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前言

在现在微服务项目中，项目大部分都是进行分层的。DO一般不会让对外的服务依赖，这时候需要在提供对外接口的模块里放 DTO 用于对象传输，也即是 DO 对象对内，DTO对象对外，DTO 可以根据业务需要变更，并不需要映射 DO 的全部属性。那这时候就会出现一个问题，我们需要将DTO转换为DO，那就只能一个个从DTO里 get 出来，然后一个个放到 DO 里去。

这种对象与对象之间的互相转换，就需要有一个专门用来解决转换问题的工具，毕竟每一个字段都 get/set 会很麻烦，并且毫无技术可言。（不仅限于DO转DTO）

导入依赖

<dependency>
	<groupId>org.projectlombok</groupId>
	<artifactId>lombok</artifactId>
</dependency>
 <!--包含必需的注释，如@Mapping-->
<dependency>
	<groupId>org.mapstruct</groupId>
	<artifactId>mapstruct-jdk8</artifactId>
	<version>1.2.0.Final</version>
</dependency>
<!--包含生成映射器实现的注释处理器-->
<dependency>
	<groupId>org.mapstruct</groupId>
	<artifactId>mapstruct-processor</artifactId>
	<version>1.2.0.Final</version>
</dependency>
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实例

属性完全相同

先创建两个类，一个 UserDTO 和 UserDO。注：这两个类一模一样

• UserDO

@Data
public class UserDO {

    private Long id;
    private String userName;
    
}
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• UserDTO

@Data
public class UserDTO {

    private Long id;
    private String userName;
    
}
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然后编写一个Java接口，接口上增加注解 @Mapper，提供 userDOToUserDTO 方法进行转换，UserDO 转换为 UserDTO。注意@Mapper的导包

• MapStructConverter

import com.gongj.mapstruct.data.UserDO;
import com.gongj.mapstruct.dto.UserDTO;
import org.mapstruct.Mapper;
import org.mapstruct.factory.Mappers;

@Mapper
public interface MapStructConverter {

    MapStructConverter INSTANCE = Mappers.getMapper(MapStructConverter.class);

    UserDTO userDOToUserDTO(UserDO userDO);
}
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当源对象(UserDO)属性 与 目标对象(UserDTO)属性名字一致时，会隐式地映射

• 编写测试类

@Test
    void contextLoads() {
        UserDO userDo = new UserDO();
        userDo.setId(8888L);
        userDo.setUserName("gongjie");
        
        System.out.println("userDo = "+ userDo.toString());
        //调用转换方法 将源对象传入 UserDO
        UserDTO userDTO = MapStructConverter.INSTANCE.userDOToUserDTO(userDo);
        System.out.println("userDTO = "+ userDTO.toString());
    }

输出结果：
userDo = UserDO(id=8888, userName=gongjie)
userDTO = UserDTO(id=8888, userName=gongjie)
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可以看到结果，已经是我想要的。那它是怎么实现的呢？在我们运行这个test方法之后，mapstruct 它会自动在 target目录下生成MapStructConverter对应的实现类。

我们来看看 MapStructConverterImpl 的内容：

@Generated(
    value = "org.mapstruct.ap.MappingProcessor",
    date = "2020-12-03T12:42:12+0800",
    comments = "version: 1.2.0.Final, compiler: javac, environment: Java 1.8.0_271 (Oracle Corporation)"
)
public class MapStructConverterImpl implements MapStructConverter {

     @Override
    public UserDTO userDOToUserDTO(UserDO userDO) {
        if ( userDO == null ) {
            return null;
        }

        UserDTO userDTO = new UserDTO();

        userDTO.setId( userDO.getId() );
        userDTO.setUserName( userDO.getUserName() );

        return userDTO;
    }
}
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看了它生成的源代码之后，是不是感觉很棒。

属性名称不一致

上面我们已经简单的使用了mapstruct。当源对象属性与目标对象属性名字一致，会自动映射对应属性。那如果属性名称不一致呢？接下来分别在 UserDTO 和 UserDO 内增加属性。

• UserDO

@Data
public class UserDO {

    /**
     * 新增
     */
    private String email;
   
}
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• UserDTO

@Data
public class UserDTO {
	
	/**
	 * 新增
 	*/
	//名称不一致
	private String userEmail;
}
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• MapStructConverter 需要进行修改，当双方对象具有不同的名称时，可以通过@Mapping注解指定其名称的映射关系

