MappedSuperclass -在子类中更改SequenceGenerator

Question

我在Hibernate中使用JPA2，并尝试为我的实体引入一个公共基类。到目前为止，它看起来像这样:

@MappedSuperclass
public abstract class BaseEntity {
    @Id
    private Long id;
    @Override
    public int hashCode() {
        // ...
    }
    @Override
    public boolean equals(Object obj) {
        // ...
    }
    public Long getId() {
        return this.id;
    }
    public void setId(Long id) {
        this.id = id;
    }
}

然而，对于每个表都有一个序列$entityname_seq，我想用它作为我的序列生成器。如何从子类中设置呢?我认为我需要重写@GeneratedValue，并创建一个新的SequenceGenerator与@SequenceGenerator。

Answer 1

是有可能的。您可以使用@SequenceGenerator注释覆盖默认的生成器名称。

基类

    @MappedSuperclass
    public abstract class PersistentEntity implements Serializable
    {
        private static final long serialVersionUID = 1L;
        @Id
        @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "default_gen")
        protected Long id = 0L;
        public Long getId()
        {
            return id;
        }
        public void setId(Long id)
        { 
            this.id = id;
        }
    }

序列(SQL)

• create sequence role_seq;

派生类

    @Entity
    @Table(name = "role")
    @SequenceGenerator(name = "default_gen", sequenceName = "role_seq", allocationSize = 1)
    public class Role extends PersistentEntity implements Serializable
    {
        private static final long serialVersionUID = 1L;
        @NotNull
        @Size(max = 32)
        private String name;
        public String getName()
        {
             return name;
        }
        public void setName(String name)
        {
             this.name = name;
        }   
    }

• 这种方法在Hibernate 4.1中工作得很好。

编辑

• 根据评论，它似乎与EclipseLink 2.6.1-RC1一起工作。

Answer 2

在JPA中不能使用注释完成。注释本身不能被重写。实体从MappedSuperClass继承所有映射信息。只有两种注释可以用来重新定义从映射的superClass继承的映射:

1. AttributeOverride覆盖列映射和
2. AssociationOverride重写连接列/表

它们都不能帮助GeneratedValue

Answer 3

使用EclipseLink，您可以使用Customizer。DescriptorCustomizer接口定义了一种自定义jpa描述符(又名持久实体)的所有信息的方法。

public class SequenceCustomizer implements DescriptorCustomizer {
    @Override
    public void customize(ClassDescriptor descriptor) throws Exception {
        descriptor.setSequenceNumberName(descriptor.getTableName());
    }
}

和映射的父类:

@MappedSuperclass
@Customizer(SequenceCustomizer.class)
public abstract class AbstractEntity implements Serializable {
    ...
}

Answer 4

我写这个，因为它变得太不可读的评论接受的答案:

我有一个BaseEntity，每个其他实体继承自:

BaseEntity.java:

@MappedSuperclass
public abstract class BaseEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "SEQ_ID")
    private Long id;

然后我有两个实体User和Order，都继承自BaseEntity，同时也有@SequenceGenerator注释:

User.java:

@SequenceGenerator(name = "SEQ_ID", sequenceName = "SEQ_USER", allocationSize = 1)
public class User extends BaseEntity { ... }

Order.java:

@SequenceGenerator(name = "SEQ_ID", sequenceName = "SEQ_ORDER", allocationSize = 1)
public class Order extends BaseEntity { ... }

它至少在H2上使用2个序列SEQ_USER和SEQ_ORDERS:

select SEQ_USER.nextval from dual;
select SEQ_ORDERS.nextval from dual;