JPA: how to make a field auto increment

Question

I use Eclipselink as my JPA provider, how can I make a field auto increment?

Answer 1

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.

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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;
    }
}

Answer 2

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

Answer 3

@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.