详解自定义SpringMVC的Http信息转换器的使用

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：

1. canRead ：判断该转换器是否能将请求内容转换成Java对象
2. canWrite ：判断该转换器是否可以将Java对象转换成返回内容
3. getSupportedMediaTypes ：获得该转换器支持的MediaType类型
4. read ：读取请求内容并转换成Java对象
5. 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。

当前Spring中已经默认提供了相当多的转换器，分别有：

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);
}

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