questions regarding transactions

Hi,

 I'm using JDBC with transactions and I've got a couple of questions:

1) Am I supposed to do "myConn.rollback()" on error? What happens if I
don't do it - will the transaction stay half-committed?! (I ask this,
because rollback() can throw a SQLException, so it's not guaranteed to
work, in my opinion).

2) When I use statements like (SELECT currval('somesequence')) in a
jdbc-transaction, will it be atomic to? Will the sequence be decremented
again when the transaction fails? Can I get wrong numbers, when there is
another transaction which increments somesequence?

3) Is it sufficient to do setAutocommit(false) and set the
transaction-level or do I have to call a statement like "START
TRANSACTION" to properly start an transaction? (I found
the latter example somewhere on the net.)

Many thanks in advance and thank you for the wonderful driver
fabian zeindl