spring boot 返回json字符串 null值转空字符串

@Configurationpublic class JacksonConfig {    @Bean    @Primary    @ConditionalOnMissingBean(ObjectMapper.class)    public ObjectMapper jacksonObjectMapper(Jackson2ObjectMapperBuilder builder) {        ObjectMapper objectMapper = builder.createXmlMapper(false).build();        objectMapper.getSerializerProvider().setNullValueSerializer(new JsonSerializer<Object>() {            @Override            public void serialize(Object o, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException, JsonProcessingException {                jsonGenerator.writeString("");            }        });        return objectMapper;    }}