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Java中基于注解的代码生成工具MapStruct映射使用详解

作者:Andya

MapStruct 作为一个基于注解的代码生成工具,为我们提供了一种更加优雅、高效的解决方案,本文主要为大家介绍了它的具体使用,感兴趣的可以了解下

介绍

在 Java 开发中,我们经常需要在不同的 Java Bean 之间进行数据映射,比如从实体类(Entity)到数据传输对象(DTO)的转换。传统的做法是手动编写大量的 setter 和 getter 方法来完成属性的赋值,这种方式不仅繁琐,而且容易出错。MapStruct 作为一个基于注解的代码生成工具,为我们提供了一种更加优雅、高效的解决方案。它在编译时自动生成映射代码,避免了运行时反射带来的性能开销,同时保证了类型安全。

优缺点

优点

缺点

核心注解及详细使用语法说明

@Mapper

import org.mapstruct.Mapper;
import org.mapstruct.factory.Mappers;

@Mapper
public interface UserMapper {
    // 通过 Mappers.getMapper 方法获取映射接口的实例
    UserMapper INSTANCE = Mappers.getMapper(UserMapper.class);

    // 定义从 UserEntity 到 UserDTO 的映射方法
    UserDTO toDTO(UserEntity entity);
    // 定义从 UserDTO 到 UserEntity 的映射方法
    UserEntity toEntity(UserDTO dto);
}

@Mapping

import org.mapstruct.Mapper;
import org.mapstruct.Mapping;
import org.mapstruct.factory.Mappers;

@Mapper
public interface UserMapper {
    UserMapper INSTANCE = Mappers.getMapper(UserMapper.class);

    // 使用 @Mapping 注解指定 entityId 映射到 id,entityName 映射到 name
    @Mapping(source = "entityId", target = "id")
    @Mapping(source = "entityName", target = "name")
    UserDTO toDTO(UserEntity entity);

    // 反向映射
    @Mapping(source = "id", target = "entityId")
    @Mapping(source = "name", target = "entityName")
    UserEntity toEntity(UserDTO dto);
}

@Mappings

import org.mapstruct.Mapper;
import org.mapstruct.Mappings;
import org.mapstruct.Mapping;
import org.mapstruct.factory.Mappers;

@Mapper
public interface UserMapper {
    UserMapper INSTANCE = Mappers.getMapper(UserMapper.class);

    @Mappings({
        @Mapping(source = "entityId", target = "id"),
        @Mapping(source = "entityName", target = "name")
    })
    UserDTO toDTO(UserEntity entity);

    @Mappings({
        @Mapping(source = "id", target = "entityId"),
        @Mapping(source = "name", target = "entityName")
    })
    UserEntity toEntity(UserDTO dto);
}

@Context

import org.mapstruct.Mapper;
import org.mapstruct.Context;
import org.mapstruct.factory.Mappers;
import java.util.Locale;

@Mapper
public interface UserMapper {
    UserMapper INSTANCE = Mappers.getMapper(UserMapper.class);

    // 使用 @Context 注解传递 Locale 对象作为上下文信息
    UserDTO toDTO(UserEntity entity, @Context Locale locale);
}

@AfterMapping

import org.mapstruct.Mapper;
import org.mapstruct.AfterMapping;
import org.mapstruct.MappingTarget;
import org.mapstruct.factory.Mappers;

@Mapper
public interface UserMapper {
    UserMapper INSTANCE = Mappers.getMapper(UserMapper.class);

    UserDTO toDTO(UserEntity entity);

    // 使用 @AfterMapping 注解定义映射完成后的自定义逻辑
    @AfterMapping
    default void afterMapping(@MappingTarget UserDTO dto, UserEntity entity) {
        // 将源对象的 firstName 和 lastName 拼接后赋值给目标对象的 fullName 属性
        dto.setFullName(entity.getFirstName() + " " + entity.getLastName());
    }
}

Demo示例

公共基本类定义

import lombok.Data;

// 用户实体类
@Data
public class UserEntity {
    private Long id;
    private String name;
    private Integer age;
    private String firstName;
    private String lastName;
    private AddressEntity address;
    private Long entityId;
    private String entityName;
}

// 用户数据传输对象类
@Data
public class UserDTO {
    private Long id;
    private String name;
    private Integer age;
    private String fullName;
    private AddressDTO address;
    private Long entityId;
    private String entityName;
}

// 地址实体类
@Data
public class AddressEntity {
    private String street;
    private String city;
}

// 地址数据传输对象类
@Data
public class AddressDTO {
    private String street;
    private String city;
}

简单映射示例

import org.mapstruct.Mapper;
import org.mapstruct.factory.Mappers;

@Mapper
public interface UserMapper {
    // 获取映射接口的实例
    UserMapper INSTANCE = Mappers.getMapper(UserMapper.class);

    // 从 UserEntity 到 UserDTO 的映射方法
    UserDTO toDTO(UserEntity entity);
    // 从 UserDTO 到 UserEntity 的映射方法
    UserEntity toEntity(UserDTO dto);
}

// 测试代码
public class MainSimpleMapping {
    public static void main(String[] args) {
        UserEntity userEntity = new UserEntity();
        userEntity.setId(1L);
        userEntity.setName("John");
        userEntity.setAge(25);

        // 使用映射接口的实例进行映射
        UserDTO userDTO = UserMapper.INSTANCE.toDTO(userEntity);
        System.out.println("简单映射示例结果:");
        System.out.println("UserDTO: id=" + userDTO.getId() + ", name=" + userDTO.getName() + ", age=" + userDTO.getAge());
    }
}

输出结果:

简单映射示例结果:
UserDTO: id=1, name=John, age=25

字段名不一致的映射示例

import org.mapstruct.Mapper;
import org.mapstruct.Mapping;
import org.mapstruct.factory.Mappers;

@Mapper
public interface UserMapper {
    UserMapper INSTANCE = Mappers.getMapper(UserMapper.class);

    @Mapping(source = "entityId", target = "id")
    @Mapping(source = "entityName", target = "name")
    UserDTO toDTO(UserEntity entity);

    @Mapping(source = "id", target = "entityId")
    @Mapping(source = "name", target = "entityName")
    UserEntity toEntity(UserDTO dto);
}

// 测试代码
public class MainFieldNameMismatch {
    public static void main(String[] args) {
        UserEntity userEntity = new UserEntity();
        userEntity.setEntityId(1L);
        userEntity.setEntityName("John");
        userEntity.setAge(25);

        UserDTO userDTO = UserMapper.INSTANCE.toDTO(userEntity);
        System.out.println("字段名不一致映射示例结果:");
        System.out.println("UserDTO: id=" + userDTO.getId() + ", name=" + userDTO.getName() + ", age=" + userDTO.getAge());
    }
}

输出结果:

字段名不一致映射示例结果:
UserDTO: id=1, name=John, age=25

嵌套对象映射示例

import org.mapstruct.Mapper;
import org.mapstruct.factory.Mappers;

@Mapper
public interface UserMapper {
    UserMapper INSTANCE = Mappers.getMapper(UserMapper.class);

    UserDTO toDTO(UserEntity entity);
    UserEntity toEntity(UserDTO dto);

    AddressDTO toDTO(AddressEntity entity);
    AddressEntity toEntity(AddressDTO dto);
}

// 测试代码
public class MainNestedObjectMapping {
    public static void main(String[] args) {
        AddressEntity addressEntity = new AddressEntity();
        addressEntity.setStreet("123 Main St");
        addressEntity.setCity("New York");

        UserEntity userEntity = new UserEntity();
        userEntity.setId(1L);
        userEntity.setName("John");
        userEntity.setAddress(addressEntity);

        UserDTO userDTO = UserMapper.INSTANCE.toDTO(userEntity);
        System.out.println("嵌套对象映射示例结果:");
        System.out.println("UserDTO: id=" + userDTO.getId() + ", name=" + userDTO.getName());
        System.out.println("AddressDTO: street=" + userDTO.getAddress().getStreet() + ", city=" + userDTO.getAddress().getCity());
    }
}

输出结果:

嵌套对象映射示例结果:
UserDTO: id=1, name=John
AddressDTO: street=123 Main St, city=New York

自定义映射逻辑示例

import org.mapstruct.Mapper;
import org.mapstruct.AfterMapping;
import org.mapstruct.MappingTarget;
import org.mapstruct.factory.Mappers;

@Mapper
public interface UserMapper {
    UserMapper INSTANCE = Mappers.getMapper(UserMapper.class);

    UserDTO toDTO(UserEntity entity);

    @AfterMapping
    default void afterMapping(@MappingTarget UserDTO dto, UserEntity entity) {
        dto.setFullName(entity.getFirstName() + " " + entity.getLastName());
    }
}

// 测试代码
public class MainCustomMappingLogic {
    public static void main(String[] args) {
        UserEntity userEntity = new UserEntity();
        userEntity.setId(1L);
        userEntity.setFirstName("John");
        userEntity.setLastName("Doe");

        UserDTO userDTO = UserMapper.INSTANCE.toDTO(userEntity);
        System.out.println("自定义映射逻辑示例结果:");
        System.out.println("UserDTO: id=" + userDTO.getId() + ", fullName=" + userDTO.getFullName());
    }
}

输出结果:

自定义映射逻辑示例结果:
UserDTO: id=1, fullName=John Doe

使用上下文示例

import org.mapstruct.Mapper;
import org.mapstruct.Context;
import org.mapstruct.factory.Mappers;
import java.util.Locale;

@Mapper
public interface UserMapper {
    UserMapper INSTANCE = Mappers.getMapper(UserMapper.class);

    UserDTO toDTO(UserEntity entity, @Context Locale locale);

    default String localize(String value, @Context Locale locale) {
        // 根据 locale 进行本地化处理
        return value;
    }
}

// 测试代码
public class MainWithContext {
    public static void main(String[] args) {
        UserEntity userEntity = new UserEntity();
        userEntity.setId(1L);
        userEntity.setName("John");

        Locale locale = Locale.US;
        UserDTO userDTO = UserMapper.INSTANCE.toDTO(userEntity, locale);
        System.out.println("使用上下文示例结果:");
        System.out.println("UserDTO: id=" + userDTO.getId() + ", name=" + userDTO.getName());
    }
}

输出结果:

使用上下文示例结果:
UserDTO: id=1, name=John

通过以上示例可以看到,使用 MapStruct 能够方便快捷地完成 Java Bean 之间的映射,同时结合 Lombok 的 @Data 注解进一步简化了代码。并且从输出结果可以直观地验证各个映射场景的正确性。

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