第一个类：

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.autoconfigure.http.HttpMessageConverters;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.converter.HttpMessageConverter;
import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;

import java.util.List;
/**
 * @desc: webMvc配置
 * @auth: zhangsy
 * @data: 2019/9/20 10:09
 */
@Configuration
public class WebConfig  implements WebMvcConfigurer {

    @Autowired
    private HttpMessageConverters httpMessageConverters;

    /**
     * MappingJackson2HttpMessageConverter 实现了HttpMessageConverter 接口，
     * httpMessageConverters.getConverters() 返回的对象里包含了MappingJackson2HttpMessageConverter
     * @return
     */
    @Bean
    public MappingJackson2HttpMessageConverter getMappingJackson2HttpMessageConverter() {
        return new MappingJackson2HttpMessageConverter(new JSONMapper());
    }

    @Override
    public void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
        converters.addAll(httpMessageConverters.getConverters());
    }
}

第二个类：JSONMapper.java

import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.module.SimpleModule;
import com.fasterxml.jackson.databind.ser.std.ToStringSerializer;

/**
 * @desc: 类型转换：主要解决问题 将controller 返回的对象中long 类型转换为 string 类型返回给
 *         前端，这样前端能看到 long 保存的真实的 数据
 *
 * @auth: zhangsy
 * @data: 2019/9/20 9:57
 */
public class JSONMapper  extends ObjectMapper {

    /**
     * @desc: 配置类型转换器
     * @auth: zhangsy
     * @data: 2019/9/20 10:09
     * @return:
     */
    public JSONMapper() {
        super();
        this.configure(JsonGenerator.Feature.WRITE_BIGDECIMAL_AS_PLAIN, true);
        this.configure(JsonGenerator.Feature.IGNORE_UNKNOWN, true);
        this.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
        this.setSerializationInclusion(JsonInclude.Include.NON_NULL);
        SimpleModule simpleModule = new SimpleModule();
        simpleModule.addSerializer(Long.class, ToStringSerializer.instance);
        simpleModule.addSerializer(Long.TYPE, ToStringSerializer.instance);
        simpleModule.addSerializer(long.class, ToStringSerializer.instance);
        registerModule(simpleModule);
    }
}

另外一种简单的方法：

只需要修改配置 https://www.csdcb.cn/article/217.html