作者：bettermann

toutiao.com/i6891531055631696395/

老铁们是不是经常为写一些实体转换的原始代码感到头疼，尤其是实体字段特别多的时候。介绍一个开源项目 mapstruct ，可以轻松优雅的进行转换，简化你的代码。当然有的人喜欢写get set，或者用BeanUtils 进行复制，代码只是工具，本文只是提供一种思路。

先贴下官网地址吧：https://mapstruct.org/

废话不多说，上代码：

pom 配置：

<properties><project.build.sourceEncoding>UTF-8</project.build.sourceEncoding><maven.compiler.source>1.8</maven.compiler.source><maven.compiler.target>1.8</maven.compiler.target><org.mapstruct.version>1.4.1.Final</org.mapstruct.version><org.projectlombok.version>1.18.12</org.projectlombok.version>
</properties><dependencies><dependency><groupId>org.mapstruct</groupId><artifactId>mapstruct</artifactId><version>${org.mapstruct.version}</version></dependency><!-- lombok dependencies should not end up on classpath --><dependency><groupId>org.projectlombok</groupId><artifactId>lombok</artifactId><version>${org.projectlombok.version}</version><scope>provided</scope></dependency><!-- idea 2018.1.1 之前的版本需要添加下面的配置，后期的版本就不需要了，可以注释掉，
我自己用的2019.3 --><dependency><groupId>org.mapstruct</groupId><artifactId>mapstruct-processor</artifactId><version>${org.mapstruct.version}</version><scope>provided</scope></dependency></dependencies><build><plugins><plugin><groupId>org.apache.maven.plugins</groupId><artifactId>maven-compiler-plugin</artifactId><version>3.8.1</version><configuration><source>1.8</source><target>1.8</target><annotationProcessorPaths><path><groupId>org.projectlombok</groupId><artifactId>lombok</artifactId><version>${org.projectlombok.version}</version></path><path><groupId>org.mapstruct</groupId><artifactId>mapstruct-processor</artifactId><version>${org.mapstruct.version}</version></path></annotationProcessorPaths></configuration></plugin></plugins></build>

关于lombok和mapstruct的版本兼容问题多说几句，maven插件要使用3.6.0版本以上、lombok使用1.16.16版本以上，另外编译的lombok mapstruct的插件不要忘了加上。否则会出现下面的错误：No property named "aaa" exists in source parameter(s). Did you mean "null"?

这种异常就是lombok编译异常导致缺少get setter方法造成的。还有就是缺少构造函数也会抛异常。

@Data
@Builder
@AllArgsConstructor
@NoArgsConstructor
public class Student {private String name;private int age;private GenderEnum gender;private Double height;private Date birthday;}
public enum GenderEnum {Male("1", "男"),Female("0", "女");private String code;private String name;public String getCode() {return this.code;}public String getName() {return this.name;}GenderEnum(String code, String name) {this.code = code;this.name = name;}
}
@Data
@Builder
@AllArgsConstructor
@NoArgsConstructor
public class StudentVO {private String name;private int age;private String gender;private Double height;private String birthday;
}
@Mapper
public interface StudentMapper {StudentMapper INSTANCE = Mappers.getMapper(StudentMapper.class);@Mapping(source = "gender.name", target = "gender")@Mapping(source = "birthday", target = "birthday", dateFormat = "yyyy-MM-dd HH:mm:ss")StudentVO student2StudentVO(Student student);}

实体类是开发过程少不了的，就算是用工具生成肯定也是要有的，需要手写的部分就是这个Mapper的接口，编译完成后会自动生成相应的实现类

然后就可以直接用mapper进行实体的转换了

public class Test {public static void main(String[] args) {Student student = Student.builder().name("小明").age(6).gender(GenderEnum.Male).height(121.1).birthday(new Date()).build();System.out.println(student);//这行代码便是实际要用的代码StudentVO studentVO = StudentMapper.INSTANCE.student2StudentVO(student);System.out.println(studentVO);}}

mapper可以进行字段映射，改变字段类型，指定格式化的方式，包括一些日期的默认处理。

可以手动指定格式化的方法：

@Mapper
public interface StudentMapper {StudentMapper INSTANCE = Mappers.getMapper(StudentMapper.class);@Mapping(source = "gender", target = "gender")@Mapping(source = "birthday", target = "birthday", dateFormat = "yyyy-MM-dd HH:mm:ss")StudentVO student2StudentVO(Student student);default String getGenderName(GenderEnum gender) {return gender.getName();}}

上面只是最简单的实体映射处理，下面介绍一些高级用法

1.List 转换

属性映射基于上面的mapping配置

@Mapper
public interface StudentMapper {StudentMapper INSTANCE = Mappers.getMapper(StudentMapper.class);@Mapping(source = "gender.name", target = "gender")@Mapping(source = "birthday", target = "birthday", dateFormat = "yyyy-MM-dd HH:mm:ss")StudentVO student2StudentVO(Student student);List<StudentVO> students2StudentVOs(List<Student> studentList);}
public static void main(String[] args) {Student student = Student.builder().name("小明").age(6).gender(GenderEnum.Male).height(121.1).birthday(new Date()).build();List<Student> list = new ArrayList<>();list.add(student);List<StudentVO> result = StudentMapper.INSTANCE.students2StudentVOs(list);System.out.println(result);
}

2.多对象转换到一个对象

@Data
@Builder
@AllArgsConstructor
@NoArgsConstructor
public class Student {private String name;private int age;private GenderEnum gender;private Double height;private Date birthday;}
@Data
@AllArgsConstructor
@Builder
@NoArgsConstructor
public class Course {private String courseName;private int sortNo;private long id;}
@Data
@Builder
@AllArgsConstructor
@NoArgsConstructor
public class StudentVO {private String name;private int age;private String gender;private Double height;private String birthday;private String course;
}
@Mapper
public interface StudentMapper {StudentMapper INSTANCE = Mappers.getMapper(StudentMapper.class);@Mapping(source = "student.gender.name", target = "gender")@Mapping(source = "student.birthday", target = "birthday", dateFormat = "yyyy-MM-dd HH:mm:ss")@Mapping(source = "course.courseName", target = "course")StudentVO studentAndCourse2StudentVO(Student student, Course course);}
public class Test {public static void main(String[] args) {Student student = Student.builder().name("小明").age(6).gender(GenderEnum.Male).height(121.1).birthday(new Date()).build();Course course = Course.builder().id(1L).courseName("语文").build();StudentVO studentVO = StudentMapper.INSTANCE.studentAndCourse2StudentVO(student, course);System.out.println(studentVO);}}

3.默认值

@Mapper
public interface StudentMapper {StudentMapper INSTANCE = Mappers.getMapper(StudentMapper.class);@Mapping(source = "student.gender.name", target = "gender")@Mapping(source = "student.birthday", target = "birthday", dateFormat = "yyyy-MM-dd HH:mm:ss")@Mapping(source = "course.courseName", target = "course")@Mapping(target = "name", source = "student.name", defaultValue = "张三")StudentVO studentAndCourse2StudentVO(Student student, Course course);}