A Derby Connection object is not garbage-collected until all other JDBC objects created from that connection are explicitly closed or are themselves garbage-collected. Once the connection is closed, no further JDBC requests can be made against objects created from the connection. Do not explicitly close the Connection object until you no longer need it for executing statements.
A session-severity or higher exception causes the connection to close and all other JDBC objects against it to be closed. System-severity exceptions cause the Derby system to shut down, which not only closes the connection but means that no new connections should be created in the current JVM.