[Java知识库]springboot-方法处理3-返回值处理器

1.返回值处理器简介

上一篇章稍微研究了参数解析器(HandlerMethodArgumentResolver)，本篇稍微研究返回值处理器(HandlerMethodReturnValueHandler)。返回值处理器主要用于根据目标方法的返回类型描述(比如参数类型/方法的注解)，把返回值写入到各种http对象中(比如response/session)；

1.1.类结构

1.2.功能说明(重点)

名称以ReturnValueHandler结尾的是单一的返回值处理器，名称以MethodProcessor结尾的既是参数解析器，也是返回值处理器；

2.返回值处理部分源码

2.1.把返回值写入body

扩展把目标方法的返回值写入body的功能(复杂的逻辑是处理各种类型响应数据的http头信息)。所有把目标方法的返回值写入body的返回值处理器都是它的子类。具体过程如下：

• 获取安全的文件扩展名(默认为请求header中Accept配置的文件扩展名+常见的安全文件扩展名)；
• 获取返回值的class对象和具体类型(如果返回值是泛型，则获取具体的泛型类型)；
• 如果是资源文件支持分段下载；
• 获取合适的MediaType。如果找不到 & 返回值不为空，则报错；
• 根据MediaType和返回类型的class对象找到支持的消息转换器。如果找不到 & 返回值不为空，则报错；
• 调用RequestResponseBodyAdviceChain.beforeBodyWrite写入之前进行预处理；
• 添加http header头信息；
• 调用HttpMessageConverter把目标方法的返回值写入body；

public abstract class AbstractMessageConverterMethodProcessor extends AbstractMessageConverterMethodArgumentResolver
		implements HandlerMethodReturnValueHandler {

	//**常见的安全文件扩展名
	private static final Set<String> SAFE_EXTENSIONS = new HashSet<>(Arrays.asList(
			"txt", "text", "yml", "properties", "csv",
			"json", "xml", "atom", "rss",
			"png", "jpe", "jpeg", "jpg", "gif", "wbmp", "bmp"));
    
    //**安全的文件类型
	private static final Set<String> SAFE_MEDIA_BASE_TYPES = new HashSet<>(
			Arrays.asList("audio", "image", "video"));
    
    //**表示所有MediaType的常量
	private static final List<MediaType> ALL_APPLICATION_MEDIA_TYPES =
			Arrays.asList(MediaType.ALL, new MediaType("application"));

	private static final Type RESOURCE_REGION_LIST_TYPE =
			new ParameterizedTypeReference<List<ResourceRegion>>() { }.getType();


	private final ContentNegotiationManager contentNegotiationManager;
    
    //**安全的文件扩展名
	private final Set<String> safeExtensions = new HashSet<>();

	//**创建实例时，获取安全的文件扩展名(默认为请求header中Accept配置的文件扩展名+常见的安全文件扩展名)
	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);
	}

    //**根据NativeWebRequest创建ServletServerHttpResponse
	protected ServletServerHttpResponse createOutputMessage(NativeWebRequest webRequest) {
		HttpServletResponse response = webRequest.getNativeResponse(HttpServletResponse.class);
		Assert.state(response != null, "No HttpServletResponse");
		return new ServletServerHttpResponse(response);
	}

	//**把目标方法的返回值写入ServletServerHttpResponse
	protected <T> void writeWithMessageConverters(T value, MethodParameter returnType, NativeWebRequest webRequest)
			throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {

		ServletServerHttpRequest inputMessage = createInputMessage(webRequest);
		ServletServerHttpResponse outputMessage = createOutputMessage(webRequest);
		writeWithMessageConverters(value, returnType, inputMessage, outputMessage);
	}

	//**把目标方法的返回值写入ServletServerHttpResponse
	@SuppressWarnings({"rawtypes", "unchecked"})
	protected <T> void writeWithMessageConverters(@Nullable T value, MethodParameter returnType,
			ServletServerHttpRequest inputMessage, ServletServerHttpResponse outputMessage)
			throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {

		Object body;  //**body内容
		Class<?> valueType; //**返回值的class对象
		Type targetType;    //**返回值类型(如果返回值是泛型，则获取具体的泛型类型)
        
		if (value instanceof CharSequence) { //**如果返回值是字符串，返回值的类型为String
			body = value.toString();
			valueType = String.class;
			targetType = String.class;
		}
		else { //**否则，获取返回值的类型
			body = value;
			valueType = getReturnValueType(body, returnType);
			targetType = GenericTypeResolver.resolveType(getGenericType(returnType), returnType.getContainingClass());
		}
        
        /**
         * 资源文件支持分段下载
         * 请求头信息：Range: bytes=100-200
         * 响应头信息：Accept-Ranges: bytes
         * 响应状态码：206
         */
		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());
				}
			}
		}
        
        //**获取合适的MediaType
		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();
			List<MediaType> acceptableTypes;
			try {
				acceptableTypes = getAcceptableMediaTypes(request);
			}
			catch (HttpMediaTypeNotAcceptableException ex) {
				int series = outputMessage.getServletResponse().getStatus() / 100;
				if (body == null || series == 4 || series == 5) {
					if (logger.isDebugEnabled()) {
						logger.debug("Ignoring error response content (if any). " + ex);
					}
					return;
				}
				throw ex;
			}
			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<>();
			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) {
				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);
			}
		}
        
        /**
         * 如果获取到了正确的MediaType & HttpMessageConverter能处理目标方法的返回值
         * 1.调用RequestResponseBodyAdviceChain.beforeBodyWrite写入之前进行预处理
         * 2.添加头信息
         * 3.调用HttpMessageConverter把目标方法的返回值写入body
         */
		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) {
							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(getSupportedMediaTypes(body.getClass()));
		}
	}

	//**获取目标方法返回值的类型
	protected Class<?> getReturnValueType(@Nullable Object value, MethodParameter returnType) {
		return (value != null ? value.getClass() : returnType.getParameterType());
	}

	//目标方法返回值是否是Resource类型
	protected boolean isResourceType(@Nullable Object value, MethodParameter returnType) {
		Class<?> clazz = getReturnValueType(value, returnType);
		return clazz != InputStreamResource.class && Resource.class.isAssignableFrom(clazz);
	}

	//**根据目标方法返回类型获取MediaType列表
	private Type getGenericType(MethodParameter returnType) {
		if (HttpEntity.class.isAssignableFrom(returnType.getParameterType())) {
			return ResolvableType.forType(returnType.getGenericParameterType()).getGeneric().getType();
		}
		else {
			return returnType.getGenericParameterType();
		}
	}

	//**根据目标方法返回类型获取MediaType列表
	@SuppressWarnings("unused")
	protected List<MediaType> getProducibleMediaTypes(HttpServletRequest request, Class<?> valueClass) {
		return getProducibleMediaTypes(request, valueClass, null);
	}

	//**获取目标方法返回值的MediaType
	@SuppressWarnings("unchecked")
	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);
		}
		List<MediaType> result = new ArrayList<>();
		for (HttpMessageConverter<?> converter : this.messageConverters) {
			if (converter instanceof GenericHttpMessageConverter && targetType != null) {
				if (((GenericHttpMessageConverter<?>) converter).canWrite(targetType, valueClass, null)) {
					result.addAll(converter.getSupportedMediaTypes(valueClass));
				}
			}
			else if (converter.canWrite(valueClass, null)) {
				result.addAll(converter.getSupportedMediaTypes(valueClass));
			}
		}
		return (result.isEmpty() ? Collections.singletonList(MediaType.ALL) : result);
	}

	private List<MediaType> getAcceptableMediaTypes(HttpServletRequest request)
			throws HttpMediaTypeNotAcceptableException {

		return this.contentNegotiationManager.resolveMediaTypes(new ServletWebRequest(request));
	}

