Hibernate/JPA在合并后使用数据库中的默认值获取列的值

Question

案例：我想在合并对象后获取映射到数据库端默认值列的字段值。

表：

CREATE TABLE IF NOT EXISTS `db`.`OBJECT` (
  `OBJECT_ID` INT NOT NULL AUTO_INCREMENT,
  `NAME` VARCHAR(64) NULL,
  `CREATED_ON` DATETIME NULL DEFAULT CURRENT_TIMESTAMP,
  `EDITED_ON` DATETIME NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
)

类别：

@Entity
@Table(name="OBJECT")
public class SomeObject {
    private int id;
    private String name;
    private Timestamp createdOn;
    private Timestamp editedOn;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "ID")
    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    @Column(name = "NAME")
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Column(name = "CREATED_ON", insertable = false, updatable = false)
    public Timestamp getCreatedOn() {
        return createdOn;
    }

    public void setCreatedOn(Timestamp createdOn) {
        this.createdOn = createdOn;
    }

    @Column(name = "EDITED_ON", insertable = false, updatable = false)
    public Timestamp getEditedOn() {
        return editedOn;
    }

    public void setEditedOn(Timestamp editedOn) {
        this.editedOn = editedOn;
    }
}

DAO：

@Transactional
public class ObjectDAO {
    @PersistenceContext
    private EntityManager entityManager;

    public someObject merge(someObject someObject) {
        return entityManager.merge(someObject);
    }
}

我想做什么：

SomeObject someObject = new SomeObject();
someObject.setName("name");
SomeObject insertedObject = objectDAO.merge(someObject);
System.out.println(insertedObject.getId());
System.out.println(insertedObject.getEditedOn());

结果：对象被插入到数据库中，所有默认值都正确生成。

问：为什么第二个println打印null，而第一个println打印插入数据库的Id的正确值？

Answer 1

经过更多的研究，我发现若我使用默认的数据库值，那个么在从合并返回的实体中就不可能有它们。就这么简单。我更改apporach，并使用PrePersist和PreUpdate注释。应该是这样的：

@Entity
@Table(name="OBJECT")
public class SomeObject {
    private int id;
    private String name;
    private Timestamp createdOn;
    private Timestamp editedOn;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "ID")
    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    @Column(name = "NAME")
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Column(name = "CREATED_ON")
    public Timestamp getCreatedOn() {
        return createdOn;
    }

    public void setCreatedOn(Timestamp createdOn) {
        this.createdOn = createdOn;
    }

    @Column(name = "EDITED_ON")
    public Timestamp getEditedOn() {
        return editedOn;
    }

    public void setEditedOn(Timestamp editedOn) {
        this.editedOn = editedOn;
    }

    @PrePersist
    public void setDates() {
        createdOn = LocalDateTime.now();
        editedOn = LocalDateTime.now();
    }

    @PreUpdate
    public void setEditedOn() {
        editedOn = LocalDateTime.now();
    }
}

为什么返回的实体中有正确的id？因为它实际上是由Hibernate生成的GeneratedValue（策略=GenerationType.IDENTITY）

Answer 2

在执行这一行之前：

System.out.printlnbject.getEditedOn（）；

您必须通过调用entityManager来“刷新”（同步）更新。冲洗（）