    @Mappings(
        {
            @Mapping(source = "email", target = "userEmail"),
    	}
    )
    UserDTO userDOToUserDTO(UserDO userDO);
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• 测试

	 @Test
    void contextLoads2() {
        UserDO userDo = new UserDO();
        userDo.setId(8888L);
        userDo.setUserName("gongjie");
        userDo.setEmail("1998@163.com");
        System.out.println("userDo = "+ userDo.toString());
        //调用转换方法 将源对象传入 UserDO
        UserDTO userDTO = MapStructConverter.INSTANCE.userDOToUserDTO(userDo);
        System.out.println("userDTO = "+ userDTO.toString());
    }
结果：
userDo = UserDO(id=8888, userName=gongjie, email=1998@163.com)
userDTO = UserDTO(id=8888, userName=gongjie, userEmail=1998@163.com)
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注意：如果新增属性之后，运行Test方法达不到想要的结果，需要将之前自动生成的实现类删除(或者将target目录删除)，然后进行重新编译

• 源码

 @Override
    public UserDTO userDOToUserDTO(UserDO userDO) {
        if ( userDO == null ) {
            return null;
        }

        UserDTO userDTO = new UserDTO();

        userDTO.setUserEmail( userDO.getEmail() );
        userDTO.setId( userDO.getId() );
        userDTO.setUserName( userDO.getUserName() );

        return userDTO;
    }
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时间格式转换

再次增加属性 birthday、time，类型分别为Date、String

• UserDO

@Data
public class UserDO {

    /**
     * 新增
     */
    private Date birthday;
    
    private String time;
   
}
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• UserDTO

增加两个属性 date、userBirthday，一个类型类型为Date、一个为String

@Data
public class UserDTO {

	/**
	 * 新增
 	*/
	// 名称与类型都不一致
    private String userBirthday;
    
	 // 名称与类型都不一致
    private Date userTime;
    
}
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• MapStructConverter的userDOToUserDTO 再次进行修改，其中userBirthday、userTime使用 dateFormat进行时间格式化。

   @Mappings({
              @Mapping(source = "birthday", target = "userBirthday",dateFormat = "yyyy-MM-dd HH:mm:ss"),
              @Mapping(source = "time", target = "userTime",dateFormat = "yyyy-MM-dd"),
              @Mapping(source = "email", target = "userEmail"),
      }
      )
      UserDTO userDOToUserDTO(UserDO userDO);
  复制代码

• 测试

	  @Test
    void test1(){
        UserDO userDo = new UserDO();
        userDo.setId(8888L);
        userDo.setUserName("gongjie");
        userDo.setBirthday(new Date());
        userDo.setTime("2021-05-09");
        userDo.setEmail("99@163.com");
        System.out.println("userDo = "+ userDo.toString());
        UserDTO userDTO = MapStructConverter.INSTANCE.userDOToUserDTO(userDo);
        System.out.println("userDTO = "+ userDTO.toString());
    }

结果：
userDo = UserDO(id=8888, userName=gongjie, birthday=Sun May 09 21:59:20 CST 2021, time=2021-05-09, email=99@163.com)
    
userDTO = UserDTO(id=8888, userName=gongjie, userBirthday=2021-05-09 21:59:20, userTime=Sun May 09 00:00:00 CST 2021, userEmail=99@163.com)
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• 源代码

  @Override
      public UserDTO userDOToUserDTO(UserDO userDO) {
          if ( userDO == null ) {
              return null;
          }
  
          UserDTO userDTO = new UserDTO();
  
          userDTO.setUserEmail( userDO.getEmail() );
          try {
              if ( userDO.getTime() != null ) {
                  userDTO.setUserTime( new SimpleDateFormat( "yyyy-MM-dd" ).parse( userDO.getTime() ) );
              }
          }
          catch ( ParseException e ) {
              throw new RuntimeException( e );
          }
          if ( userDO.getBirthday() != null ) {
            userDTO.setUserBirthday( new SimpleDateFormat( "yyyy-MM-dd HH:mm:ss" ).format( userDO.getBirthday() ) );
          }
          userDTO.setId( userDO.getId() );
          userDTO.setUserName( userDO.getUserName() );
  
          return userDTO;
      }
  复制代码

  源代码中是使用 SimpleDateFormat进行时间格式化的。

忽略属性

一般来说 DO 的 id 是不会返回给 DTO 的，我们可以使用 ignore属性来忽略转换改字段。

 @Mappings({
            @Mapping(source = "id",target = "id",ignore = true),
            @Mapping(source = "birthday", target = "userBirthday",dateFormat = "yyyy-MM-dd HH:mm:ss"),
            @Mapping(source = "time", target = "userTime",dateFormat = "yyyy-MM-dd"),
            @Mapping(source = "email", target = "userEmail"),
    }
    )
    UserDTO userDOToUserDTO(UserDO userDO);

