在SpringMVC中，可以使用@RequestBody和@ResponseBody两个注解，分别完成请求报文到对象和对象到响应报文的转换，底层这种灵活的消息转换机制。使用系统默认配置的HttpMessageConverter进行解析，然后把相应的数据绑定到要返回的对象上。
HttpInputMessage
这个类是SpringMVC内部对一次Http请求报文的抽象，在HttpMessageConverter的read()方法中，有一个HttpInputMessage的形参，它正是SpringMVC的消息转换器所作用的受体“请求消息”的内部抽象，消息转换器从“请求消息”中按照规则提取消息，转换为方法形参中声明的对象。
package org.springframework.http;import java.io.IOException;import java.io.InputStream;public interface HttpInputMessage extends HttpMessage { InputStream getBody() throws IOException;}HttpOutputMessage
在HttpMessageConverter的write()方法中，有一个HttpOutputMessage的形参，它正是SpringMVC的消息转换器所作用的受体“响应消息”的内部抽象，消息转换器将“响应消息”按照一定的规则写到响应报文中。
package org.springframework.http;import java.io.IOException;import java.io.OutputStream;public interface HttpOutputMessage extends HttpMessage { OutputStream getBody() throws IOException;}HttpMessageConverter
/* * Copyright 2002-2010 the original author or authors. * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */package org.springframework.http.converter;import java.io.IOException;import java.util.List;import org.springframework.http.HttpInputMessage;import org.springframework.http.HttpOutputMessage;import org.springframework.http.MediaType;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;}HttpMessageConverter 接口提供了5个方法：
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。
当前Spring中已经默认提供了相当多的转换器，分别有：
| 名称 | 作用 | 读支持MediaType | 写支持MediaType |
|---|---|---|---|
| ByteArrayHttpMessageConverter | 数据与字节数组的相互转换 | / | application/octet-stream |
| StringHttpMessageConverter | 数据与String类型的相互转换 | text/* | text/plain |
| FormHttpMessageConverter | 表单与MultiValueMap<string, string=””>的相互转换 | application/x-www-form-urlencoded | application/x-www-form-urlencoded |
| SourceHttpMessageConverter | 数据与javax.xml.transform.Source的相互转换 | text/xml和application/xml | text/xml和application/xml |
| MarshallingHttpMessageConverter | 使用SpringMarshaller/Unmarshaller转换XML数据 | text/xml和application/xml | text/xml和application/xml |
| MappingJackson2HttpMessageConverter | 使用Jackson的ObjectMapper转换Json数据 | application/json | application/json |
| MappingJackson2XmlHttpMessageConverter | 使用Jackson的XmlMapper转换XML数据 | application/xml | application/xml |
| BufferedImageHttpMessageConverter | 数据与java.awt.image.BufferedImage的相互转换 | Java I/O API支持的所有类型 | Java I/O API支持的所有类型 |
HttpMessageConverter匹配过程：
@RequestBody注解时： 根据Request对象header部分的Content-Type类型，逐一匹配合适的HttpMessageConverter来读取数据。
private Object readWithMessageConverters(MethodParameter methodParam, HttpInputMessage inputMessage, Class paramType) throws Exception { MediaType contentType = inputMessage.getHeaders().getContentType(); if (contentType == null) { StringBuilder builder = new StringBuilder(ClassUtils.getShortName(methodParam.getParameterType())); String paramName = methodParam.getParameterName(); if (paramName != null) { builder.append(' '); builder.append(paramName); } throw new HttpMediaTypeNotSupportedException("Cannot extract parameter (" + builder.toString() + "): no Content-Type found"); } List<MediaType> allSupportedMediaTypes = new ArrayList<MediaType>(); if (this.messageConverters != null) { for (HttpMessageConverter<?> messageConverter : this.messageConverters) { allSupportedMediaTypes.addAll(messageConverter.getSupportedMediaTypes()); if (messageConverter.canRead(paramType, contentType)) { if (logger.isDebugEnabled()) { logger.debug("Reading [" + paramType.getName() + "] as /"" + contentType + "/" using [" + messageConverter + "]"); } return messageConverter.read(paramType, inputMessage); } } } throw new HttpMediaTypeNotSupportedException(contentType, allSupportedMediaTypes); }@ResponseBody注解时：根据Request对象header部分的Accept属性（逗号分隔），逐一按accept中的类型，去遍历找到能处理的HttpMessageConverter。
