You should explicitly close Statements, ResultSets, and Connections when you no longer need them, unless you declare them in a try-with-resources statement (available in JDK 7 and after).
Connections to Derby are resources external to an application, and the garbage collector will not close them automatically.
To close a Statement, ResultSet, or Connection object that is not declared in a try-with-resources statement, use its close method. If auto-commit is disabled, you must explicitly commit or roll back active transactions before you close the connection.
Statements, result sets, and connections extend AutoCloseable in JDK 7 and after. If you declare a connection in a try-with-resources statement and there is an error that the code does not catch, the JRE will attempt to close the connection automatically.
Note that a transaction-severity or higher exception causes Derby to abort an in-flight transaction. But a statement-severity exception does NOT roll back the transaction. Also note that Derby throws an exception if an attempt is made to close a connection with an in-flight transaction. Suppose now that a Connection is declared in a try-with-resources statement, a transaction is in-flight, and an unhandled statement-severity error occurs inside the try-with-resources block. In this situation, Derby will raise a follow-on exception as the JRE exits the try-with-resources block. (For details on error severity levels, see the documentation of the derby.stream.error.logSeverityLevel property in the Java DB Reference Manual.)
It is therefore always best to catch errors inside the try-with-resources block and to either roll back or commit, as appropriate, to ensure that there is no pending transaction when leaving the try-with-resources block. This action also improves application portability, since DBMSs differ in their semantics when trying to close a connection with a pending transaction.