结果：
userDo = UserDO(id=8888, userName=gongjie, birthday=Sun May 09 22:03:15 CST 2021, time=2021-05-09, email=99@163.com)
// DTO中 id属性为 null     
userDTO = UserDTO(id=null, userName=gongjie, userBirthday=2021-05-09 22:03:15, userTime=Sun May 09 00:00:00 CST 2021, userEmail=99@163.com)
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集合转换

MapStructConverter 新增方法 userDOToUserDTOs：

  List<UserDTO> userDOToUserDTOs(List<UserDO> userDO);
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• 测试

	void test2(){

        ArrayList<UserDO> objects = new ArrayList<>();
        UserDO userDo = new UserDO();
        userDo.setId(8888L);
        userDo.setUserName("gongjie");
        userDo.setBirthday(new Date());
        userDo.setTime("2021-05-09");
        userDo.setEmail("99@163.com");
        System.out.println("userDo = "+ userDo.toString());
        objects.add(userDo);
        List<UserDTO> userDTOS = MapStructConverter.INSTANCE.userDOToUserDTOs(objects);
        System.out.println("userDTO = "+ userDTOS.toString());
    }
结果：
userDo = UserDO(id=8888, userName=gongjie, birthday=Sun May 09 22:04:47 CST 2021, time=2021-05-09, email=99@163.com)
    
userDTO = [UserDTO(id=null, userName=gongjie, userBirthday=2021-05-09 22:04:47, userTime=Sun May 09 00:00:00 CST 2021, userEmail=99@163.com)]
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• 源代码

@Override
   public List<UserDTO> userDOToUserDTOs(List<UserDO> userDO) {
       if ( userDO == null ) {
           return null;
       }

       List<UserDTO> list = new ArrayList<UserDTO>( userDO.size() );
       for ( UserDO userDO1 : userDO ) {
           list.add( userDOToUserDTO( userDO1 ) );
       }

       return list;
   }

@Override
   public UserDTO userDOToUserDTO(UserDO userDO) {
       if ( userDO == null ) {
           return null;
       }

       UserDTO userDTO = new UserDTO();

       userDTO.setUserEmail( userDO.getEmail() );
       try {
           if ( userDO.getTime() != null ) {
               userDTO.setUserTime( new SimpleDateFormat( "yyyy-MM-dd" ).parse( userDO.getTime() ) );
           }
       }
       catch ( ParseException e ) {
           throw new RuntimeException( e );
       }
       if ( userDO.getBirthday() != null ) {
           userDTO.setUserBirthday( new SimpleDateFormat( "yyyy-MM-dd HH:mm:ss" ).format( userDO.getBirthday() ) );
       }
       userDTO.setUserName( userDO.getUserName() );

       return userDTO;
   }
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可以看到 userDOToUserDTOs方法内部其实就是调用了 userDOToUserDTO方法。为什么呢？因为 mapStruct 是根据类型去匹配的。userDOToUserDTOs的target类型、source类型 与 userDOToUserDTO的target类型、source类型是一致的。

多层对象转换

UserDO 对象中 新增属性 CardDO属性。

• CardDO

  @Data
  public class CardDO {
  
     private Long userCardId;
     private String userCardName;
      
  }
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• UserDO

@Data
public class UserDO {

    /**
     * 新增
     */
    private CardDO card;
}
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• UserDTO

@Data
public class UserDTO {
	/**
     * 新增
     */
    //对应 CardDO 里的 userCardName
    private String cardName;
}
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• MapStructConverter的userDOToUserDTO 进行修改，增加Mapping映射，注意写法，使用 UserDO里的 card属性调用。

