JPA将查询结果转换为DTO对象

前言

JPA支持使用@Query自定义查询，查询的结果需要字节用DTO对象接收，如果使用HQL的查询语句，可以将直接将DTO对象的构造方法传入hql中，直接转为DTO对象；而如果使用native sql查询的方式，只能将返回结果用Object[]对象接收，然后DTO设置对象的构造来接收Object[]里面的参数完成DTO对象的转换。

例子

mysql数据库表

用户表

CREATE TABLE `pos_user` (
  `id` bigint(20) NOT NULL AUTO_INCREMENT,
  `user_name` varchar(50) CHARACTER SET utf8 COLLATE utf8_bin DEFAULT NULL,
  `user_pwd` varchar(255) DEFAULT NULL,
  `user_type` int(11) DEFAULT NULL,
  `parent_id` bigint(20) DEFAULT NULL,
  `user_status` int(11) DEFAULT NULL,
  `distributor_id` bigint(20) DEFAULT NULL,
  `creator_identity_type` int(2) DEFAULT NULL,
  `creator_id` bigint(20) DEFAULT NULL,
  `create_date` varchar(50) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=22 DEFAULT CHARSET=utf8;

设备表

CREATE TABLE `pos_device` (
  `id` bigint(20) NOT NULL AUTO_INCREMENT,
  `imei` varchar(120) NOT NULL,
  `mac` varchar(120) NOT NULL,
  `unique_code` varchar(120) CHARACTER SET utf8 COLLATE utf8_bin DEFAULT NULL,
  `type` varchar(100) DEFAULT NULL,
  `system_version` varchar(100) DEFAULT NULL,
  `distributor_id` bigint(20) DEFAULT NULL,
  `creator_identity_type` int(2) DEFAULT NULL,
  `creator_id` bigint(20) DEFAULT NULL,
  `create_date` varchar(50) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=31 DEFAULT CHARSET=utf8;

用户和设备关联表

CREATE TABLE `pos_user_device_relation` (
  `id` bigint(20) NOT NULL AUTO_INCREMENT,
  `device_id` bigint(20) DEFAULT NULL,
  `user_id` bigint(20) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=8 DEFAULT CHARSET=utf8;

可以看到用户和设备关联表中有用户id和设备id

联合查询的需求

想列出pos_user_device_relation表中所有pos_user的distributor_id=1的所有用户和设备，要求返回的信息包括用户的username、type信息和设备的imei、mac等信息。

sql语句

SELECT
pdr.id,
pdr.device_id,
pd.imei,
pd.mac,
pd.unique_code,
pd.type,
pd.system_version,
pdr.user_id,
pu.user_name,
pu.user_type
FROM
pos_user_device_relation pdr, pos_user pu, pos_device pd
WHERE pdr.device_id = pd.id AND pdr.user_id = pu.id AND pdr.user_id in (select id from pos_user where distributor_id=1) limit 0,10

查询可以正常得到结果，结果行是这样的：

+----+-----------+---------------------+-------------------+--------------------------+----------+----------------+---------+---------------+-----------+
| id | device_id | imei                | mac               | unique_code              | type     | system_version | user_id | user_name     | user_type |
+----+-----------+---------------------+-------------------+--------------------------+----------+----------------+---------+---------------+-----------+

如何在JPA中映射为DTO对象

DTO对象字段定义如下：

private Long posUserDeviceId;
private Long deviceId;
private String deviceImei;
private String deviceMac;
private String deviceUniqueCode;
private String deviceType;
private String deviceSystemVersion;
private Long userId;
private String username;
private PosUserEntityConstants.UserType userType;

对象中的PosUserEntityConstants.UserType是一个自定义转换类型，通过继承AttributeConverter将Integer转换为UserType的枚举。

方法一：使用HQL的方法

Repository的查询代码如下：

@Query(
        value = "SELECT
" +
                "new com.hengbao.ethiopiatelecomrecharge.dao.dto.PosUserDeviceRelationDto(
" +
                "pdr.id,
" +
                "pdr.deviceId,
" +
                "pd.imei,
" +
                "pd.mac,
" +
                "pd.uniqueCode,
" +
                "pd.type,
" +
                "pd.systemVersion,
" +
                "pdr.userId,
" +
                "pu.userName,
" +
                "pu.userType
" +
                ") 
" +
                "FROM 
" +
                "PosUserDeviceRelationEntity pdr, PosUserEntity pu, PosDeviceEntity pd 
" +
                "WHERE pdr.deviceId = pd.id AND pdr.userId = pu.id AND pdr.userId in (select id from PosUserEntity where distributorId=?1)",
        countQuery = "SELECT count(*) FROM 
" +
                "PosUserDeviceRelationEntity pdr, PosUserEntity pu, PosDeviceEntity pd 
" +
                "WHERE pdr.deviceId = pd.id AND pdr.userId = pu.id AND pdr.userId in (select id from PosUserEntity where distributorId=?1)"
)
Page<PosUserDeviceRelationDto> findUserAndDeviceInfoByDistributorId(Long distributorId, Pageable pageable);

