Implementing Data Masking in Java Using Custom Annotations and Jackson Serializer

During a recent security audit a "user information leakage" issue was found, prompting the need for data masking in a backend system.

Data masking hides sensitive fields by encrypting or obfuscating them before they are returned to the client. Instead of modifying each controller, a custom annotation approach provides an efficient and extensible solution.

The solution consists of two strategies: masking at the query level or during serialization; the article chooses the latter using Jackson.

1. Define an enum PrivacyTypeEnum that lists the types of private data (CUSTOMER, NAME, ID_CARD, PHONE, EMAIL).

import lombok.Getter;

/**
 * 隐私数据类型枚举
 */
@Getter
public enum PrivacyTypeEnum {

  /** 自定义（此项需设置脱敏的范围）*/
  CUSTOMER,

  /** 姓名 */
  NAME,

  /** 身份证号 */
  ID_CARD,

  /** 手机号 */
  PHONE,

  /** 邮箱 */
  EMAIL,
}

2. Create a custom annotation PrivacyEncrypt that references the enum and allows configuration of prefix/suffix lengths and mask symbol.

import com.fasterxml.jackson.annotation.JacksonAnnotationsInside;
import com.fasterxml.jackson.databind.annotation.JsonSerialize;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * 自定义数据脱敏注解
 */
@Target(ElementType.FIELD) // 作用在字段上
@Retention(RetentionPolicy.RUNTIME) // class文件中保留，运行时也保留，能通过反射读取到
@JacksonAnnotationsInside // 表示自定义自己的注解PrivacyEncrypt
@JsonSerialize(using = PrivacySerializer.class) // 该注解使用序列化的方式
public @interface PrivacyEncrypt {

    /**
     * 脱敏数据类型（没给默认值，所以使用时必须指定type）
     */
    PrivacyTypeEnum type();

    /**
     * 前置不需要打码的长度
     */
    int prefixNoMaskLen() default 1;

    /**
     * 后置不需要打码的长度
     */
    int suffixNoMaskLen() default 1;

    /**
     * 用什么打码
     */
    String symbol() default "*";
}

3. Implement a custom Jackson serializer PrivacySerializer that applies the appropriate masking logic based on the annotation values.

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.BeanProperty;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.SerializerProvider;
import com.fasterxml.jackson.databind.ser.ContextualSerializer;
import com.zk.common.core.domain.enumerate.PrivacyTypeEnum;
import com.zk.common.core.utils.PrivacyUtil;
import java.io.IOException;
import java.util.Objects;
import lombok.AllArgsConstructor;
import lombok.NoArgsConstructor;
import org.apache.commons.lang3.StringUtils;

@NoArgsConstructor
@AllArgsConstructor
public class PrivacySerializer extends JsonSerializer
implements ContextualSerializer {

  // 脱敏类型
  private PrivacyTypeEnum privacyTypeEnum;
  // 前几位不脱敏
  private Integer prefixNoMaskLen;
  // 最后几位不脱敏
  private Integer suffixNoMaskLen;
  // 用什么打码
  private String symbol;

  @Override
  public void serialize(final String origin, final JsonGenerator jsonGenerator,
      final SerializerProvider serializerProvider) throws IOException {
    if (StringUtils.isNotBlank(origin) && null != privacyTypeEnum) {
      switch (privacyTypeEnum) {
        case CUSTOMER:
          jsonGenerator.writeString(PrivacyUtil.desValue(origin, prefixNoMaskLen, suffixNoMaskLen, symbol));
          break;
        case NAME:
          jsonGenerator.writeString(PrivacyUtil.hideChineseName(origin));
          break;
        case ID_CARD:
          jsonGenerator.writeString(PrivacyUtil.hideIDCard(origin));
          break;
        case PHONE:
          jsonGenerator.writeString(PrivacyUtil.hidePhone(origin));
          break;
        case EMAIL:
          jsonGenerator.writeString(PrivacyUtil.hideEmail(origin));
          break;
        default:
          throw new IllegalArgumentException("unknown privacy type enum " + privacyTypeEnum);
      }
    }
  }

  @Override
  public JsonSerializer
createContextual(final SerializerProvider serializerProvider,
      final BeanProperty beanProperty) throws JsonMappingException {
    if (beanProperty != null) {
      if (Objects.equals(beanProperty.getType().getRawClass(), String.class)) {
        PrivacyEncrypt privacyEncrypt = beanProperty.getAnnotation(PrivacyEncrypt.class);
        if (privacyEncrypt == null) {
          privacyEncrypt = beanProperty.getContextAnnotation(PrivacyEncrypt.class);
        }
        if (privacyEncrypt != null) {
          return new PrivacySerializer(privacyEncrypt.type(), privacyEncrypt.prefixNoMaskLen(),
              privacyEncrypt.suffixNoMaskLen(), privacyEncrypt.symbol());
        }
      }
      return serializerProvider.findValueSerializer(beanProperty.getType(), beanProperty);
    }
    return serializerProvider.findNullValueSerializer(null);
  }
}

4. Provide a utility class PrivacyUtil with static methods for masking phone numbers, emails, ID cards, Chinese names, and a generic desValue method.

public class PrivacyUtil {

    /**
     * 隐藏手机号中间四位
     */
    public static String hidePhone(String phone) {
        return phone.replaceAll("(\\d{3})\\d{4}(\\d{4})", "$1****$2");
    }

    /**
     * 隐藏邮箱
     */
    public static String hideEmail(String email) {
        return email.replaceAll("(\\w?)(\\w+)(\\w)(@\\w+\\.[a-z]+(\\.[a-z]+)?)", "$1****$3$4");
    }

    /**
     * 隐藏身份证
     */
    public static String hideIDCard(String idCard) {
        return idCard.replaceAll("(\\d{4})\\d{10}(\\w{4})", "$1*****$2");
    }

    /**
     * 【中文姓名】只显示第一个汉字，其他隐藏为星号，比如：任**
     */
    public static String hideChineseName(String chineseName) {
        if (chineseName == null) {
            return null;
        }
        return desValue(chineseName, 1, 0, "*");
    }

    /**
     * 对字符串进行脱敏操作
     * @param origin 原始字符串
     * @param prefixNoMaskLen 左侧需要保留几位明文字段
     * @param suffixNoMaskLen 右侧需要保留几位明文字段
     * @param maskStr 用于遮罩的字符串, 如'*'
     * @return 脱敏后结果
     */
    public static String desValue(String origin, int prefixNoMaskLen, int suffixNoMaskLen, String maskStr) {
        if (origin == null) {
            return null;
        }
        StringBuilder sb = new StringBuilder();
        for (int i = 0, n = origin.length(); i < n; i++) {
            if (i < prefixNoMaskLen) {
                sb.append(origin.charAt(i));
                continue;
            }
            if (i > (n - suffixNoMaskLen - 1)) {
                sb.append(origin.charAt(i));
                continue;
            }
            sb.append(maskStr);
        }
        return sb.toString();
    }

    public static void main(String[] args) {
        System.out.println(hideChineseName("张三三"));
    }
}

5. Apply the @PrivacyEncrypt annotation to fields in data classes (e.g., phone, email) to automatically mask them during JSON serialization.

@Data
public class People {

    private Integer id;
    private String name;
    private Integer sex;
    private Integer age;

    @PrivacyEncrypt(type = PrivacyTypeEnum.PHONE) // 隐藏手机号
    private String phone;

    @PrivacyEncrypt(type = PrivacyTypeEnum.EMAIL) // 隐藏邮箱
    private String email;

    private String sign;
}

After integrating the annotation, all responses are automatically masked, eliminating the need for repetitive manual encryption.