前文通过阅读源码，深入分析了DispatcherServlet及相关组件的工作流程，本文不再阅读源码，介绍一下扩展HttpMessageConverter的方式。

HttpMessageConverter工作方式及扩展方式

前文介绍过，HttpMessageConverter是读写请求体和响应体的组件。

RequestResponseBodyMethodProcessor(用于解析请求参数、处理返回值)从内置的HttpMessageConverter查找支持当前请求体、响应体的实例，然后调用read、write来读写数据。

Spring内置的HttpMessageConverter在装配RequestResponseBodyMethodProcessor的时候创建，具体代码在WebMvcConfigurationSupport类addDefaultHttpMessageConverters方法中。

开发者如果要扩展使用自己的HttpMessageConverter实现，可以编写组件实现WebMvcConfigurer接口，在extendMessageConverters方法中注入自己的HttpMessageConverter实现类对象。

自定义HttpMessageConverter

需求描述

系统需要对响应体进行加密、对请求体解密操作。

思路：

1. 编写类实现HttpMessageConverter接口，read方法中先对请求体解密，之后在做json反序列化
2. write方法先做json序列化，之后再加密
3. 通过WebMvcConfigurer注册

编写HttpMessageConverter实现类

public class MyMappingJackson2HttpMessageConverter implements GenericHttpMessageConverter<Object> {private static final String S_KEY = "1234567890123456";private static final String IV_PARAMETER = "abcdefghijklmnop";// 用来做json序列化和反序列化，自己编写代码也可以，此处直接使用MappingJackson2HttpMessageConverter来做private final MappingJackson2HttpMessageConverter jackson2HttpMessageConverter;// 读写字符串private final StringHttpMessageConverter stringHttpMessageConverter;public MyMappingJackson2HttpMessageConverter(MappingJackson2HttpMessageConverter jackson2HttpMessageConverter) {this.jackson2HttpMessageConverter = jackson2HttpMessageConverter;this.stringHttpMessageConverter = new StringHttpMessageConverter(StandardCharsets.UTF_8);}@Overridepublic boolean canRead(Class<?> clazz, MediaType mediaType) {return jackson2HttpMessageConverter.canRead(clazz, mediaType);}@Overridepublic boolean canWrite(Class<?> clazz, MediaType mediaType) {return jackson2HttpMessageConverter.canWrite(clazz, mediaType);}@Overridepublic List<MediaType> getSupportedMediaTypes() {return jackson2HttpMessageConverter.getSupportedMediaTypes();}@Overridepublic Object read(Class<?> clazz, HttpInputMessage inputMessage)throws IOException, HttpMessageNotReadableException {// 读取请求原始字节byte[] bytes = readBytes(inputMessage);// 解密byte[] decryptBytes = AesUtil.decrypt(bytes, S_KEY, IV_PARAMETER);// 封装HttpInputMessage供下面反序列化使用HttpInputMessage in = new MyHttpInputMessage(inputMessage, decryptBytes);// json反序列化return jackson2HttpMessageConverter.read(clazz, in);}@Overridepublic void write(Object o, MediaType contentType, HttpOutputMessage outputMessage)throws IOException, HttpMessageNotWritableException {ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();// json序列化jackson2HttpMessageConverter.write(o, contentType, new MyHttpOutputMessage(outputMessage, byteArrayOutputStream));byte[] bytes = byteArrayOutputStream.toByteArray();// 加密byte[] encryptStr = AesUtil.encrypt(bytes, S_KEY, IV_PARAMETER);// 将响应写出去this.stringHttpMessageConverter.write(new String(encryptStr, StandardCharsets.UTF_8), contentType, outputMessage);}@Overridepublic boolean canRead(Type type, Class<?> contextClass, MediaType mediaType) {return jackson2HttpMessageConverter.canRead(type, contextClass, mediaType);}@Overridepublic Object read(Type type, Class<?> contextClass, HttpInputMessage inputMessage)throws IOException, HttpMessageNotReadableException {byte[] bytes = readBytes(inputMessage);byte[] decryptBytes = AesUtil.decrypt(bytes, S_KEY, IV_PARAMETER);HttpInputMessage in = new MyHttpInputMessage(inputMessage, decryptBytes);return jackson2HttpMessageConverter.read(type, contextClass, in);}@Overridepublic boolean canWrite(Type type, Class<?> clazz, MediaType mediaType) {return jackson2HttpMessageConverter.canWrite(type, clazz, mediaType);}@Overridepublic void write(Object o, Type type, MediaType contentType, HttpOutputMessage outputMessage)throws IOException, HttpMessageNotWritableException {ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();jackson2HttpMessageConverter.write(o, type, contentType, new MyHttpOutputMessage(outputMessage, byteArrayOutputStream));byte[] bytes = byteArrayOutputStream.toByteArray();byte[] encryptStr = AesUtil.encrypt(bytes, S_KEY, IV_PARAMETER);this.stringHttpMessageConverter.write(new String(encryptStr, StandardCharsets.UTF_8), contentType, outputMessage);}private byte[] readBytes(HttpInputMessage inputMessage) throws IOException {long contentLength = inputMessage.getHeaders().getContentLength();ByteArrayOutputStream bos =new ByteArrayOutputStream(contentLength >= 0 ? (int) contentLength : StreamUtils.BUFFER_SIZE);StreamUtils.copy(inputMessage.getBody(), bos);return bos.toByteArray();}private static class MyHttpInputMessage implements HttpInputMessage {private final HttpInputMessage originalHttpInputMessage;private final byte[] buf;public MyHttpInputMessage(HttpInputMessage originalHttpInputMessage, byte[] buf) {this.originalHttpInputMessage = originalHttpInputMessage;this.buf = buf;}@Overridepublic InputStream getBody() throws IOException {return new ByteArrayInputStream(this.buf);}@Overridepublic HttpHeaders getHeaders() {HttpHeaders headers = this.originalHttpInputMessage.getHeaders();headers.setContentLength(this.buf.length);return headers;}}private static class MyHttpOutputMessage implements HttpOutputMessage {private final HttpOutputMessage originalHttpOutputMessage;private final OutputStream outputStream;public MyHttpOutputMessage(HttpOutputMessage originalHttpOutputMessage,OutputStream outputStream) {this.originalHttpOutputMessage = originalHttpOutputMessage;this.outputStream = outputStream;}@Overridepublic OutputStream getBody() throws IOException {return this.outputStream;}@Overridepublic HttpHeaders getHeaders() {return this.originalHttpOutputMessage.getHeaders();}}
}

注入HttpMessageConverter实现类对象

@Component
public class MyWebMvcConfigurer implements WebMvcConfigurer {@Overridepublic void extendMessageConverters(List<HttpMessageConverter<?>> converters) {MappingJackson2HttpMessageConverter converter = null;for (HttpMessageConverter<?> messageConverter : converters) {if (messageConverter instanceof MappingJackson2HttpMessageConverter) {converter = (MappingJackson2HttpMessageConverter) messageConverter;break;}}if (converter != null) {// 注入MyMappingJackson2HttpMessageConverterMyMappingJackson2HttpMessageConverter myMappingJackson2HttpMessageConverter =new MyMappingJackson2HttpMessageConverter(converter);converters.add(0, myMappingJackson2HttpMessageConverter);}}
}