前后端交互时将返回值中的null置为空字符串的8种方式

返回给前端的Json对象中有很多字段没有值，就会有很多的null值（如下图）

前端判断处理会比较麻烦，所以打算在后端回传数据时做一下处理：

1、字符串null值置为空字符串

2、空List转为[]

3、封装数字类型null转为0

4、Boolean类型null转为false

先看想要达到的效果：

 

 一、通过继承WebMvcConfigurationSupport，重写configureMessageConverters方法实现

@Configuration
public class MyJsonConfig extends WebMvcConfigurationSupport {
    @Override
    public void configureMessageConverters(List> converters) {
        FastJsonHttpMessageConverter converter = new FastJsonHttpMessageConverter();
        FastJsonConfig config = new FastJsonConfig();
        config.setSerializerFeatures(
                //json格式输出
                SerializerFeature.PrettyFormat,
                // 保留map为空的字段
                SerializerFeature.WriteMapNullValue,
                // 将String类型的null转成""形式
                SerializerFeature.WriteNullStringAsEmpty,
                // 将Number类型的null转成0
                SerializerFeature.WriteNullNumberAsZero,
                // 将List类型的null转成[],而不是""
                SerializerFeature.WriteNullListAsEmpty,
                // Boolean类型的null转成false
                SerializerFeature.WriteNullBooleanAsFalse,
                // 处理可能循环引用的问题
                SerializerFeature.DisableCircularReferenceDetect);
        converter.setFastJsonConfig(config);
        converter.setDefaultCharset(StandardCharsets.UTF_8);
        List mediaTypeList = new ArrayList<>();
        mediaTypeList.add(MediaType.APPLICATION_JSON);
        converter.setSupportedMediaTypes(mediaTypeList);
        converters.add(converter);
    }
}

二、通过实现WebMvcConfigurer，重写configureMessageConverters方法实现

@Configuration
public class BaseMvcConfig implements WebMvcConfigurer {
    @Override
    public void configureMessageConverters(List> converters) {
        MappingJackson2HttpMessageConverter jackson2HttpMessageConverter = new MappingJackson2HttpMessageConverter();
        ObjectMapper objectMapper = new ObjectMapper();
        /**
         * 序列换成Json时,将所有的Long变成String
         * 因为js中得数字类型不能包括所有的java Long值
         */
        SimpleModule simpleModule = new SimpleModule();
        simpleModule.addSerializer(Long.class, ToStringSerializer.instance);
        simpleModule.addSerializer(Long.TYPE, ToStringSerializer.instance);
 
        // 所有的double类型返回保留三位小数
        simpleModule.addSerializer(Double.class, CustomerDoubleSerialize.instance);
        simpleModule.addSerializer(Double.TYPE, CustomerDoubleSerialize.instance);
 
        objectMapper.registerModule(simpleModule);
        jackson2HttpMessageConverter.setObjectMapper(objectMapper);
        converters.add(jackson2HttpMessageConverter);
    }
}

三、在Springboot启动类中注入Bean---FastJson

@Bean
public HttpMessageConverters fastJsonConverters() {
	FastJsonHttpMessageConverter4 fastConverter = new FastJsonHttpMessageConverter4();
	FastJsonConfig fastJsonConfig = new FastJsonConfig();
	fastJsonConfig.setSerializerFeatures(SerializerFeature.PrettyFormat, SerializerFeature.IgnoreNonFieldGetter,
			SerializerFeature.WriteMapNullValue, SerializerFeature.WriteNullStringAsEmpty);
	fastConverter.setFastJsonConfig(fastJsonConfig);
	List supportedMediaTypes = new ArrayList();
	supportedMediaTypes.add(new MediaType("text", "json", Charset.forName("utf8")));
	supportedMediaTypes.add(new MediaType("application", "json", Charset.forName("utf8")));
	fastConverter.setSupportedMediaTypes(supportedMediaTypes);
	HttpMessageConverter converter = fastConverter;
	return new HttpMessageConverters(converter);
}

四、在Springboot启动类中注入Bean---Jackson

@Bean
public MappingJackson2HttpMessageConverter mappingJackson2HttpMessageConverter() {
	MappingJackson2HttpMessageConverter mappingJackson2HttpMessageConverter = new MappingJackson2HttpMessageConverter();
	List supported = new ArrayList<>();
	supported.add(MediaType.APPLICATION_JSON_UTF8);
	mappingJackson2HttpMessageConverter.setSupportedMediaTypes(supported);
 
	// JsonMapper
	ObjectMapper objectMapper = new ObjectMapper();
 
	// null值的处理，必须要放在第一步，否则会将下面的日期格式化覆盖掉  null ==> ""
	objectMapper.getSerializerProvider().setNullValueSerializer(new JsonSerializer