Spring JPA自定义查询结果的接收方式

//构建实体类
@Getter
@Setter
@Entity
@Builder
@NoArgsConstructor(access = AccessLevel.PROTECTED)
@AllArgsConstructor(access = AccessLevel.PRIVATE)
@Table(name = "user")
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String username;
    private Integer age;
    private String address;
}
//继承JPA
public interface UserRepository extends JpaRepository<User,Long> {
}
//调用测试
@RunWith(SpringRunner.class)
@SpringBootTest
class UserTest {
    @Autowired
    UserRepository userRepository;
    @Test
    void testUser() {
        User newUser = User.builder()
				        .id(1L)
				        .username("zhangsan")
				        .age(18)
				        .address("北京市").build();
        User savedUser = userRepository.save(newUser);
        assertEquals("zhangsan",savedUser.getUsername());
    }
}

//创建个性化的DTO用于接收
@Value
public class UserDTO {
    Long id;
    String username;
}
//写sql语句
public interface UserRepository extends JpaRepository<User,Long> {
    //@Query("select u from User u where u.id =?1")
    //是像这样，还是怎么写呢？
    UserDTO findUserDTOByID(Long id);
}
//调用
@RunWith(SpringRunner.class)
@SpringBootTest
class UserTest {
    @Autowired
    UserRepository userRepository;

    @Test
    void testUserDTO() {
        UserDTO dto = userRepository.findUserDTOByID(1L);
        assertEquals("zhangsan",dto.getUsername());
        
    }
}

//1.意图User实体自动映射某些属性到UserDTO
public interface UserRepository extends JpaRepository<User,Long> {
    @Query("select u from User u where u.id =?1")
    UserDTO findUserDTOByID(Long id);
    //但是会报org.springframework.core.convert.ConverterNotFoundException错误
}
//2.意图挑出User的某些字段会自动映射到UserDTO
public interface UserRepository extends JpaRepository<User,Long> {
    @Query("select u.id,u.username from User u where u.id =?1")
    UserDTO findUserDTOByID(Long id);
    //但是会报org.springframework.core.convert.ConverterNotFoundException错误
}
//3.意图挑出User的某些字段会自动映射到UserDTO
public interface UserDTORepository extends JpaRepository<UserDTO,Long> {
    @Query("select u.id,u.username from User u where u.id =?1")
    UserDTO findByID(Long id);
    //但是这会生成一个新表
}

//1.使用Object接收
public interface UserRepository extends JpaRepository<User,Long> {
    @Query("select u.id,u.username from User u where u.id =?1")
    Object findUserDTOByID(Long id);
    //倒是能接收到结果，但是丢失了属性名称，必须数组的索引访问，不方便
}
//2.使用全限定名接收
public interface UserRepository extends JpaRepository<User,Long> {
    @Query("select new com.example.admin.ums.domain.user.UserDTO(id,username) from User u where u.id =?1")
    UserDTO findUserDTOByID(Long id);
    //能接收到结果，但是UserDTO必须有构造函数，带上所有参数，也不方便
}
//3.定义接口来接收，使用的是projections接口投影机制
public interface IUser {
	//定义这些getter方法才能接收结果
    Long getId();
    String getUsername();
}
//用接口接收查询结果
public interface UserRepository extends JpaRepository<User,Long> {
    @Query("select u.id as id ,u.username as username  from User u where u.id =?1")
    IUser findUserDTOByID(Long id);
    //能查询到结果，但是必须用接口接收，可能不习惯
}
//4.泛型动态查询投影
public interface UserRepository extends JpaRepository<User,Long> {
    <T> T  findById(Long id, Class<T> type);
    //只需要输入ID和类类型就能查到结果，这个利用了方法名的查询生成器机制，不用专门写@Query
    //同时使用动态查询投影，所以不用输入很多参数，方便了很多，强烈推荐
}
//5.用Map接收查询的结果
public interface UserRepository extends JpaRepository<User,Long> {
    @Query("select u.id as id ,u.username as username, u.address as address  from User u where u.id =?1")
    Map<String,Object> findUserDTO(Long id);
    //需要写JPQL语句，必须用as取别名，否则就没有key值
    //优点是不用构造DTO直接输出给前端
    //缺点是查出来的不是对象，不方便再处理业务逻辑，若参数很多，就会很繁琐    
}