JPA: how to make a field auto increment

I use Eclipselink as my JPA provider, 开发者_如何学编程how can I make a field auto increment?

I just want an auto-increment field for 1 entity.

Yes, I get that, this is what you wrote in your question. But it can be interpreted in several ways and repeating the same thing without clarifying doesn't help much :)

So does Pascal's way work or I have to do what Bytecode suggest, query select (max) counter + 1 from MyEntity, to get the next value, then set it to the design field, the persist?

If you want to increment a field per entity record (e.g. a "counter" for Order with id=1, another one for Order with id=2), what I suggested would work.

If you want a behavior similar to a primary key (i.e. an auto-incremented column), it won't. And in that case, standard JPA doesn't offer any particular facility (standard JPA only allows GenereatedValue on Id annotated field). The only way I can think of would be to insert another dedicated entity just to get the primary key from it.

Your exact requirement is unclear but I'm assuming you're talking about a random field, not the primary key. In that case, you could maybe use life cycle callbacks methods:

@Entity
public class MyEntity {
    ...
    private int counter;
    ...

    @PrePersist
    @PreUpdate
    protected void increment() {
         counter = counter + 1;
    }
}

JPA 2.0 allows you to set auto_increment in a non-id field as long as you define the auto increment column in your schema - Hibernate / EclipseLink will not do it for you:

`non_id` bigint(20) DEFAULT NULL AUTO_INCREMENT,

and then in your entity:

 @Column(name = "non_id")
 @GeneratedValue
 public void setId(Long id) {
   this.id = id;
 }

will do the trick.

@Harry: unfortunately it looks like that EclipseLink cannot make non-ID fields auto-incremented. But looks like OpenJPA does have this feature:

OpenJPA allows you to use the GeneratedValue annotation on any field, not just identity fields. Before using the IDENTITY generation strategy, however, read Section 3.4, “ Autoassign / Identity Strategy Caveats ” in the Reference Guide.

If you are using mysql (not sure if it works for other DBs) and make the column auto increment from DB side, following code should work

@Column(name = "counter", unique = true, insertable = false, updatable = false)
@Generated(value = GenerationTime.INSERT)
private Long counter;

Some JPA impls allow you to set GeneratedValue on the field whether it is PK or not