Spring RESTful API中如何避免Long类型序列化为String？

@restcontroller
@requestmapping(value = "/api", produces = "application/json")
public class myrestcontroller {

spring.jackson.serialization.write_dates_as_timestamps=false

@springbootapplication
public class myapplication {
    public static void main(string[] args) {
        springapplication.run(myapplication.class, args);
    }
    @bean
    public encoder feignencoder() {
        return new springencoder(new mappingjackson2httpmessageconverter(
                new jackson2objectmapperbuilder().build()
        ));
    }
}

import com.fasterxml.jackson.databind.ObjectMapper;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

import java.math.BigInteger;

@RestController
public class MyController {
    // 确保注入的不是容器里的 ObjectMapper
    @Autowired private ObjectMapper objectMapper;
    @GetMapping("/demo")
    public String demo() {
        return objectMapper.writeValueAsString(BigInteger.valueOf(1L));
    }
}