可以看到HQL的方法将PosUserDeviceRelationDto的构造器直接传入到HQL语句中，省去了我们自行转换的麻烦。那么PosUserDeviceRelationDto中也要重写一个相应的构造器：
由于项目中使用了lombok，所有最终dto的代码只是在类上面加上了一些注解，@AllArgsConstructor的注解会自动生成一个全参数的构造器，构造器的顺序和字段定义顺序一致，类代码如下：

@Getter
@Setter
@NoArgsConstructor
@AllArgsConstructor
@ToString
public class PosUserDeviceRelationDto implements Serializable {
    /**
     * 版本号
     */
    private static final long serialVersionUID = 1L;

    private Long posUserDeviceId;

    private Long deviceId;
    private String deviceImei;
    private String deviceMac;
    private String deviceUniqueCode;
    private String deviceType;
    private String deviceSystemVersion;

    private Long userId;
    private String username;
    private PosUserEntityConstants.UserType userType;
}

方法二：使用native query的方式查询并转换为dto

Repository的查询代码如下：

@Query(
        value = "SELECT
" +
                "pdr.id,
" +
                "pdr.device_id,
" +
                "pd.imei,
" +
                "pd.mac,
" +
                "pd.unique_code,
" +
                "pd.type,
" +
                "pd.system_version,
" +
                "pdr.user_id,
" +
                "pu.user_name,
" +
                "pu.user_type
" +
                "FROM
" +
                "pos_user_device_relation pdr, pos_user pu, pos_device pd
" +
                "WHERE pdr.device_id = pd.id AND pdr.user_id = pu.id AND pdr.user_id in (select id from pos_user where distributor_id=?1)",
        countQuery = "SELECT count(*) FROM
" +
                "pos_user_device_relation pdr, pos_user pu, pos_device pd
" +
                "WHERE pdr.device_id = pd.id AND pdr.user_id = pu.id AND pdr.user_id in (select id from pos_user where distributor_id=?1)",
        nativeQuery = true
)
Page<Object[]> findUserAndDeviceInfoByDistributorId2(Long distributorId, Pageable pageable);

可以看到这样只能用Object[]来接收结果集，而不能直接将返回参数定义为PosUserDeviceRelationDto对象，否则会报no converter的异常。
那如何将Object[]的结果集转换为PosUserDeviceRelationDto对象呢？
首先先看一下Object[]每个对象的类型：BigInteger BigInteger String String String String String BigInteger String Integer
这是可以发现虽然mysql数据库定义的是bigint(20)类型，但是结果集是BigInteger，不能直接用Long接收，所以专门定义一个dto的构造器如下：

public PosUserDeviceRelationDto(BigInteger posUserDeviceId,
                                BigInteger deviceId,
                                String deviceImei,
                                String deviceMac,
                                String deviceUniqueCode,
                                String deviceType,
                                String deviceSystemVersion,
                                BigInteger userId,
                                String username,
                                Integer userType) {
    this.posUserDeviceId = posUserDeviceId == null ? null : posUserDeviceId.longValue();
    this.deviceId = deviceId == null ? null : deviceId.longValue();
    this.deviceImei = deviceImei;
    this.deviceMac = deviceMac;
    this.deviceUniqueCode = deviceUniqueCode;
    this.deviceType = deviceType;
    this.deviceSystemVersion = deviceSystemVersion;
    this.userId = userId == null ? null : userId.longValue();
    this.username = username;
    // UserTypeConverter是继承自javax.persistence.AttributeConverter的类型转换器
    this.userType = new PosUserEntityConstants.UserTypeConverter().convertToEntityAttribute(userType);
}

然后直接调用构造即可：

Page<Object[]> userAndDeviceInfoByDistributorId2 = posUserDeviceRelationRepository.findUserAndDeviceInfoByDistributorId2(1L, PageRequest.of(0, 10));
for (Object[] objects : userAndDeviceInfoByDistributorId2.getContent()) {
    // 转换成dto的方法一：将objects中的所有参数强转为对应类型，传递到dto的构造器中；dto对象定义好对应的构造器
    PosUserDeviceRelationDto dto1 = new PosUserDeviceRelationDto(
            (BigInteger) objects[0],
            (BigInteger) objects[1],
            (String    ) objects[2],
            (String    ) objects[3],
            (String    ) objects[4],
            (String    ) objects[5],
            (String    ) objects[6],
            (BigInteger) objects[7],
            (String    ) objects[8],
            (Integer   ) objects[9]);
    System.out.println(dto1);

