Java Database Connectivity (JDBC)

Hi there!
I want to ask what I should do in the following case: I have to handle mutliple rows of data to insert OR to update into the database.

The first question is about how to decide whether I should take INSERT or UPDATE. I read here in the forum that I could take a SELECT-statement before, and, if it isn't null, I could update the resultset..if it is null I can make an INSERT-statement.

But now I a have multiple rows to update or to insert which I want to handle as a transaction (with a batch), so I don't want to check each row the way I described above. Does anyone has a hint ?

Thanks a lot in advance.