一篇文章教你弄懂SpringMvc中的HttpMessageConverter

文章目录

一、HttpMessageConverter介绍
二、自定义HttpMessageConverter

写在前面：
我是「境里婆娑」。我还是从前那个少年，没有一丝丝改变，时间只不过是考验，种在心中信念丝毫未减，眼前这个少年，还是最初那张脸，面前再多艰险不退却。
写博客的目的就是分享给大家一起学习交流，如果您对 Java感兴趣，可以关注我，我们一起学习。

导语：相信使用过Spring的开发人员都用过@RequestBody、@ResponseBody注解，可以直接将输入解析成Json、将输出解析成Json，但HTTP 请求和响应是基于文本的，意味着浏览器和服务器通过交换原始文本进行通信，而这里其实就是HttpMessageConverter发挥着作用。

一、HttpMessageConverter介绍

Http请求响应报文其实都是字符串，当请求报文到java程序会被封装为一个ServletInputStream流，开发人员再读取报文，响应报文则通过ServletOutputStream流，来输出响应报文。

从流中只能读取到原始的字符串报文，同样输出流也是。那么在报文到达SpringMVC / SpringBoot出去，都存在一个字符串到java对象的转化问题。这一过程，在SpringMVC / SpringBoot中，是通过HttpMessageConverter来解决的。

HttpMessageConverter接口提供了5个方法：

canRead：判断该转换器是否能将请求内容转换成Java对象
canWrite：判断该转换器是否可以将Java对象转换成返回内容
getSupportedMediaTypes：获得该转换器支持的MediaType类型
read：读取请求内容并转换成Java对象
write：将Java对象转换后写入返回内容
其中read和write方法的参数分别有有HttpInputMessage和HttpOutputMessage对象，这两个对象分别代表着一次Http通讯中的请求和响应部分，可以通过getBody方法获得对应的输入流和输出流。
其中read和write方法的参数分别有有HttpInputMessage和HttpOutputMessage对象，这两个对象分别代表着一次Http通讯中的请求和响应部分，可以通过getBody方法获得对应的输入流和输出流。

public interface HttpMessageConverter<T> {boolean canRead(Class<?> clazz, MediaType mediaType);boolean canWrite(Class<?> clazz, MediaType mediaType);List<MediaType> getSupportedMediaTypes();T read(Class<? extends T> clazz, HttpInputMessage inputMessage)throws IOException, HttpMessageNotReadableException;void write(T t, MediaType contentType, HttpOutputMessage outputMessage)throws IOException, HttpMessageNotWritableException;
}

缺省配置
平常我们在发送http请求时，没有配置任何 MessageConverter，但是数据前后传递依旧好用，是因为 SpringMVC 启动时会自动配置一些HttpMessageConverter，在 WebMvcConfigurationSupport 类中添加了缺省 MessageConverter：

protected final void addDefaultHttpMessageConverters(List<HttpMessageConverter<?>> messageConverters) {StringHttpMessageConverter stringConverter = new StringHttpMessageConverter();stringConverter.setWriteAcceptCharset(false);messageConverters.add(new ByteArrayHttpMessageConverter());messageConverters.add(stringConverter);messageConverters.add(new ResourceHttpMessageConverter());messageConverters.add(new SourceHttpMessageConverter<Source>());messageConverters.add(new AllEncompassingFormHttpMessageConverter());if (romePresent) {messageConverters.add(new AtomFeedHttpMessageConverter());messageConverters.add(new RssChannelHttpMessageConverter());}if (jackson2XmlPresent) {ObjectMapper objectMapper = Jackson2ObjectMapperBuilder.xml().applicationContext(this.applicationContext).build();messageConverters.add(new MappingJackson2XmlHttpMessageConverter(objectMapper));}else if (jaxb2Present) {messageConverters.add(new Jaxb2RootElementHttpMessageConverter());}if (jackson2Present) {ObjectMapper objectMapper = Jackson2ObjectMapperBuilder.json().applicationContext(this.applicationContext).build();messageConverters.add(new MappingJackson2HttpMessageConverter(objectMapper));}else if (gsonPresent) {messageConverters.add(new GsonHttpMessageConverter());}}