网上还能搜到另外一种解决方法，就是通过反射的方法简化dto的转化步骤(https://blog.csdn.net/qq_36144258/article/details/80296512)，但是csdn上这个存在bug，如果返回的objects数组中有一个值为null，那么getClass()方法获取类的类型就会报错，所以改为将每个参数的类型直接传入进去，可以这样使用反射其实省不了多少工夫了：

Page<Object[]> userAndDeviceInfoByDistributorId2 = posUserDeviceRelationRepository.findUserAndDeviceInfoByDistributorId2(1L, PageRequest.of(0, 10));
for (Object[] objects : userAndDeviceInfoByDistributorId2.getContent()) {
    // 转换成dto的方法二：反射的方法直接调用构造
    PosUserDeviceRelationDto dto2 = caseDto(objects, new Class[]{BigInteger.class,
                    BigInteger.class,
                    String.class,
                    String.class,
                    String.class,
                    String.class,
                    String.class,
                    BigInteger.class,
                    String.class,
                    Integer.class},
            PosUserDeviceRelationDto.class);
    System.out.println(dto2);
}

/**
 * https://blog.csdn.net/qq_36144258/article/details/80296512
 * 网页中直接使用objectArray中获取每一个class，但是这样有一个问题，就是如果获取的objectArray中有一个空值的话，不能获取到class，
 * 导致不能获取到对象的构造器
 * @param objectArray
 * @param objectClassArray
 * @param dtoClass
 * @param <T>
 * @return
 */
private <T> T caseDto(Object[] objectArray, Class[] objectClassArray, Class<T> dtoClass) throws Exception {
    Constructor<T> constructor = dtoClass.getConstructor(objectClassArray);
    return constructor.newInstance(objectArray);
}

例子涉及的部分源代码

Repository

@Query(
        value = "SELECT
" +
                "new com.hengbao.ethiopiatelecomrecharge.dao.dto.PosUserDeviceRelationDto(
" +
                "pdr.id,
" +
                "pdr.deviceId,
" +
                "pd.imei,
" +
                "pd.mac,
" +
                "pd.uniqueCode,
" +
                "pd.type,
" +
                "pd.systemVersion,
" +
                "pdr.userId,
" +
                "pu.userName,
" +
                "pu.userType
" +
                ") 
" +
                "FROM 
" +
                "PosUserDeviceRelationEntity pdr, PosUserEntity pu, PosDeviceEntity pd 
" +
                "WHERE pdr.deviceId = pd.id AND pdr.userId = pu.id AND pdr.userId in (select id from PosUserEntity where distributorId=?1)",
        countQuery = "SELECT count(*) FROM 
" +
                "PosUserDeviceRelationEntity pdr, PosUserEntity pu, PosDeviceEntity pd 
" +
                "WHERE pdr.deviceId = pd.id AND pdr.userId = pu.id AND pdr.userId in (select id from PosUserEntity where distributorId=?1)"
)
Page<PosUserDeviceRelationDto> findUserAndDeviceInfoByDistributorId(Long distributorId, Pageable pageable);
@Query(
        value = "SELECT
" +
                "pdr.id,
" +
                "pdr.device_id,
" +
                "pd.imei,
" +
                "pd.mac,
" +
                "pd.unique_code,
" +
                "pd.type,
" +
                "pd.system_version,
" +
                "pdr.user_id,
" +
                "pu.user_name,
" +
                "pu.user_type
" +
                "FROM
" +
                "pos_user_device_relation pdr, pos_user pu, pos_device pd
" +
                "WHERE pdr.device_id = pd.id AND pdr.user_id = pu.id AND pdr.user_id in (select id from pos_user where distributor_id=?1)",
        countQuery = "SELECT count(*) FROM
" +
                "pos_user_device_relation pdr, pos_user pu, pos_device pd
" +
                "WHERE pdr.device_id = pd.id AND pdr.user_id = pu.id AND pdr.user_id in (select id from pos_user where distributor_id=?1)",
        nativeQuery = true
)
Page<Object[]> findUserAndDeviceInfoByDistributorId2(Long distributorId, Pageable pageable);

