Can't say that I use Connection.isClosed often if at all, nor that I fully understand the use cases here.
Technically speaking, if the underlying connection isn't closed, then PoolableConnection still has an open channel to the database. Does that mean isClosed should return true? I think you're suggesting that PoolableConnection.isClosed should return true when the connection is in the pool, false when it's out of the pool (and the underlying connection isn't closed either). Sounds reasonable to me.
