Transations in postgres

I'm using Postgres 6.4/Linux  with Java/Windows as client.
When I do a transaction for update and I shutdown the client before
commit/rollback, then the tables used in the transaction are still locked.

            How can avoid this? Is there any mechanism that rollbacks the
        transaction in
        case of repentine shutdown of the cliente?

                    Thanks.

                                    Rober


--
Roberto Moreda
Resp. Dpto. Informática Handem/San Luis
Tlf +34 981 779000
Fax +34 981 779022
Pol. Piadela Sur, Autovía A6 Sal.567
15300 Betanzos (A Coruña) - España