    @Mappings({
            @Mapping(source = "id",target = "id",ignore = true),
            @Mapping(source = "birthday", target = "userBirthday",dateFormat = "yyyy-MM-dd HH:mm:ss"),
            @Mapping(source = "time", target = "userTime",dateFormat = "yyyy-MM-dd"),
            @Mapping(source = "email", target = "userEmail"),
            @Mapping(source = "card.userCardName", target = "cardName")
    }
    )
    UserDTO userDOToUserDTO(UserDO userDO);
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• 测试

//多层对象转换
    @Test
    void test3(){


        UserDO userDo = new UserDO();
        userDo.setId(8888L);
        userDo.setUserName("gongjie");
        userDo.setBirthday(new Date());
        userDo.setTime("2021-05-09");
        userDo.setEmail("99@163.com");

        CardDO cardDO = new CardDO();
        cardDO.setUserCardId(999L);
        cardDO.setUserCardName("ZSYH");
        userDo.setCard(cardDO);
        
        System.out.println("userDo = "+ userDo.toString());
        UserDTO userDTO = MapStructConverter.INSTANCE.userDOToUserDTO(userDo);
        System.out.println("userDTO = "+ userDTO.toString());
    }
结果：
userDo = UserDO(id=8888, userName=gongjie, birthday=Sun May 09 22:09:33 CST 2021, time=2021-05-09, email=99@163.com, card=CardDO(userCardId=999, userCardName=ZSYH))
    
userDTO = UserDTO(id=null, userName=gongjie, userBirthday=2021-05-09 22:09:33, userTime=Sun May 09 00:00:00 CST 2021, userEmail=99@163.com, cardName=ZSYH)
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多对一

MapStruct 可以将几种类型的对象映射为另外一种类型，比如：将多个 DO 对象转换为 DTO

• MapStructConverter 需要进行修改，增加新方法 toUserDTO，注意source属性的写法，使用方法属性的名称调用。

@Mappings({
             @Mapping(source = "userDO.birthday", target = "userBirthday",dateFormat = "yyyy-MM-dd HH:mm:ss"),
            @Mapping(source = "userDO.time", target = "userTime",dateFormat = "yyyy-MM-dd"),
            @Mapping(source = "userDO.email", target = "userEmail"),
            @Mapping(source = "cardDO.userCardName", target = "cardName")
    }
    )
    UserDTO toUserDTO(UserDO userDO, CardDO cardDO);
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• 测试

@Test
    void test4(){
        UserDO userDo = new UserDO();
        userDo.setId(8888L);
        userDo.setUserName("gongjie");
        userDo.setBirthday(new Date());
        userDo.setTime("2021-05-09");
        userDo.setEmail("99@163.com");

        CardDO cardDO = new CardDO();
        cardDO.setUserCardId(999L);
        cardDO.setUserCardName("ZSYH");

        UserDTO userDTO = MapStructConverter.INSTANCE.toUserDTO(userDo,cardDO);
        System.out.println("userDTO = "+ userDTO.toString());
    }

输出结果：
userDTO = UserDTO(id=8888, userName=gongjie, userBirthday=2021-05-09 22:12:04, userTime=Sun May 09 00:00:00 CST 2021, userEmail=99@163.com, cardName=ZSYH)
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更新已有对象

在前面的例子中都是通过类似于UserDTO userDOToUserDTO(UserDO userDO);这样的方法来进行转换。MapStruct 提供了另外一种方式来转换另外一个对象中的属性。那就是@MappingTarget注解。

• MapStructConverter

   @Mappings({
            @Mapping(source = "id",target = "id",ignore = true),
            @Mapping(source = "birthday", target = "userBirthday",dateFormat = "yyyy-MM-dd HH:mm:ss"),
            @Mapping(source = "time", target = "userTime",dateFormat = "yyyy-MM-dd"),
            @Mapping(source = "email", target = "userEmail"),
            @Mapping(source = "card.userCardName", target = "cardName")
    }
    )
    void testMappingTarget(UserDO userDO,@MappingTarget UserDTO userDTO);
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• 测试

