java - 当 ID 为 null 并在数据库中自动生成时持久化实体

Question

我将 EclipseLink 与 JavaDB 一起使用，每次我尝试保留新用户实体时，我都会收到与新用户实体的 ID 字段为空这一事实相关的错误。我知道它是空的——这应该是因为 ID 字段是作为数据库中的身份生成的。还是我错了？我已经尝试将 @GeneratedValue 添加到实体，但无论使用何种生成策略，都会导致错误。我也看过 here , here , 和 here但这些问题似乎与我的问题并不完全相关。下面的异常总是在调用 EntityManager.persist(user) 时发生。

我还尝试删除@NotNull 和@GeneratedValue，奇怪的是，它通过了持久调用——但只是在提交时失败。据我所知，我已经把一切都设置好了。所以问题是:为什么这个应用程序无法添加新用户，我需要做什么来修复它？

用户实体:

 @Entity(name = "User")
 @Table(name = "USERS")
 public class User implements Serializable {
     private static final long serialVersionUID = 1L;
     @Id
     @Basic(optional = false)
     @NotNull
     @Column(name = "IDENTIFIER", nullable = false,unique = true)
     private Long identifier;
     // class shortened for brevity
}

对应这个SQL脚本:

create table "USERSDBADMIN".USERS
(
        IDENTIFIER BIGINT not null primary key GENERATED ALWAYS AS IDENTITY 
            (START WITH 0, INCREMENT BY 1),
        USERNAME VARCHAR(50) not null,
        EMAIL VARCHAR(150) not null,
        LASTLOGIN DATE default NULL
);

用户创建方法(来自 JPA Controller ):

public void create(User user) throws PreexistingEntityException, RollbackFailureException, Exception {
    EntityManager em = null;
    try {
        utx.begin();
        em = getEntityManager();
        em.persist(user);
        utx.commit();
    } catch (Exception ex) {
        try {
            utx.rollback();
        } catch (Exception re) {
            throw new RollbackFailureException("An error occurred attempting to roll back the transaction.", re);
        }
        if (findUser(user.getIdentifier()) != null) {
            throw new PreexistingEntityException("User " + user + " already exists.", ex);
        }
        throw ex;
    } finally {
        if (em != null) {
            em.close();
        }
    }
}

GenerationStrategy.Identity 错误(基本上是说 ID 字段为空):

ex = (javax.validation.ConstraintViolationException) javax.validation.ConstraintViolationException: Bean Validation constraint(s) violated while executing Automatic Bean Validation on callback event:'prePersist'. Please refer to embedded ConstraintViolations for details.

GenerationStrategy.Sequence 错误(Table 和 Auto 产生相同的异常):

ex = (org.eclipse.persistence.exceptions.DatabaseException) Exception [EclipseLink-4002] (Eclipse Persistence Services - 2.5.2.v20140319-9ad6abd): org.eclipse.persistence.exceptions.DatabaseException. Internal Exception: java.sql.SQLSyntaxErrorException: SEQUENCE 'SEQ_GEN_SEQUENCE' does not exist. Call: VALUES(NEXT VALUE FOR SEQ_GEN_SEQUENCE) Query: ValueReadQuery(sql="VALUES(NEXT VALUE FOR SEQ_GEN_SEQUENCE)")

Answer 1

不要对使用GenerationType.IDENTITY 策略生成的@Id 属性使用@NotNull 约束。问题是 @NotNull 和其他 JSR-303 约束在提交之前被验证。直到提交完成后，自动递增的标识符才可用。

试试这个，不再关心 ID 初始化:

 @Id
 @GeneratedValue(strategy = GenerationType.IDENTITY)
 @Column(name = "IDENTIFIER", nullable = false, unique = true)
 private Long identifier;