 是则将 返回值类型和目标类型设置为 String.class
	if (value instanceof CharSequence) {
		body = value.toString();
		valueType = String.class;
		targetType = String.class;
	}
	// 不是 CharSequence 类型,一般是我们的自定义类
	else {
		body = value;
		valueType = getReturnValueType(body, returnType);
		targetType = GenericTypeResolver.resolveType(getGenericType(returnType), returnType.getContainingClass());
	}
	
	// 返回值类型是否是实现了 Resource 接口的资源
	// 这里我就不分析了
	if (isResourceType(value, returnType)) {
		outputMessage.getHeaders().set(HttpHeaders.ACCEPT_RANGES, "bytes");
		if (value != null && inputMessage.getHeaders().getFirst(HttpHeaders.RANGE) != null &&
				outputMessage.getServletResponse().getStatus() == 200) {
			Resource resource = (Resource) value;
			try {
				List<HttpRange> httpRanges = inputMessage.getHeaders().getRange();
				outputMessage.getServletResponse().setStatus(HttpStatus.PARTIAL_CONTENT.value());
				body = HttpRange.toResourceRegions(httpRanges, resource);
				valueType = body.getClass();
				targetType = RESOURCE_REGION_LIST_TYPE;
			}
			catch (IllegalArgumentException ex) {
				outputMessage.getHeaders().set(HttpHeaders.CONTENT_RANGE, "bytes */" + resource.contentLength());
				outputMessage.getServletResponse().setStatus(HttpStatus.REQUESTED_RANGE_NOT_SATISFIABLE.value());
			}
		}
	}

	// 选中的媒体类型
	MediaType selectedMediaType = null;
	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 {
		HttpServletRequest request = inputMessage.getServletRequest();
		// 可接受的媒体类型
		List<MediaType> acceptableTypes = getAcceptableMediaTypes(request);
		// 可产生的媒体类型
		List<MediaType> producibleTypes = getProducibleMediaTypes(request, valueType, targetType);

		if (body != null && producibleTypes.isEmpty()) {
			throw new HttpMessageNotWritableException(
					"No converter found for return value of type: " + valueType);
		}
		// 将要被使用的媒体类型
		List<MediaType> mediaTypesToUse = new ArrayList<>();
		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);

		for (MediaType mediaType : mediaTypesToUse) {
			// 该媒体类型是否是具体的
			// 也就是不包含类似于 * 这样的通配符
			if (mediaType.isConcrete()) {
				// 选中要使用的媒体类型
				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();
		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)) {
				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) {
						// 使用类型转换器将请求写入到 response body 中
						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);
	}
}

该方法中通过调用 getAcceptableMediaTypes() 方法获取到 acceptableTypes,getProducibleMediaTypes() 方法获取到 producibleTypes,然后调用 isCompatibleWith() 方法比较 acceptableTypes 和 producibleTypes,获取到两者都兼容的类型。最后通过调用 isConcrete() 获取到一个具体使用的媒体类型。

AbstractMessageConverterMethodProcessor # getProducibleMediaTypes

protected List<MediaType> getProducibleMediaTypes(
			HttpServletRequest request, Class<?> valueClass, @Nullable Type targetType) {

	Set<MediaType> mediaTypes =
			(Set<MediaType>) request.getAttribute(HandlerMapping.PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE);
	if (!CollectionUtils.isEmpty(mediaTypes)) {
		return new ArrayList<>(mediaTypes);
	}
	else if (!this.allSupportedMediaTypes.isEmpty()) {
		List<MediaType> result = new ArrayList<>();
		// 遍历类型转化器,获取支持的媒体类型
		for (HttpMessageConverter<?> converter : this.messageConverters) {
			if (converter instanceof GenericHttpMessageConverter && targetType != null) {
				if (((GenericHttpMessageConverter<?>) converter).canWrite(targetType, valueClass, null)) {
					result.addAll(converter.getSupportedMediaTypes());
				}
			}
			else if (converter.canWrite(valueClass, null)) {
				result.addAll(converter.getSupportedMediaTypes());
			}
		}
		return result;
	}
	else {
		return Collections.singletonList(MediaType.ALL);
	}
}

