Statement.cancel() doesn't seem to work

Hi, I'm trying to cancel a long-running statement (takes about 400
seconds) by calling PreparedStatement.cancel() from another thread,
but it's not working.  Specifically, the call to
PreparedStatement.cancel() completes normally, but the executing
statement seems to complete normally (i.e., it still takes the same
amount of time to complete, and it does not throw any exception).  The
sql being executed is a delete ("delete from qos_data where
report_time < ?").  Is PreparedStatement.cancel() not implemented?

Thanks,
Jim