【小家Spring】Spring MVC容器的web九大组件之---HandlerAdapter源码详解---HttpMessageConverter的匹配规则（选择原理）(上)

public class WebMvcConfigurationSupport implements ApplicationContextAware, ServletContextAware {
  ...
  protected final List<HttpMessageConverter<?>> getMessageConverters() {
    if (this.messageConverters == null) {
      this.messageConverters = new ArrayList<>();
      // 调用者自己配置消息转换器
      // 若调用者自己没有配置，那就走系统默认的转换器们~~~~~
      configureMessageConverters(this.messageConverters);
      if (this.messageConverters.isEmpty()) {
        addDefaultHttpMessageConverters(this.messageConverters);
      }
      // 不管调用者配不配置，通过扩展接口进来的转换器都会添加进来
      // 因为复写此个protected方法也是我们最为常用的自定义消息转换器的一个手段~~~~~
      extendMessageConverters(this.messageConverters);
    }
    return this.messageConverters;
  }
  ...
  // 大多数情况下，我们并不需要配置。因此看看系统默认的addDefaultHttpMessageConverters(this.messageConverters);
  protected final void addDefaultHttpMessageConverters(List<HttpMessageConverter<?>> messageConverters) {
    StringHttpMessageConverter stringHttpMessageConverter = new StringHttpMessageConverter();
    stringHttpMessageConverter.setWriteAcceptCharset(false);  // see SPR-7316
    messageConverters.add(new ByteArrayHttpMessageConverter());
    messageConverters.add(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()));
    }
  }
}

        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-databind</artifactId>
            <version>2.9.8</version>
        </dependency>

  // 返回值为string类型
    @ResponseBody
    @RequestMapping(value = "/hello", method = RequestMethod.GET)
    public String helloGet() throws Exception {
      // 请注意：我这里又有中文  又有英文
        return "哈喽，world";
    }
  // 返回值是个对象，希望被转换为
    @ResponseBody
    @RequestMapping(value = "/hello/json", method = RequestMethod.GET)
    public Parent helloGetJson() throws Exception {
        return new Parent("fsx", 18);
    }

public class RequestMappingHandlerAdapter extends AbstractHandlerMethodAdapter
    implements BeanFactoryAware, InitializingBean {
  ...
  public RequestMappingHandlerAdapter() {
    StringHttpMessageConverter stringHttpMessageConverter = new StringHttpMessageConverter();
    stringHttpMessageConverter.setWriteAcceptCharset(false);  // see SPR-7316
    this.messageConverters = new ArrayList<>(4);
    this.messageConverters.add(new ByteArrayHttpMessageConverter());
    this.messageConverters.add(stringHttpMessageConverter);
    try {
      this.messageConverters.add(new SourceHttpMessageConverter<>());
    }
    catch (Error err) {
      // Ignore when no TransformerFactory implementation is available
    }
    this.messageConverters.add(new AllEncompassingFormHttpMessageConverter());
  }
  ...
}

public class RequestResponseBodyMethodProcessor extends AbstractMessageConverterMethodProcessor {
  ...
  @Override
  public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
      ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
      throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {
    mavContainer.setRequestHandled(true);
    ServletServerHttpRequest inputMessage = createInputMessage(webRequest);
    ServletServerHttpResponse outputMessage = createOutputMessage(webRequest);
    // Try even with null return value. ResponseBodyAdvice could get involved.
    // 这里找到消息转换器，来把返回的结果写进response里面~~~
    // 该方法位于父类`AbstractMessageConverterMethodArgumentResolver`中，通用的利用转换器处理返回值的方法
    writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);
  }
  ...
}