private void writeWithMessageConverters(Object returnValue, HttpInputMessage inputMessage, HttpOutputMessage outputMessage) throws IOException, HttpMediaTypeNotAcceptableException { List<MediaType> acceptedMediaTypes = inputMessage.getHeaders().getAccept(); if (acceptedMediaTypes.isEmpty()) { acceptedMediaTypes = Collections.singletonList(MediaType.ALL); } MediaType.sortByQualityValue(acceptedMediaTypes); Class<?> returnValueType = returnValue.getClass(); List<MediaType> allSupportedMediaTypes = new ArrayList<MediaType>(); if (getMessageConverters() != null) { for (MediaType acceptedMediaType : acceptedMediaTypes) { for (HttpMessageConverter messageConverter : getMessageConverters()) { if (messageConverter.canWrite(returnValueType, acceptedMediaType)) { messageConverter.write(returnValue, acceptedMediaType, outputMessage); if (logger.isDebugEnabled()) { MediaType contentType = outputMessage.getHeaders().getContentType(); if (contentType == null) { contentType = acceptedMediaType; } logger.debug("Written [" + returnValue + "] as /"" + contentType + "/" using [" + messageConverter + "]"); } this.responseArgumentUsed = true; return; } } } for (HttpMessageConverter messageConverter : messageConverters) { allSupportedMediaTypes.addAll(messageConverter.getSupportedMediaTypes()); } } throw new HttpMediaTypeNotAcceptableException(allSupportedMediaTypes); }自定义一个JSON转换器
class CustomJsonHttpMessageConverter implements HttpMessageConverter { //Jackson的Json映射类 private ObjectMapper mapper = new ObjectMapper(); //该转换器的支持类型：application/json private List supportedMediaTypes = Arrays.asList(MediaType.APPLICATION_JSON); /** * 判断转换器是否可以将输入内容转换成Java类型 * @param clazz 需要转换的Java类型 * @param mediaType 该请求的MediaType * @return */ @Override public boolean canRead(Class clazz, MediaType mediaType) { if (mediaType == null) { return true; } for (MediaType supportedMediaType : getSupportedMediaTypes()) { if (supportedMediaType.includes(mediaType)) { return true; } } return false; } /** * 判断转换器是否可以将Java类型转换成指定输出内容 * @param clazz 需要转换的Java类型 * @param mediaType 该请求的MediaType * @return */ @Override public boolean canWrite(Class clazz, MediaType mediaType) { if (mediaType == null || MediaType.ALL.equals(mediaType)) { return true; } for (MediaType supportedMediaType : getSupportedMediaTypes()) { if (supportedMediaType.includes(mediaType)) { return true; } } return false; } /** * 获得该转换器支持的MediaType * @return */ @Override public List getSupportedMediaTypes() { return supportedMediaTypes; } /** * 读取请求内容，将其中的Json转换成Java对象 * @param clazz 需要转换的Java类型 * @param inputMessage 请求对象 * @return * @throws IOException * @throws HttpMessageNotReadableException */ @Override public Object read(Class clazz, HttpInputMessage inputMessage) throws IOException, HttpMessageNotReadableException { return mapper.readValue(inputMessage.getBody(), clazz); } /** * 将Java对象转换成Json返回内容 * @param o 需要转换的对象 * @param contentType 返回类型 * @param outputMessage 回执对象 * @throws IOException * @throws HttpMessageNotWritableException */ @Override public void write(Object o, MediaType contentType, HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException { mapper.writeValue(outputMessage.getBody(), o); }}自定义MappingJackson2HttpMessage
从 MappingJackson2HttpMessageConverter 的父类 AbstractHttpMessageConverter 中的 write 方法可以看出，该方法通过 writeInternal 方法向返回结果的输出流中写入数据，所以只需要重写该方法即可:
@Beanpublic MappingJackson2HttpMessageConverter mappingJackson2HttpMessageConverter() { return new MappingJackson2HttpMessageConverter() { //重写writeInternal方法，在返回内容前首先进行加密 @Override protected void writeInternal(Object object, HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException { //使用Jackson的ObjectMapper将Java对象转换成Json String ObjectMapper mapper = new ObjectMapper(); String json = mapper.writeValueAsString(object); LOGGER.error(json); //加密 String result = json + "加密了！"; LOGGER.error(result); //输出 outputMessage.getBody().write(result.getBytes()); } };} 在这之后还需要将这个自定义的转换器配置到Spring中，这里通过重写 WebMvcConfigurer 中的 configureMessageConverters 方法添加自定义转换器：
//添加自定义转换器@Overridepublic void configureMessageConverters(List<httpmessageconverter<?>> converters) { converters.add(mappingJackson2HttpMessageConverter()); super.configureMessageConverters(converters);}以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持VeVb武林网。
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