How are JDBC connections implemented?

Question

I have a project in which I used HikariCP for JDBC connection pooling. And HikariCP works just great for my needs. It also logs the stats of the pool like below.

2014-12-03 10:16:08 DEBUG HikariPool:559 - Before cleanup pool stats loginPool (total=8, inUse=0, avail=8, waiting=0)
  2014-12-03 10:16:08 DEBUG HikariPool:559 - After cleanup pool stats loginPool (total=7, inUse=1, avail=7, waiting=0)

Just for experimental purposes I closed all the MySQL connections for the configured database using MySQL Workbench . But, still I see HikariCP logging the stats like before though there are no actual connections to the database. When there was a request for connection it immediately established the connections(initial 8), so everything works great.

So, my question is how does these connections are managed or implemented? I think the reason why HikariCP logs stats, as if there were connections, is because it has valid in memory references to connections, which are actually non existent(with database).

Is my understanding correct?

Answer 1

The connection pool created 8 connections at startup. You say you disconnected them using the workbench. Most connection pools won't know the connection is disconnected until it gets used.

Your assumption is correct. You manually killed the connections but the pool has a handle to 8 sockets which it assumes are connected. Given time your connection pool may have checked the connections for validity and attempted to reconnect them. I can't speak for HikariCP but this is what modern connections pools do.

Answer 2

When you close the connections using MySQL Workbench, you are closing them on the server end. On the JDBC (client) side, the previously established connections will remain in existence until the client code attempts to use them. At that point, they will be found to be "broken"; ie the client will get exceptions when it tries to use them.

The client-side JDBC Connection objects only get closed or recycled when they are returned to the connection pool by your Java application code.