在JSON构建中基于变量动态确定其他变量：以国家代码与电话前缀为例

{
  "BasicData": {
    "country": "United Kingdom"
  },
  "Phone": {
    "Phone prefix": "+44"
  }
}

// Country.java
public class Country {
    private String displayName;
    private String dialingCode;

    public Country(String displayName, String dialingCode) {
        this.displayName = displayName;
        this.dialingCode = dialingCode;
    }

    public String getDisplayName() {
        return displayName;
    }

    public String getDialingCode() {
        return dialingCode;
    }

    // 可以根据需要添加setter或equals/hashCode方法
}

// CountryService.java
import java.util.HashMap;
import java.util.Map;
import java.util.Optional;

public class CountryService {   
    private static final Map<String, Country> data = new HashMap<>();

    static {
        // 使用ISO国家代码作为键，存储国家数据
        data.put("gb", new Country("United Kingdom", "+44"));
        data.put("us", new Country("United States", "+1"));
        data.put("cn", new Country("China", "+86"));
        // ... 更多国家数据
    }

    /**
     * 根据ISO国家代码查找对应的国家信息。
     * @param isoCode ISO国家代码（例如 "gb", "us"）。
     * @return 包含国家信息的Optional对象，如果未找到则为Optional.empty()。
     */
    public Optional<Country> find(String isoCode) {
        if (isoCode == null || isoCode.trim().isEmpty()) {
            return Optional.empty();
        }
        // 查找时忽略大小写
        return Optional.ofNullable(data.get(isoCode.toLowerCase()));
    }        
}

// Message.java (表示最终JSON结构的POJO)
public class Message {
    public BasicData BasicData;
    public Phone Phone;

    // 内部类 BasicData
    public static class BasicData {
        public String country;
        public BasicData(String country) { this.country = country; }
    }

    // 内部类 Phone
    public static class Phone {
        public String Phone_prefix; // 注意JSON键名与Java字段名的映射
        public Phone(String phonePrefix) { this.Phone_prefix = phonePrefix; }
    }

    // Message的构造函数或其他方法
}

// ResponseBuilder.java (辅助构建Message对象)
public class ResponseBuilder {
    private Message message = new Message();

    public ResponseBuilder basicData(String countryDisplayName) {
        message.BasicData = new Message.BasicData(countryDisplayName);
        return this;
    }

    public ResponseBuilder phone(String phonePrefix) {
        message.Phone = new Message.Phone(phonePrefix);
        return this;
    }

    public Message build() {
        return message;
    }
}

// Controller.java (处理请求并构建响应)
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.http.HttpStatus;
import org.springframework.web.server.ResponseStatusException;

@RestController
public class Controller {

    private final CountryService countryService = new CountryService(); // 实际应用中通过依赖注入

    @GetMapping("/message")
    public Message handleRequest(@RequestParam String countryIsoCode) {
        // 查找国家信息
        Country country = countryService.find(countryIsoCode)
            .orElseThrow(() -> new ResponseStatusException(
                HttpStatus.BAD_REQUEST, "Invalid or unsupported country ISO code: " + countryIsoCode));

        // 使用构建器模式构造响应对象
        return new ResponseBuilder()
            .basicData(country.getDisplayName())
            .phone(country.getDialingCode())
            .build();
    }
}