Springboot接收前端的Json但是为null问题

{
    "id": 94,
    "nickname": "王明",
    "username": "萧强",
    "password": "nulla",
    "email": "p.tplfsayobt@qq.com",
    "avatar": "http://dummyimage.com/100x100",
    "type": 47,
    "createTime": "2007-12-03 08:23:55",
    "updateTime": "1972-02-26 05:02:56"
}

    @PostMapping(value = "/register")
    public Result registerUser(User user) {
        userService.register(user);
        return ResultGenerator.getSuccessResult(user);
    }

    @PostMapping(value = "/register")
    public Result registerUser(@RequestBody User user) {
        userService.register(user);
        return ResultGenerator.getSuccessResult(user);
    }

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializerProvider;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Primary;
import org.springframework.http.converter.json.Jackson2ObjectMapperBuilder;

import java.io.IOException;

/**
 * @author : Nan
 * @date : 2021/06/20
 * @description :
 */
@Configuration
public 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, IOException {
                // 这里可以给任意值 将返回null的值替换 但是目前不知道还想返回null怎么办
                jsonGenerator.writeString("--");
            }
        });
        return objectMapper;
    }
}