2.1.10 Freeing DBMS Resources

It is recommended that programmers explicitly close connections and statements they have created when they are no longer needed.
A programmer writing code in the Java programming language and not using any outside resources does not need to worry about memory management. The garbage collector automatically removes objects when they are no longer being used and frees the memory they were using. When memory is running low, it will recycle discarded objects, making the memory they currently occupy available for quick reuse.
However, if an application uses external resources, as it does when it accesses a DBMS with the JDBC API, the garbage collector has no way of knowing the status of those resources. It will still recycle discarded objects, but if there is lots of free memory in the Java heap, it may garbage collect infrequently, even though the (small) amount of Java garbage is holding open large amounts of expensive database resources. Therefore, it is recommended that programmers explicitly close all connections (with the method Connection.close) and statements (with the method Statement.close) as soon as they are no longer needed, thereby freeing DBMS resources as early as possible. This applies especially to applications that are intended to work with different DBMSs because of variations from one DBMS to another.
Note that the method Connection.isClosed is guaranteed to return true only when it is called after the method Connection.close has been called. As a result, a programmer cannot depend on this method to indicate whether a connection is valid or not. Instead, a typical client can determine that a connection is invalid by catching the exception that is thrown when a JDBC operation is attempted.