JPA or Hibernate to generate a (non primary key) column value, not starting from 1

I want a JPA/Hibernate (preferably JPA) annotation that can generate the value of a column, that is not a primary key and it doesn't start from 1.

From what I have seen JPA cannot do that with @GeneratedValue and @SequenceGenerator and @TableGenerator. Or with anything else.

I have seen a solution with an extra table, which I find is not elegant.

I can live with a Hibernate annotation, because I already have hibernate annotations.

I want to use @Generated but I cannot make it work and people claim that it is possible.

@Generated(GenerationTime.INSERT)
private long invoiceNumber;//invoice number

Update: an extra requirement, if the transaction is rolled back, we can't have a gap in the numbering. Anyone?

Here's what worked for me - we coded all of it in the service. Here's the entity:

@Entity
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
public class Registrant extends AbstractEntity {
    //....
    private long invoiceNumber;//invoice number

    @Entity
    public static class InvoiceNumberGenerator {
        @Id
        @GeneratedValue
        private int id;
        private long counter;

        public int getId() {
            return id;
        }

        public void setId(int id) {
            this.id = id;
        }

        public long getCounter() {
            return counter;
        }

        public void setCounter(long counter) {
            this.counter = counter;
        }
    }
}

And then we have a service that does the magic (actually there's no magic, all is done manually):

public synchronized Registrant save(Registrant registrant) {
    long counter = getInvoiceNumber();
    registrant.setInvoiceNumber(counter);

    return registrantRepository.save(registrant);
}

private long getInvoiceNumber() {
    //mist: get the invoice number from the other table
    long count = registrantInvoiceNumberGeneratorRepository.count();
    if(count > 1) {
        throw new RuntimeException(": InvoiceNumberGenerator table has more than one row. Fix that");
    }

    Registrant.InvoiceNumberGenerator generator;
    if(count == 0) {
        generator = new Registrant.InvoiceNumberGenerator();
        generator.setCounter(1000001);
        generator = registrantInvoiceNumberGeneratorRepository.save(generator);
    } else {
        generator = registrantInvoiceNumberGeneratorRepository.findFirstByOrderByIdAsc();
    }


    long counter = generator.getCounter();
    generator.setCounter(counter+1);
    registrantInvoiceNumberGeneratorRepository.save(generator);
    return counter;
}

Note the synchronized method - so that nobody can get the same number.

I can't believe there's nothing automatic that can do that.