@Test
    void test5(){
        UserDO userDo = new UserDO();
        userDo.setId(8888L);
        userDo.setUserName("gongjie");
        userDo.setBirthday(new Date());
        userDo.setTime("2021-05-09");
        userDo.setEmail("99@163.com");
        CardDO cardDO = new CardDO();
        cardDO.setUserCardId(999L);
        cardDO.setUserCardName("ZSYH");
        userDo.setCard(cardDO);
        UserDTO userDTO = new UserDTO();
        MapStructConverter.INSTANCE.testMappingTarget(userDo,userDTO);
        System.out.println("userDTO = "+ userDTO.toString());
    }
结果：
userDTO = UserDTO(id=null, userName=gongjie, userBirthday=2021-05-09 22:24:23, userTime=Sun May 09 00:00:00 CST 2021, userEmail=99@163.com, cardName=ZSYH)
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expression表达式

目前，Java 是唯一支持的“表达式语言”，表达式必须以 Java 的形式给出，表达式中引用的任何类型都必须通过它们的全限定名称给出，此属性不能与source()， defaultValue()或constant()一起使用。就直接举例了，使用场景呢，你们会遇到的。

• MapStructConverter增加方法：testExpression，给 userEmail 拼接字符。

 @Mappings({
            @Mapping(source = "id",target = "id",ignore = true),
            @Mapping(source = "birthday", target = "userBirthday",dateFormat = "yyyy-MM-dd HH:mm:ss"),
            @Mapping(source = "time", target = "userTime",dateFormat = "yyyy-MM-dd"),
            @Mapping( target = "userEmail",expression = "java( com.gongj.mapstruct.utils.JoinUtil.join(userDO.getEmail()) )"),
            @Mapping(source = "card.userCardName", target = "cardName")
    }
    )
    UserDTO testExpression(UserDO userDO);
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• JoinUtil

public class JoinUtil {

    public static String join(String oldStr){
        return oldStr + "====expression====";
    }
}
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• 测试

@Test
    void test6(){
        UserDO userDo = new UserDO();
        userDo.setId(8888L);
        userDo.setUserName("gongjie");
        userDo.setBirthday(new Date());
        userDo.setTime("2021-05-09");
        userDo.setEmail("99@163.com");

        System.out.println("userDo = "+ userDo.toString());
        UserDTO userDTO = MapStructConverter.INSTANCE.testExpression(userDo);
       System.out.println("userDTO = "+ userDTO.toString());
    }
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在测试之前，需要将该方法进行注释List<UserDTO> userDOToUserDTOs(List<UserDO> userDO);，不然会出现以下异常：

错误:(30,19)java:模糊的映射方法发现的集合元素映射到
com.gongj.mapstruct.dto.UserDTO userDOToUserDTO(com.gongj.mapstruct.data.UserDO userDO), com.gongj.mapstruct.dto.UserDTO testExpression(com.gongj.mapstruct.data.UserDO userDO).
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我们可以发现 userDOToUserDTO 方法 与 testExpression 方法的target类型、source类型是一模一样的。在 集合转换 的例子中，我们已经知道 userDOToUserDTOs 方法会去调用 userDOToUserDTO 方法，是因为target类型、source类型一致。然后现在我们又加了一个与 userDOToUserDTO target类型、source类型一致的方法。导致 userDOToUserDTOs 方法不知道调用谁，所以抛出异常。

• 源代码

可以看到生成的源代码中，是以全限定名称调用方法的。

当然，如果觉得java( com.gongj.mapstruct.utils.JoinUtil.join(userDO.getEmail()) )这种写法太容易出错了，可以换一种写法，如下：

• MapStructConverter进行修改，@Mapper注解增加属性 imports，导入目标类。

@Mapper(imports = {JoinUtil.class})

 @Mappings({
            @Mapping(source = "id",target = "id",ignore = true),
            @Mapping(source = "birthday", target = "userBirthday",dateFormat = "yyyy-MM-dd HH:mm:ss"),
            @Mapping(source = "time", target = "userTime",dateFormat = "yyyy-MM-dd"),
            @Mapping( target = "userEmail",expression = "java( JoinUtil.join(userDO.getEmail()) )"),
            @Mapping(source = "card.userCardName", target = "cardName")
    }
    )
    UserDTO testExpression(UserDO userDO);
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简写为：java( JoinUtil.join(userDO.getEmail()) )，是不是很棒呀！！！结果还是一致的，各位自己去看吧！

自定义类型转换方法

可能各位还会觉得上面那种写法不过优雅，那我可以使用这种方式。还是调用 JoinUtil.join方法吧！当然这里是随便举例哈，展示使用方式。

• MapStructConverter进行修改，@Mapper注解增加属性 uses，导入目标类。并增加方法：testCustom

@Mapper(imports = {JoinUtil.class},uses = {JoinUtil.class})


 @Mappings({
            @Mapping(source = "id",target = "id",ignore = true),
            @Mapping(source = "birthday", target = "userBirthday",dateFormat = "yyyy-MM-dd HH:mm:ss"),
            @Mapping(source = "time", target = "userTime",dateFormat = "yyyy-MM-dd"),
            @Mapping( target = "userEmail"),
            @Mapping(source = "card.userCardName", target = "cardName")
    }
    )
    UserDTO testCustom(UserDO userDO);
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• 测试

@Test
    void test7(){
        UserDO userDo = new UserDO();
        userDo.setId(8888L);
        userDo.setUserName("gongjie");
        userDo.setBirthday(new Date());
        userDo.setTime("2021-05-09");
        userDo.setEmail("99@163.com");

        System.out.println("userDo = "+ userDo.toString());
        UserDTO userDTO = MapStructConverter.INSTANCE.testCustom(userDo);
        System.out.println("userDTO = "+ userDTO.toString());
    }
结果：
userDo = UserDO(id=8888, userName=gongjie, birthday=Mon May 10 20:53:54 CST 2021, time=2021-05-09, email=99@163.com, card=null)
    
userDTO = UserDTO(id=null, userName=gongjie====expression====, userBirthday=2021-05-10 20:53:54, userTime=Sun May 09 00:00:00 CST 2021, userEmail=99@163.com====expression====, cardName=null)
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然后你会发现userName与userEmail都进行字符拼接。我们看下源代码是怎么实现的。

• 源代码

可以看到，其实是三个属性都调用了 join方法，只不过 userCardName属性我没有进行赋值，所以没有进入if分支代码块。那是为什么呢！其实还是因为类型一致造成的， target类型、source类型两两一致，然后又与 join方法匹配。所有匹配的属性都执行了我们自定义的方法，一个非常有用的功能，可以统一处理相同类型的属性。

执行指定自定义转换方法

自定义转换方法在很多时候都是很有用的，都在某场景下就不适合了。还是上述例子，我只想让 userEmail进行字符拼接。除了expression方式还有其他的吗？

@Qualifier注解

• 首先创建JoinUtilAnnotation注解

import org.mapstruct.Qualifier;

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

@Qualifier
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.CLASS)
public @interface JoinUtilAnnotation {
}
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为什么要创建一个注解类呢？那是因为 Qualifier注解只能标注在 注解上。注意导包哦！

• 将JoinUtil拷贝一份，并重命名为 JoinUtil2

public class JoinUtil2 {

    @JoinUtilAnnotation
    public static String join(String oldStr){
        return oldStr + "====expression====";
    }

}
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• MapStructConverter进行修改，uses属性的值修改为 JoinUtil2，再编写一个方法：testQualifier，在userEmail映射关系上新增属性 qualifiedBy并指向 JoinUtilAnnotation注解。

@Mapper(imports = {JoinUtil.class},uses = {JoinUtil2.class})

  @Mappings({
            @Mapping(source = "id",target = "id",ignore = true),
            @Mapping(source = "birthday", target = "userBirthday",dateFormat = "yyyy-MM-dd HH:mm:ss"),
            @Mapping(source = "time", target = "userTime",dateFormat = "yyyy-MM-dd"),
            @Mapping(source="email",target = "userEmail",qualifiedBy = JoinUtilAnnotation.class),
            @Mapping(source = "card.userCardName", target = "cardName")
    }
    )
    UserDTO testQualifier(UserDO userDO);
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• 测试

  @Test
    void test8(){
        UserDO userDo = new UserDO();
        userDo.setId(8888L);
        userDo.setUserName("gongjie");
        userDo.setBirthday(new Date());
        userDo.setTime("2021-05-09");
        userDo.setEmail("99@163.com");

        System.out.println("userDo = "+ userDo.toString());
        UserDTO userDTO = MapStructConverter.INSTANCE.testQualifier(userDo);
        System.out.println("userDTO = "+ userDTO.toString());
    }
结果：
userDo = UserDO(id=8888, userName=gongjie, birthday=Mon May 10 22:16:48 CST 2021, time=2021-05-09, email=99@163.com, card=null)
    
userDTO = UserDTO(id=null, userName=gongjie, userBirthday=2021-05-10 22:16:48, userTime=Sun May 09 00:00:00 CST 2021, userEmail=99@163.com====expression====, cardName=null)
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• 源代码