HttpMessageConverter匹配过程
当使用 @RequestBody和 @ResponseBody注解时，RequestMappingHandlerAdapter就使用它们来进行读取或者写入相应格式的数据。

@RequestBody 据Request对象header部分的Content-Type类型，逐一匹配合适的HttpMessageConverter来读取数据。底层调用的是RequestResponseBodyMethodProcessor readWithMessageConverters方法

protected <T> Object readWithMessageConverters(HttpInputMessage inputMessage, MethodParameter parameter,Type targetType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {MediaType contentType;boolean noContentType = false;try {contentType = inputMessage.getHeaders().getContentType();}catch (InvalidMediaTypeException ex) {throw new HttpMediaTypeNotSupportedException(ex.getMessage());}if (contentType == null) {noContentType = true;contentType = MediaType.APPLICATION_OCTET_STREAM;}Class<?> contextClass = parameter.getContainingClass();Class<T> targetClass = (targetType instanceof Class ? (Class<T>) targetType : null);if (targetClass == null) {ResolvableType resolvableType = ResolvableType.forMethodParameter(parameter);targetClass = (Class<T>) resolvableType.resolve();}HttpMethod httpMethod = (inputMessage instanceof HttpRequest ? ((HttpRequest) inputMessage).getMethod() : null);Object body = NO_VALUE;EmptyBodyCheckingHttpInputMessage message;try {message = new EmptyBodyCheckingHttpInputMessage(inputMessage);for (HttpMessageConverter<?> converter : this.messageConverters) {Class<HttpMessageConverter<?>> converterType = (Class<HttpMessageConverter<?>>) converter.getClass();GenericHttpMessageConverter<?> genericConverter =(converter instanceof GenericHttpMessageConverter ? (GenericHttpMessageConverter<?>) converter : null);if (genericConverter != null ? genericConverter.canRead(targetType, contextClass, contentType) :(targetClass != null && converter.canRead(targetClass, contentType))) {if (message.hasBody()) {HttpInputMessage msgToUse =getAdvice().beforeBodyRead(message, parameter, targetType, converterType);body = (genericConverter != null ? genericConverter.read(targetType, contextClass, msgToUse) :((HttpMessageConverter<T>) converter).read(targetClass, msgToUse));body = getAdvice().afterBodyRead(body, msgToUse, parameter, targetType, converterType);}else {body = getAdvice().handleEmptyBody(null, message, parameter, targetType, converterType);}break;}}}catch (IOException ex) {throw new HttpMessageNotReadableException("I/O error while reading input message", ex, inputMessage);}if (body == NO_VALUE) {if (httpMethod == null || !SUPPORTED_METHODS.contains(httpMethod) ||(noContentType && !message.hasBody())) {return null;}throw new HttpMediaTypeNotSupportedException(contentType, this.allSupportedMediaTypes);}MediaType selectedContentType = contentType;Object theBody = body;LogFormatUtils.traceDebug(logger, traceOn -> {String formatted = LogFormatUtils.formatValue(theBody, !traceOn);return "Read \"" + selectedContentType + "\" to [" + formatted + "]";});return body;}

@ResponseBody 底层调用的AbstractMessageConverterMethodProcessor

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());}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());}}}MediaType selectedMediaType = null;MediaType contentType = outputMessage.getHeaders().getContentType();if (contentType != null && contentType.isConcrete()) {if (logger.isDebugEnabled()) {logger.debug("Found 'Content-Type:" + contentType + "' in response");}selectedMediaType = contentType;}else {HttpServletRequest request = inputMessage.getServletRequest();List<MediaType> acceptableTypes = getAcceptableMediaTypes(request);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);}}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) {throw new HttpMediaTypeNotAcceptableException(this.allSupportedMediaTypes);}}

二、自定义HttpMessageConverter

一般springboot会有默认的消息管理器，StringHttpMessageConverter(字符串转换器),FastJsonHttpMessageConverter ,MappingJackson2HttpMessageConverter等等。

