You can read the specs on JDBC connections and threads here: http://java.sun.com/j2se/1.3/docs/guide/jdbc/spec/jdbc-spec.frame9.html
From the above location:
We require that all operations on all the java.sql objects be multi-thread safe and able to cope correctly with having several threads simultaneously calling the same object.
Some drivers may allow more concurrent execution than others. Developers can assume fully concurrent execution; if the driver requires some form of synchronization, it will provide it. The only difference visible to the developer will be that applications will run with reduced concurrency.