spring boot jackson 实现为null的字段不返回 ,有两个好处，第一减少返回的量，第二也可以隐藏字段,防止暴露

在实现WebMvcConfigurer类中实现configureMessageConverters方法，如下

@Override
	public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
		ObjectMapper objectMapper = new ObjectMapper();
		objectMapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);
		MappingJackson2HttpMessageConverter jsonConverter = new MappingJackson2HttpMessageConverter(objectMapper);
		converters.add(jsonConverter);
	}