Question

我正在尝试实现一个自动增加“版本”列值的DAO。该版本不是主要关键。问题是我之前尝试过“@GeneratedValue”值，但它没有用，而且sql给出了一条消息这只允许一个主键列，所以我不得不使用lastUpdate值并增加它是一个值，但这不起作用，并且它继续为插入值传递1。

这是我的代码：

@Repository
public class VersionDAO {

    @PersistenceContext
    private EntityManager em;

    public void create(Version version) {
        Version lastUpdate = (Version) em.createQuery("SELECT c FROM Version c ORDER BY c.uploadedtimestamp DESC")//
        .setMaxResults(1).getResultList();
        int ver = lastUpdate.getVersion() + 1;
        version.setVersion(ver);

        em.persist(version);
    }

    public void delete(Version version) {
        em.remove(version);
    }
}

@Entity
@Table(name = "version")
public class Version implements Serializable {
    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id")
    private Integer id;


    @Column(name = "version")   
    private Integer version;

    @Column(name="uploadedtimestamp")
    @Temporal(TemporalType.TIMESTAMP)
    private Date uploadedtimestamp = new Date();        

    public Integer getId() {
        return id;
    }

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

    public Integer getVersion() {
        return version;
    }

    public void setVersion(Integer version) {
        this.version = version;
    }

    @Override
    public boolean equals(Object obj) {
        if (obj == null) {
            return false;
        }
        if (obj == this) {
            return true;
        }
        if (obj.getClass() != getClass()) {
            return false;
        }
        Version cdi = (Version) obj;
        return new EqualsBuilder().append(getId(), cdi.getId()).isEquals();
    }

    @Override
    public int hashCode() {
        return new HashCodeBuilder(17, 37).append(getId()).toHashCode();
    }

    public Date getUploadedtimestamp() {
        return uploadedtimestamp;
    }

    public void setUploadedtimestamp(Date uploadedtimestamp) {
        this.uploadedtimestamp = uploadedtimestamp;
    }
}

Answer 1

在JPA 2.0中，有@Version注释。

您只需将注释添加到您希望成为版本字段的字段中，如下所示：

@Entity
public class MyEntity implements Serializable {    

    @Id
    @GeneratedValue
    private Long id;

    private String name;

    @Version
    private Long version;

    //...
}

看看：Java - JPA - @Version annotation

Answer 2

尝试选择最大版本，并更新值+ 1.

public void incrementVersion() {
        Version lastUpdate = (Version) em.createQuery("SELECT max (c.version) FROM X c")          .setMaxResults(1).getResultList();
        int ver = lastUpdate.getVersion() + 1;
        lastUpdate.setVersion(ver);

        em.persist(lastUpdate);
    }

Answer 3

我认为你不应该通过JPA来解决这个问题。您可以为表格创建触发器以自动增加＆＃34;版本＆＃34;每次更新行时的值。

如果你以MySQL为例：

CREATE TRIGGER version_trigger BEFORE UPDATE ON `your_table`
FOR EACH ROW
SET NEW.version = (OLD.version + 1)

JPA将列的值增加1

3 个答案: