MappedSuperclass - Change SequenceGenerator in Sub

问题:

I'm using JPA2 with Hibernate and try to introduce a common base class for my entities. So far it looks like that:

@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;
    }
}

However, for every table theres a sequence $entityname_seq which I want to use as my sequence generator. How can I set that from my subclass? I think I need to override @GeneratedValue and create a new SequenceGenerator with @SequenceGenerator.

回答1:

Yes, it is possible. You can override the default generator name with the @SequenceGenerator annotation.

• Base class

    @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;
        }
    }

• Sequence (SQL)

  create sequence role_seq;

• Derived class

    @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;
        }   
    }

• This approach worked fine in Hibernate 4.1.x, but it didn't in EclipseLink 2.x.

edit

• As per the comment, it seems to be working with EclipseLink 2.6.1-RC1.

回答2:

In JPA that cannot be done with annotations. Annotation itself cannot be overridden. Entity inherits all the mapping information from MappedSuperClass. There is only two annotations that can be used to redefine mappings inherited from mapped superClass:

1. AttributeOverride to override column mappings and
2. AssociationOverride to override join columns / table.

Neither of them helps with GeneratedValue.

回答3:

With EclipseLink, you can use a Customizer. DescriptorCustomizer interface defines a way to customize all the information about a jpa descriptor (aka a persistent entity).

public class SequenceCustomizer implements DescriptorCustomizer {

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

and in your mapped superclass:

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