@Configuration
public class WebConfig extends WebMvcConfigurationSupport {
    @Bean
    public HttpMessageConverter<String> responseBodyConverter() {
        // 框架默认的StringHttpMessageConverter编码是ISO-8859-1，@Response注解当接口返回的是字符串时会中文乱码
        return new StringHttpMessageConverter(Charset.forName("UTF-8"));
    }
    /**
     * 消息转换器
     * @param converters
     */
    @Override
    protected void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
        super.extendMessageConverters(converters);
        // 请注意顺序，因为其实底层就是用list进行存储的所有可以通过指定下标来指定顺序
        // 目前这种写法可以解决问题，但是因为我们指定的StringHttpMessageConverter
        // 框架也指定了StringHttpMessageConverter，所有我们要将自己的排在前面，否者依然无法使用自定义的消息转换器
        converters.add(0, responseBodyConverter());
    }
}

public interface HttpMessageConverter<T> {
  /**
   * Indicates whether the given class can be read by this converter.
   * @param clazz the class to test for readability
   * @param mediaType the media type to read (can be {@code null} if not specified);
   * typically the value of a {@code Content-Type} header.
   * @return {@code true} if readable; {@code false} otherwise
   */
  // 是否支持读mediaType类型的消息
  boolean canRead(Class<?> clazz, @Nullable MediaType mediaType);
  /**
   * Indicates whether the given class can be written by this converter.
   * @param clazz the class to test for writability
   * @param mediaType the media type to write (can be {@code null} if not specified);
   * typically the value of an {@code Accept} header.
   * @return {@code true} if writable; {@code false} otherwise
   */
  // 是否支持写mediaType类型的消息
  boolean canWrite(Class<?> clazz, @Nullable MediaType mediaType);
  /**
   * Return the list of {@link MediaType} objects supported by this converter.
   * @return the list of supported media types, potentially an immutable copy
   */
  // 支持消息类型集合
  List<MediaType> getSupportedMediaTypes();
  /**
   * Read an object of the given type from the given input message, and returns it.
   * @param clazz the type of object to return. This type must have previously been passed to the
   * {@link #canRead canRead} method of this interface, which must have returned {@code true}.
   * @param inputMessage the HTTP input message to read from
   * @return the converted object
   * @throws IOException in case of I/O errors
   * @throws HttpMessageNotReadableException in case of conversion errors
   */
  // 具体实现读，这里可以修改我们的请求消息
  T read(Class<? extends T> clazz, HttpInputMessage inputMessage)
      throws IOException, HttpMessageNotReadableException;
  /**
   * Write an given object to the given output message.
   * @param t the object to write to the output message. The type of this object must have previously been
   * passed to the {@link #canWrite canWrite} method of this interface, which must have returned {@code true}.
   * @param contentType the content type to use when writing. May be {@code null} to indicate that the
   * default content type of the converter must be used. If not {@code null}, this media type must have
   * previously been passed to the {@link #canWrite canWrite} method of this interface, which must have
   * returned {@code true}.
   * @param outputMessage the message to write to
   * @throws IOException in case of I/O errors
   * @throws HttpMessageNotWritableException in case of conversion errors
   */
  // 具体实现写，可以修改我们的响应消息
  void write(T t, @Nullable MediaType contentType, HttpOutputMessage outputMessage)
      throws IOException, HttpMessageNotWritableException;
}

public abstract class AbstractHttpMessageConverter<T> implements HttpMessageConverter<T>{
  // ......
}

public class FormHttpMessageConverter implements HttpMessageConverter<MultiValueMap<String, ?>> {
  // ......
}

public class BufferedImageHttpMessageConverter implements HttpMessageConverter<BufferedImage>
  // ......
}

public interface GenericHttpMessageConverter<T> extends HttpMessageConverter<T> {
  // ...... 
}

public class StringHttpMessageConverter extends AbstractHttpMessageConverter<String> {
    // ...... 
} 

public class MappingJackson2HttpMessageConverter extends AbstractJackson2HttpMessageConverter {
    // ......
}

public class WebMvcConfigurationSupport implements ApplicationContextAware, ServletContextAware {
  // ......
  // 初始话消息转换器集合
  protected final List<HttpMessageConverter<?>> getMessageConverters() {
        if (this.messageConverters == null) {
            this.messageConverters = new ArrayList<>();
      // 1、加载消息转换器
            configureMessageConverters(this.messageConverters);
            if (this.messageConverters.isEmpty()) {
        // 2、如果消息转换器集合为空那么久系统默认加载
                addDefaultHttpMessageConverters(this.messageConverters);
            }
      // 3、扩展开发者自己的加载器
            extendMessageConverters(this.messageConverters);
        }
        return this.messageConverters;
    }
  
  protected final void addDefaultHttpMessageConverters(List<HttpMessageConverter<?>> messageConverters) {
        messageConverters.add(new ByteArrayHttpMessageConverter());
        messageConverters.add(new StringHttpMessageConverter());
        messageConverters.add(new ResourceHttpMessageConverter());
        messageConverters.add(new ResourceRegionHttpMessageConverter());
        try {
            messageConverters.add(new SourceHttpMessageConverter<>());
        }
        catch (Throwable ex) {
            // Ignore when no TransformerFactory implementation is available...
        }
        messageConverters.add(new AllEncompassingFormHttpMessageConverter());

        if (romePresent) {
            messageConverters.add(new AtomFeedHttpMessageConverter());
            messageConverters.add(new RssChannelHttpMessageConverter());
        }

        if (jackson2XmlPresent) {
            Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.xml();
            if (this.applicationContext != null) {
                builder.applicationContext(this.applicationContext);
            }
            messageConverters.add(new MappingJackson2XmlHttpMessageConverter(builder.build()));
        }
        else if (jaxb2Present) {
            messageConverters.add(new Jaxb2RootElementHttpMessageConverter());
        }

        if (jackson2Present) {
            Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.json();
            if (this.applicationContext != null) {
                builder.applicationContext(this.applicationContext);
            }
            messageConverters.add(new MappingJackson2HttpMessageConverter(builder.build()));
        }
        else if (gsonPresent) {
            messageConverters.add(new GsonHttpMessageConverter());
        }
        else if (jsonbPresent) {
            messageConverters.add(new JsonbHttpMessageConverter());
        }

        if (jackson2SmilePresent) {
            Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.smile();
            if (this.applicationContext != null) {
                builder.applicationContext(this.applicationContext);
            }
            messageConverters.add(new MappingJackson2SmileHttpMessageConverter(builder.build()));
        }
        if (jackson2CborPresent) {
            Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.cbor();
            if (this.applicationContext != null) {
                builder.applicationContext(this.applicationContext);
            }
            messageConverters.add(new MappingJackson2CborHttpMessageConverter(builder.build()));
        }
    }
  // ......
}