	//**获取具体的MediaType
	private MediaType getMostSpecificMediaType(MediaType acceptType, MediaType produceType) {
		MediaType produceTypeToUse = produceType.copyQualityValue(acceptType);
		return (MediaType.SPECIFICITY_COMPARATOR.compare(acceptType, produceTypeToUse) <= 0 ? acceptType : produceTypeToUse);
	}

	//**如果是不安全的文件扩展名 & 状态码为2xx，则header添加("Content-disposition", "inline;filename=f.txt");
	private void addContentDispositionHeader(ServletServerHttpRequest request, ServletServerHttpResponse response) {
		HttpHeaders headers = response.getHeaders();
		if (headers.containsKey(HttpHeaders.CONTENT_DISPOSITION)) {
			return;
		}

		try {
			int status = response.getServletResponse().getStatus();
			if (status < 200 || (status > 299 && status < 400)) {
				return;
			}
		}
		catch (Throwable ex) {
			// ignore
		}

		HttpServletRequest servletRequest = request.getServletRequest();
		String requestUri = UrlPathHelper.rawPathInstance.getOriginatingRequestUri(servletRequest);

		int index = requestUri.lastIndexOf('/') + 1;
		String filename = requestUri.substring(index);
		String pathParams = "";

		index = filename.indexOf(';');
		if (index != -1) {
			pathParams = filename.substring(index);
			filename = filename.substring(0, index);
		}

		filename = UrlPathHelper.defaultInstance.decodeRequestString(servletRequest, filename);
		String ext = StringUtils.getFilenameExtension(filename);

		pathParams = UrlPathHelper.defaultInstance.decodeRequestString(servletRequest, pathParams);
		String extInPathParams = StringUtils.getFilenameExtension(pathParams);

		if (!safeExtension(servletRequest, ext) || !safeExtension(servletRequest, extInPathParams)) {
			headers.add(HttpHeaders.CONTENT_DISPOSITION, "inline;filename=f.txt");
		}
	}
    
    //**判断文件扩展名是否安全
	@SuppressWarnings("unchecked")
	private boolean safeExtension(HttpServletRequest request, @Nullable String extension) {
		if (!StringUtils.hasText(extension)) {
			return true;
		}
		extension = extension.toLowerCase(Locale.ENGLISH);
		if (this.safeExtensions.contains(extension)) {
			return true;
		}
		String pattern = (String) request.getAttribute(HandlerMapping.BEST_MATCHING_PATTERN_ATTRIBUTE);
		if (pattern != null && pattern.endsWith("." + extension)) {
			return true;
		}
		if (extension.equals("html")) {
			String name = HandlerMapping.PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE;
			Set<MediaType> mediaTypes = (Set<MediaType>) request.getAttribute(name);
			if (!CollectionUtils.isEmpty(mediaTypes) && mediaTypes.contains(MediaType.TEXT_HTML)) {
				return true;
			}
		}
		MediaType mediaType = resolveMediaType(request, extension);
		return (mediaType != null && (safeMediaType(mediaType)));
	}

    //**根据文件扩展名获取MediaType
	@Nullable
	private MediaType resolveMediaType(ServletRequest request, String extension) {
		MediaType result = null;
		String rawMimeType = request.getServletContext().getMimeType("file." + extension);
		if (StringUtils.hasText(rawMimeType)) {
			result = MediaType.parseMediaType(rawMimeType);
		}
		if (result == null || MediaType.APPLICATION_OCTET_STREAM.equals(result)) {
			result = MediaTypeFactory.getMediaType("file." + extension).orElse(null);
		}
		return result;
	}
    
    //**判断MediaType是否安全
	private boolean safeMediaType(MediaType mediaType) {
		return (SAFE_MEDIA_BASE_TYPES.contains(mediaType.getType()) ||
				mediaType.getSubtype().endsWith("+xml"));
	}
}

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