Incorrect synchronization of ConnectionPool.connectionMap

[jakubjanecek]

Access to field connectionMap (which is not thread-safe) in class ConnectionPool is not synchronized properly. The problem is in method createConnection where new connections are created and put into the map however more threads can access the map at once - the locking mechanism on the connection does not protect the map. We encountered the problem in production resulting in infinite loop (caused by corrupted internal state of the map).

I suggest to replace HashMap with ConcurrentHashMap.