该方法中通过遍历类型转换器,根据类型转换器获取到支持的媒体类型。常见的类型转化器有 StringHttpMessageConverter 支持转换为 String 类型,MappingJackson2HttpMessageConverter 支持转换为 json 类型,MappingJackson2XmlHttpMessageConverter 支持转换为 XML 类型。
以转换为 JSON 数据为例。我们最终选择的媒体类型就是 “application/json” ,然后调用 AbstractGenericHttpMessageConverter # write 方法将数据写入到 response body 中。

AbstractGenericHttpMessageConverter # write

public final void write(final T t, @Nullable final Type type, @Nullable MediaType contentType,
			HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException {

	final HttpHeaders headers = outputMessage.getHeaders();
	// 添加响应头
	// 设置 Content-Type 为	application/json
	addDefaultHeaders(headers, t, contentType);

	if (outputMessage instanceof StreamingHttpOutputMessage) {
		StreamingHttpOutputMessage streamingOutputMessage = (StreamingHttpOutputMessage) outputMessage;
		streamingOutputMessage.setBody(outputStream -> writeInternal(t, type, new HttpOutputMessage() {
			@Override
			public OutputStream getBody() {
				return outputStream;
			}
			@Override
			public HttpHeaders getHeaders() {
				return headers;
			}
		}));
	}
	else {
		// 写入数据到 response body 中
		writeInternal(t, type, outputMessage);
		outputMessage.getBody().flush();
	}
}

该方法中设置了响应头的 Content-Type 为 application/json,然后调用 writeInternal() 方法写数据

AbstractJackson2HttpMessageConverter # writeInternal

protected void writeInternal(Object object, @Nullable Type type, HttpOutputMessage outputMessage)
			throws IOException, HttpMessageNotWritableException {
	// 获取媒体类型为 application/json
	MediaType contentType = outputMessage.getHeaders().getContentType();
	// 获取 JSON 数据的编码为 UTF-8
	JsonEncoding encoding = getJsonEncoding(contentType);
	// 获取到 HttpServletResponse 的输出流对象
	// 用于将数据写入到 response body 中
	OutputStream outputStream = StreamUtils.nonClosing(outputMessage.getBody());
	// 生成 JSON 数据的类
	JsonGenerator generator = this.objectMapper.getFactory().createGenerator(outputStream, encoding);
	try {
		writePrefix(generator, object);

		Object value = object;
		Class<?> serializationView = null;
		FilterProvider filters = null;
		JavaType javaType = null;

		if (object instanceof MappingJacksonValue) {
			MappingJacksonValue container = (MappingJacksonValue) object;
			value = container.getValue();
			serializationView = container.getSerializationView();
			filters = container.getFilters();
		}
		if (type != null && TypeUtils.isAssignable(type, value.getClass())) {
			// 获取 java 类型,一般是我们自定义的类
			javaType = getJavaType(type, null);
		}
		// 用于操作可序列化对象的类
		// 我们自定义的类一定要实现 Serializable 接口,并设置 get/set 方法
		ObjectWriter objectWriter = (serializationView != null ?
				this.objectMapper.writerWithView(serializationView) : this.objectMapper.writer());
		if (filters != null) {
			objectWriter = objectWriter.with(filters);
		}
		if (javaType != null && javaType.isContainerType()) {
			objectWriter = objectWriter.forType(javaType);
		}
		SerializationConfig config = objectWriter.getConfig();
		if (contentType != null && contentType.isCompatibleWith(MediaType.TEXT_EVENT_STREAM) &&
				config.isEnabled(SerializationFeature.INDENT_OUTPUT)) {
			objectWriter = objectWriter.with(this.ssePrettyPrinter);
		}
		// 写入数据
		objectWriter.writeValue(generator, value);

		writeSuffix(generator, object);
		generator.flush();
		generator.close();
	}
	catch (InvalidDefinitionException ex) {
		throw new HttpMessageConversionException("Type definition error: " + ex.getType(), ex);
	}
	catch (JsonProcessingException ex) {
		throw new HttpMessageNotWritableException("Could not write JSON: " + ex.getOriginalMessage(), ex);
	}
}

