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Whan happens when I close a Connection application obtained from a connection Pool? How does a connection pool maintain the Connections that I had closed through the application?

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Whan happens when I close a Connection application obtained from a connection Pool? How does a connection pool maintain the Connections that I had closed through the application?

Answer1
It is the magic of polymorphism, and of Java interface vs. implementation types. Two objects can both be "instanceof" the same interface type, even though they are not of the same implementation type.
When you call "getConnection()" on a pooled connection cache manager object, you get a "logical" connection, something which implements the java.sql.Connection interface.
But it is not the same implementation type as you would get for your Connection, if you directly called getConnection() from a (non-pooled/non-cached) datasource.
So the "close()" that you invoke on the "logical" Connection is not the same "close()" method as the one on the actual underlying "physical" connection hidden by the pool cache manager.
The close() method of the "logical" connection object, while it satisfies the method signature of close() in the java.sql.Connection interface, does not actually close the underlying physical connection.

Answer2
Typically a connection pool keeps the active/in-use connections in a hashtable or other Collection mechanism. I've seen some that use one stack for ready-for-use, one stack for in-use.
When close() is called, whatever the mechanism for indicating inuse/ready-for-use, that connection is either returned to the pool for ready-for-use or else physically closed. Connections pools should have a minimum number of connections open. Any that are closing where the minimum are already available should be physically closed.
Some connection pools periodically test their connections to see if queries work on the ready-for-use connections or they may test that on the close() method before returning to the ready-for-use pool.

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