休眠:是否可以修改生成的值

Question

我有一个使用 uuid 作为PRIMARY KEY的实体，它被生成

@Id
@Column(name = "uuid")
@GeneratedValue(generator = "uuid2")
@GenericGenerator(name = "uuid2", strategy = "uuid2")
private UUID uuid;

如何告诉 Hibernate 使用显式值，或者我可以修改 Hibernate 生成的值？

编辑：我的问题是，在大多数情况下，生成值很好，但在某些情况下，我想创建一个具有显式uuid的实体。

我的解决方法（迪帕利·瓦萨尼在下面建议

）

@Id
@Column(name = "uuid")
private UUID uuid;
/**
 * if defined, this will be used instead of the generated one
 */
private transient UUID preferredUuid;
@PrePersist
private void initializeUuid() {
    if (preferredUuid != null) {
        uuid = preferredUuid;
    }
    else {
        uuid = UUID.randomUUID();
    }
}

Answer 1

选项 1： 请参考这个问题：

JPA 是否有休眠"@GenericGenerator"之类的东西来生成自定义 ID？

答案是：

https://stackoverflow.com/a/7462096/1982680

他们使用过@PrePersist

你可以像这样使用它：

  @PrePersist
  public void ensureUuid() {
    uuid= ...
  }

选项 2 ：创建自定义序列

import java.util.UUID;
import java.util.Vector;
import org.eclipse.persistence.config.SessionCustomizer;
import org.eclipse.persistence.internal.databaseaccess.Accessor;
import org.eclipse.persistence.internal.sessions.AbstractSession;
import org.eclipse.persistence.sequencing.Sequence;
import org.eclipse.persistence.sessions.Session;
public class UUIDSequence extends Sequence implements SessionCustomizer {
    public UUIDSequence() {
        super();
    }
    public UUIDSequence(String name) {
        super(name);
    }
    @Override
    public Object getGeneratedValue(Accessor accessor,
            AbstractSession writeSession, String seqName) {
        return UUID.randomUUID().toString().toUpperCase();
    }
    @Override
    public Vector getGeneratedVector(Accessor accessor,
            AbstractSession writeSession, String seqName, int size) {
        return null;
    }
    @Override
    protected void onConnect() {
    }
    @Override
    protected void onDisconnect() {
    }
    @Override
    public boolean shouldAcquireValueAfterInsert() {
        return false;
    }
    @Override
    public boolean shouldOverrideExistingValue(String seqName,
            Object existingValue) {
        return ((String) existingValue).isEmpty();
    }
    @Override
    public boolean shouldUseTransaction() {
        return false;
    }
    @Override
    public boolean shouldUsePreallocation() {
        return false;
    }
    public void customize(Session session) throws Exception {
        UUIDSequence sequence = new UUIDSequence("system-uuid");
        session.getLogin().addSequence(sequence);
    }
}

注册序列

定制器可以在 API 中指定为持久性单元属性：

properties.put(PersistenceUnitProperties.SESSION_CUSTOMIZER,
                "eclipselink.example.UUIDSequence");

或在 XML 中：

<property name="eclipselink.session.customizer" value="eclipselink.example.UUIDSequence"/>

使用序列

@Id
@GeneratedValue(generator="system-uuid")
@Column(name="uuid")
private UUID uuid;