遇到MapStruct后，再也不手写PO，DTO，VO对象之间的转换了

遇到MapStruct后，再也不手写PO，DTO，VO对象之间的转换了。

介绍
在工作中，我们经常要进行各种对象之间的转换。
PO：persistent object 持久对象，对应数据库中的一条记录
VO：view object 表现层对象，最终返回给前端的对象
DTO：data transfer object数据传输对象，如dubbo服务之间传输的对象
如果这些对象的属性名相同还好，可以用如下工具类赋值
Spring BeanUtils
Cglib BeanCopier
避免使用Apache BeanUtils，性能较差
如果属性名不同呢？如果是将多个PO对象合并成一个VO对象呢？好在有MapStruct神器，可以帮助我们快速转换
在pom文件中加入如下依赖即可

>
    >org.mapstruct>
    >mapstruct-jdk8>
    >1.2.0.CR1>
>
>
    >org.mapstruct>
    >mapstruct-processor>
    >1.2.0.CR1>
    >provided>
>

对象互转

@Data
@Builder
public class StudentPO {
    private Integer id;
    private String name;
    private Integer age;
    private String className;
}
@Data
public class StudentVO {
    private Integer id;
    private String studentName;
    private Integer studentAge;
    private String schoolName;
}
@Mapper
public interface StudentMapper {
    StudentMapper INSTANCE = Mappers.getMapper(StudentMapper.class);
    @Mappings({
            @Mapping(source = "name", target = "studentName"),
            @Mapping(source = "age", target = "studentAge")
    })
    StudentVO po2Vo(StudentPO studentPO);
}

1.新建一个Mapper接口，上面加上@Mapper注解
1.新建一个成员变量INSTANCE
1.用@Mapping注解指定映射关系，名字相同的就不用再指定了，会自动映射
测试效果如下，名字不同且没有指定映射关系的会被设置为null

@Test
public void studentPo2Vo() {
    StudentPO studentPO = StudentPO.builder().id(10).name("test")
            .age(24).className("教室名").build();
    StudentVO studentVO = StudentMapper.INSTANCE.po2Vo(studentPO);
    // StudentVO(id=10, studentName=test, studentAge=24, schoolName=null)
    System.out.println(studentVO);
}

List互转

@Mapper
public interface StudentMapper {
    StudentMapper INSTANCE = Mappers.getMapper(StudentMapper.class);
    @Mappings({
            @Mapping(source = "name", target = "studentName"),
            @Mapping(source = "age", target = "studentAge")
    })
    StudentVO po2Vo(StudentPO studentPO);
    List> poList2VoList(List> studentPO);
}

List类型互转的映射规则会用单个对象的映射规则，看测试效果

@Test
public void poList2VoList() {
    List> studentPOList = new ArrayList<>();
    for (int i = 1; i <= 2; i++) {
        StudentPO studentPO = StudentPO.builder().id(i).name(String.valueOf(i)).age(i).build();
        studentPOList.add(studentPO);
    }
    List> studentVOList = StudentMapper.INSTANCE.poList2VoList(studentPOList);
    // [StudentVO(id=1, studentName=1, studentAge=1, schoolName=null),
    // StudentVO(id=2, studentName=2, studentAge=2, schoolName=null)]
    System.out.println(studentVOList);
}

多个对象映射一个对象
我们用SchoolPO和StudentPO来映射SchoolStudentVO

@Data
@Builder
public class SchoolPO {
    private String name;
    private String location;
}
@Data
@Builder
public class StudentPO {
    private Integer id;
    private String name;
    private Integer age;
    private String className;
}

@Data
public class SchoolStudentVO {
    private String schoolName;
    private String studentName;
}
@Mapper
public interface StudentMapper {
    @Mappings({
            @Mapping(source = "schoolPO.name", target = "schoolName"),
            @Mapping(source = "studentPO.name", target = "studentName")
    })
    SchoolStudentVO mergeVo(SchoolPO schoolPO, StudentPO studentPO);
}

测试例子如下

@Test
public void mergeVo() {
    SchoolPO schoolPO = SchoolPO.builder().name("学校名字").build();
    StudentPO studentPO = StudentPO.builder().name("学生名字").build();
    SchoolStudentVO schoolStudentVO = StudentMapper.INSTANCE.mergeVo(schoolPO, studentPO);
    // SchoolStudentVO(schoolName=学校名字, studentName=学生名字)
    System.out.println(schoolStudentVO);
}

当然还有其他的骚操作，这里就简单介绍一个比较实用的技巧，有兴趣的可以看官方的example：
https://github.com/mapstruct/mapstruct-examples

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