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:
@Bean
public 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
@Override
public void configureMessageConverters(List<httpmessageconverter<?>> converters) {
converters.add(mappingJackson2HttpMessageConverter());
super.configureMessageConverters(converters);
}