目的是达到了，但是总感觉有点复杂，还要创建一个注解。还不如直接使用 expression。

@Named注解

• MapStructConverter 内增加一个不带 public 的JoinUtil3类，在要被执行的方法上加注解 @Named并指定名称。

class JoinUtil3{

    @Named("join123")
    public static String join(String oldStr){
        return oldStr + "====expression====";
    }
    
	@Named("join1234")
    public static String join1(String oldStr){
        return oldStr + "====另外一种拼接方式====";
    }
}
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• MapStructConverter 进行修改，uses属性指向 JoinUtil3，再增加一个方法：testNamed，在userEmail映射关系上新增qualifiedByName属性，该属性的值要与 @Named的值保持一致，根据该值来判断需要执行那个方法。

@Mapper(imports = {JoinUtil.class},uses = {JoinUtil3.class})

  @Mappings({
            @Mapping(source = "id",target = "id",ignore = true),
            @Mapping(source = "birthday", target = "userBirthday",dateFormat = "yyyy-MM-dd HH:mm:ss"),
            @Mapping(source = "time", target = "userTime",dateFormat = "yyyy-MM-dd"),
            @Mapping(source="email",target = "userEmail",qualifiedByName = "join123"),
            @Mapping(source = "card.userCardName", target = "cardName")
    }
    )
    UserDTO testNamed(UserDO userDO);
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• 测试

	@Test
    void test9(){
        UserDO userDo = new UserDO();
        userDo.setId(8888L);
        userDo.setUserName("gongjie");
        userDo.setBirthday(new Date());
        userDo.setTime("2021-05-09");
        userDo.setEmail("99@163.com");

        System.out.println("userDo = "+ userDo.toString());
        UserDTO userDTO = MapStructConverter.INSTANCE.testNamed(userDo);
        System.out.println("userDTO = "+ userDTO.toString());
    }

结果：
userDo = UserDO(id=8888, userName=gongjie, birthday=Mon May 10 22:41:43 CST 2021, time=2021-05-09, email=99@163.com, card=null)
    
userDTO = UserDTO(id=null, userName=gongjie, userBirthday=2021-05-10 22:41:43, userTime=Sun May 09 00:00:00 CST 2021, userEmail=99@163.com====expression====, cardName=null)
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注册到Spring

@Mapper注解中有个属性为 componentModel，componentModel的值有四种：

• default: 默认值，不使用组件类型, 通过Mappers.getMapper(Class)方式获取实例对象

• cdi: 生成的映射是一个应用程序范围的 cdi 的bean，可以通过@Inject进行检索

• spring: 生成的实现类上面会自动添加一个@Component注解，可以通过 Spring 的 @Autowired方式进行注入

• jsr330: 生成的实现类上会添加@javax.inject.Named注解，可以通过 @Inject 注解获取。

@Mapper(imports = {JoinUtil.class},uses = {JoinUtil3.class},componentModel = "spring")
public interface MapStructConverter {

    MapStructConverter INSTANCE = Mappers.getMapper(MapStructConverter.class);
}

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不过我一般两种都写，都可以使用。嘿嘿！

• 源代码

• 测试

   @Autowired
    private MapStructConverter mapStructConverterAutowired;

    @Test
    void test9(){
        UserDO userDo = new UserDO();
        userDo.setId(8888L);
        userDo.setUserName("gongjie");
        userDo.setBirthday(new Date());
        userDo.setTime("2021-05-09");
        userDo.setEmail("99@163.com");

        System.out.println("userDo = "+ userDo.toString());
        UserDTO userDTO = mapStructConverterAutowired.testNamed(userDo);
        System.out.println("userDTO = "+ userDTO.toString());
    }
结果：
userDo = UserDO(id=8888, userName=gongjie, birthday=Tue May 11 00:07:31 CST 2021, time=2021-05-09, email=99@163.com, card=null)
    
userDTO = UserDTO(id=null, userName=gongjie, userBirthday=2021-05-11 00:07:31, userTime=Sun May 09 00:00:00 CST 2021, userEmail=99@163.com====另外一种拼接方式====, cardName=null)    
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