今天我要说的是如何自定义消息管理器-FastJsonHttpMessageConverter
1.引用fastjson依赖

<dependency><groupId>com.alibaba</groupId><artifactId>fastjson</artifactId><version>1.2.58</version>
</dependency>

2. 创建配置文件JsonHttpMessageConfiguration.java

@Configuration
public class MyJsonHttpMessageConfiguration {@Bean@ConditionalOnMissingBean(FastJsonHttpMessageConverter.class)public FastJsonHttpMessageConverter fastJsonHttpMessageConverter() {//1.需要定义一个convert转换消息的对象FastJsonHttpMessageConverter fastJsonHttpMessageConverter = new FastJsonHttpMessageConverter();//2.添加fastJson的配置信息;FastJsonConfig config = new FastJsonConfig();//添加配置-config.setSerializerFeatures()  方法中可以添加多个配置（以，分开）config.setSerializerFeatures(SerializerFeature.WriteMapNullValue,SerializerFeature.DisableCircularReferenceDetect);//添加配置-config.setSerializerFeatures()  方法中可以添加多个配置（以，分开）config.setSerializerFeatures(SerializerFeature.WriteMapNullValue,SerializerFeature.DisableCircularReferenceDetect);fastJsonHttpMessageConverter.setFastJsonConfig(config);fastJsonHttpMessageConverter.setSupportedMediaTypes(getSupportedMediaTypes());fastJsonHttpMessageConverter.setDefaultCharset(Charset.forName("UTF-8"));return fastJsonHttpMessageConverter;}/***  配置全局支持的媒体类型*/private List<MediaType> getSupportedMediaTypes() {List<MediaType> supportedMediaTypes = new ArrayList<>();supportedMediaTypes.add(MediaType.APPLICATION_JSON);supportedMediaTypes.add(MediaType.APPLICATION_ATOM_XML);supportedMediaTypes.add(MediaType.APPLICATION_FORM_URLENCODED);supportedMediaTypes.add(MediaType.APPLICATION_OCTET_STREAM);supportedMediaTypes.add(MediaType.APPLICATION_PDF);supportedMediaTypes.add(MediaType.APPLICATION_RSS_XML);supportedMediaTypes.add(MediaType.APPLICATION_XHTML_XML);supportedMediaTypes.add(MediaType.APPLICATION_XML);supportedMediaTypes.add(MediaType.IMAGE_GIF);supportedMediaTypes.add(MediaType.IMAGE_JPEG);supportedMediaTypes.add(MediaType.IMAGE_PNG);supportedMediaTypes.add(MediaType.TEXT_EVENT_STREAM);supportedMediaTypes.add(MediaType.TEXT_HTML);supportedMediaTypes.add(MediaType.TEXT_MARKDOWN);supportedMediaTypes.add(MediaType.TEXT_PLAIN);supportedMediaTypes.add(MediaType.TEXT_XML);return supportedMediaTypes;}
}

SerializerFeature配置的属性解释

属性名称	含义
QuoteFieldNames	输出key时是否使用双引号,默认为true
UseSingleQuotes	使用单引号而不是双引号,默认为false
WriteMapNullValue	是否输出值为null的字段,默认为false应用场景：前端必须需要所有字段
UseISO8601DateFormat	Date使用ISO8601格式输出，默认为false
WriteNullListAsEmpty	List字段如果为null,输出为[],而不是null
WriteNullStringAsEmpty	字符类型字段如果为null,输出为"",而不是null
WriteNullNumberAsZero	数值字段如果为null,输出为0,而非null
WriteNullBooleanAsFalse	Boolean字段如果为null,输出为false,而非null
SkipTransientField	如果是true，类中的Get方法对应的Field是transient，序列化时将会被忽略。默认为true
SortField	按字段名称排序后输出。默认为false

到此SpringMvc中的HttpMessageConverter介绍完毕。如果还有不明白的可以留言。
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由于本人水平有限，难免有不足，恳请各位大佬不吝赐教！