How to Detect JDBC Connection Leakages with C3P0

First of all, I must say that the whole credit for discovering this feature goes to my colleague İlker Çelik. In a big codebase with lots of 3rd party libraries and frameworks interacting with JDBC connections, it might be difficult to trap JDBC connection leakages, which are open connections left in the application.

C3P0 connection pool has a nice option to kill unreturned connections left in the application. The unreturnedConnectionTimeout attribute is for this purpose. If you give it a positive integer value, it waits for the given seconds and then destroys those connections whose close method has not been called yet. In order to see who is responsible for the leakage, you must use it in combination with the debugUnreturnedConnectionStackTraces attribute.

If you give a positive value to unreturnedConnectionTimeout, C3P0 will capture the stack trace via an exception at the point where your application opens the connection, and setting debugUnreturnedConnectionStackTraces to true will show this stack trace. You can find detailed information about this feature of C3P0 here.