原理

我们知道springboot的mvc自动配置在WebMvcAutoConfiguration配置类中

在该类中，有一个WebMvcAutoConfigurationAdapter类，实现了WebMvcConfigurer接口，并实现了configureMessageConverters方法，该方法就可以添加一些MessageConverter.

		@Override
		public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
			this.messageConvertersProvider
					.ifAvailable((customConverters) -> converters.addAll(customConverters.getConverters()));
		}

customConverters.getConverters()实现：getConverters方法就是一个普通的get方法，那么converters的初始化可能就在该类创建的时候赋值的。

找到该类的构造方法：

	public HttpMessageConverters(boolean addDefaultConverters, Collection<HttpMessageConverter<?>> converters) {
        //调用getDefaultConverters()方法，并赋值给converters
		List<HttpMessageConverter<?>> combined = getCombinedConverters(converters,
				addDefaultConverters ? getDefaultConverters() : Collections.emptyList());
		combined = postProcessConverters(combined);
		this.converters = Collections.unmodifiableList(combined);
	}

getDefaultConverters方法，内部会调用父类的getMessageConverters方法

调用本类的addDefaultHttpMessageConverters方法

addDefaultHttpMessageConverters方法：这里会配置一些默认的消息转换器，并且从成员变量判断是否需要添加其他的消息转换器。

protected final void addDefaultHttpMessageConverters(List<HttpMessageConverter<?>> messageConverters) {
		messageConverters.add(new ByteArrayHttpMessageConverter());
		messageConverters.add(new StringHttpMessageConverter());
		messageConverters.add(new ResourceHttpMessageConverter());
		messageConverters.add(new ResourceRegionHttpMessageConverter());
		if (!shouldIgnoreXml) {
			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 (!shouldIgnoreXml) {
			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 (kotlinSerializationJsonPresent) {
			messageConverters.add(new KotlinSerializationJsonHttpMessageConverter());
		}
		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()));
		}
	}

成员变量的初始化：这里判断一些场景是否存在，如果存在，则赋值为true，进而导入相关消息转换器

自定义消息转换器

通过上文分析，我们只需要实现WebMvcConfigurer接口，并实现了configureMessageConverters方法，就可以达到自定义消息转换器的目的。

例如，我不想用jackson了，想用fastjson的消息转换器，我们可以添加fastjson相关的MessageConverter就可以了

@Configuration
public class WebConfig implements WebMvcConfigurer {


    @Override
    public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
        FastJsonHttpMessageConverter fastConverter = new FastJsonHttpMessageConverter();
        List<MediaType> fastMediaTypes = new ArrayList<>();
        fastMediaTypes.add(MediaType.TEXT_HTML);
        fastMediaTypes.add(MediaType.APPLICATION_JSON);
        fastConverter.setSupportedMediaTypes(fastMediaTypes);
        FastJsonConfig fastJsonConfig = new FastJsonConfig();
        fastJsonConfig.setSerializerFeatures(
                SerializerFeature.WriteMapNullValue,
                SerializerFeature.WriteNullStringAsEmpty,
                SerializerFeature.WriteNullListAsEmpty,
                SerializerFeature.WriteDateUseDateFormat);
        SerializeConfig serializeConfig = SerializeConfig.globalInstance;
        serializeConfig.put(BigInteger.class, ToStringSerializer.instance);
        serializeConfig.put(Long.class, ToStringSerializer.instance);
        serializeConfig.put(Long.TYPE, ToStringSerializer.instance);
        fastJsonConfig.setSerializeConfig(serializeConfig);
        fastConverter.setFastJsonConfig(fastJsonConfig);
        converters.add(fastConverter);
    }
}

@Data
public class Person {
    private String userName;
    private Integer age;
    //使用fastjson的注解进行转换
    @JSONField(format = "yyyy-MM-dd")
    private Date birth;
    private Pet pet;
}