DTO类

@Getter
@Setter
@NoArgsConstructor
@AllArgsConstructor
@ToString
public class PosUserDeviceRelationDto implements Serializable {
    /**
     * 版本号
     */
    private static final long serialVersionUID = 1L;

    private Long posUserDeviceId;

    private Long deviceId;
    private String deviceImei;
    private String deviceMac;
    private String deviceUniqueCode;
    private String deviceType;
    private String deviceSystemVersion;

    private Long userId;
    private String username;
    private PosUserEntityConstants.UserType userType;

    public PosUserDeviceRelationDto(BigInteger posUserDeviceId,
                                    BigInteger deviceId,
                                    String deviceImei,
                                    String deviceMac,
                                    String deviceUniqueCode,
                                    String deviceType,
                                    String deviceSystemVersion,
                                    BigInteger userId,
                                    String username,
                                    Integer userType) {
        this.posUserDeviceId = posUserDeviceId == null ? null : posUserDeviceId.longValue();
        this.deviceId = deviceId == null ? null : deviceId.longValue();
        this.deviceImei = deviceImei;
        this.deviceMac = deviceMac;
        this.deviceUniqueCode = deviceUniqueCode;
        this.deviceType = deviceType;
        this.deviceSystemVersion = deviceSystemVersion;
        this.userId = userId == null ? null : userId.longValue();
        this.username = username;
        // UserTypeConverter是继承自javax.persistence.AttributeConverter的类型转换器
        this.userType = new PosUserEntityConstants.UserTypeConverter().convertToEntityAttribute(userType);
    }
}

test测试类：

@Test
public void testFindUserAndDeviceInfoByDistributorId() throws Exception {
    System.out.println("-----------------hql query-----------------");
    Page<PosUserDeviceRelationDto> userAndDeviceInfoByDistributorId = posUserDeviceRelationRepository.findUserAndDeviceInfoByDistributorId(1L, PageRequest.of(0, 10));

    System.out.println("count=" + userAndDeviceInfoByDistributorId.getTotalElements());
    if(userAndDeviceInfoByDistributorId.getContent() != null) {
        for (PosUserDeviceRelationDto dto : userAndDeviceInfoByDistributorId.getContent()) {
            System.out.println(dto);
        }
    }

    System.out.println("-----------------native sql query-----------------");
    Page<Object[]> userAndDeviceInfoByDistributorId2 = posUserDeviceRelationRepository.findUserAndDeviceInfoByDistributorId2(1L, PageRequest.of(0, 10));
    System.out.println("count=" + userAndDeviceInfoByDistributorId2.getTotalElements());
    if(userAndDeviceInfoByDistributorId2.getContent() != null) {
        for (Object[] objects : userAndDeviceInfoByDistributorId2.getContent()) {
            for (Object obj : objects) {
                System.out.print(obj + "(" + (obj == null ? null : obj.getClass().getSimpleName()) + ") ");
            }
            System.out.println();


        }

        // 转换为dto 方法一
        System.out.println("-----转换dto的第一种方法-----");
        for (Object[] objects : userAndDeviceInfoByDistributorId2.getContent()) {
            // 转换成dto的方法一：将objects中的所有参数强转为对应类型，传递到dto的构造器中；dto对象定义好对应的构造器
            PosUserDeviceRelationDto dto1 = new PosUserDeviceRelationDto(
                    (BigInteger) objects[0],
                    (BigInteger) objects[1],
                    (String    ) objects[2],
                    (String    ) objects[3],
                    (String    ) objects[4],
                    (String    ) objects[5],
                    (String    ) objects[6],
                    (BigInteger) objects[7],
                    (String    ) objects[8],
                    (Integer   ) objects[9]);
            System.out.println(dto1);
        }

        // 转换为dto 方法二
        System.out.println("-----转换dto的第二种方法-----");
        for (Object[] objects : userAndDeviceInfoByDistributorId2.getContent()) {
            // 转换成dto的方法二：反射的方法直接调用构造
            PosUserDeviceRelationDto dto2 = caseDto(objects, new Class[]{BigInteger.class,
                            BigInteger.class,
                            String.class,
                            String.class,
                            String.class,
                            String.class,
                            String.class,
                            BigInteger.class,
                            String.class,
                            Integer.class},
                    PosUserDeviceRelationDto.class);
            System.out.println(dto2);
        }
    }
}

/**
 * https://blog.csdn.net/qq_36144258/article/details/80296512
 * 网页中直接使用objectArray中获取每一个class，但是这样有一个问题，就是如果获取的objectArray中有一个空值的话，不能获取到class，
 * 导致不能获取到对象的构造器
 * @param objectArray
 * @param objectClassArray
 * @param dtoClass
 * @param <T>
 * @return
 */
private <T> T caseDto(Object[] objectArray, Class[] objectClassArray, Class<T> dtoClass) throws Exception {
    Constructor<T> constructor = dtoClass.getConstructor(objectClassArray);
    return constructor.newInstance(objectArray);
}