该方法中通过调用 outputMessage.getBody() 方法获取到了 HttpServletResponse 的输出流对象,用于将数据输出到 response body 中。并设置了 JSON 数据的编码格式为 UTF-8,然后通过 ObjectWriter 对象操作我们自定义的可序列化的对象,将该对象转换为 JSON 格式输出到 response body 中。

AbstractJackson2HttpMessageConverter # getJsonEncoding

protected JsonEncoding getJsonEncoding(@Nullable MediaType contentType) {
	if (contentType != null && contentType.getCharset() != null) {
		Charset charset = contentType.getCharset();
		JsonEncoding encoding = ENCODINGS.get(charset.name());
		if (encoding != null) {
			return encoding;
		}
	}
	return JsonEncoding.UTF8;
}

设置 JSON 数据的编码格式为 UTF-8

ServletServerHttpResponse # getBody

public OutputStream getBody() throws IOException {
	this.bodyUsed = true;
	writeHeaders();
	return this.servletResponse.getOutputStream();
}

获取 HttpServletResponse 的输出流

到这里我们已经实现了将数据转化为 JSON 格式输出到 response body 中了。

还记得前面提到的 mavContainer.setRequestHandled(true) 这个方法吗,前面我说了调用了这个方法后,就不再做视图解析了,我们这里再具体分析一下。

RequestMappingHandlerAdapter # invokeHandlerMethod

protected ModelAndView invokeHandlerMethod(HttpServletRequest request,
			HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {

	...
	invocableMethod.invokeAndHandle(webRequest, mavContainer);
	...
	return getModelAndView(mavContainer, modelFactory, webRequest);
	... 
}

可以看到 getModelAndView() 方法是在 invokeAndHandle() 方法之后调用了,也就是在调用 getModelAndView() 方法前,我们已经调用了 mavContainer.setRequestHandled(true) 方法了。getModelAndView() 方法就是做视图解析的,我们来看一下该方法。

RequestMappingHandlerAdapter # getModelAndView

private ModelAndView getModelAndView(ModelAndViewContainer mavContainer,
			ModelFactory modelFactory, NativeWebRequest webRequest) throws Exception {

	modelFactory.updateModel(webRequest, mavContainer);
	// 是否已经完全处理了,若为 true,则直接返回 null
	//  mavContainer.setRequestHandled(true) 已设置为 true 了
	if (mavContainer.isRequestHandled()) {
		return null;
	}
	
	// 下面的代码是做视图解析
	ModelMap model = mavContainer.getModel();
	ModelAndView mav = new ModelAndView(mavContainer.getViewName(), model, mavContainer.getStatus());
	if (!mavContainer.isViewReference()) {
		mav.setView((View) mavContainer.getView());
	}
	if (model instanceof RedirectAttributes) {
		Map<String, ?> flashAttributes = ((RedirectAttributes) model).getFlashAttributes();
		HttpServletRequest request = webRequest.getNativeRequest(HttpServletRequest.class);
		if (request != null) {
			RequestContextUtils.getOutputFlashMap(request).putAll(flashAttributes);
		}
	}
	return mav;
}

可以看到,该方法一开始就调用了 mavContainer.isRequestHandled() 方法,如果为 true,则返回 null,并进行下面的视图解析。而 mavContainer.setRequestHandled(true) 方法已经将其设置为 true 了。这就是为什么加了 @ResponseBody 注解的方法不做视图解析的原因。

四、总结

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