The implementation of DatabaseConnection.getJDBCConnection() calls
java.sql.Connection.close(), which can possibly block due to thread
synchronization or network activity. It does so in order to return a null value
if the connection is closed.

However, this approach has several problems:

1. Connection.isClosed() is only guaranteed to return false if
Connection.close() method has been previously calleed. Therefore in order to be
reliable, the implementation of DC.getJDBCConnection() can only call
C.isClosed() after the connection has been closed (e.g., by invoking the
Disconnect action in the Database Explorer's UI). But then the implementation
doesn't need to call C.isClosed() at all, since the fact that the connection has
been closed is known (it is even displayed in the UI).

2. The Javadoc of DC.getJDBCConnection(), which states that a null value will be
returned if the connection is disconnected, suggests this method can be used to
find out the connections state. Imagine a combo box containing all connections
with a connected/disconnected icon next to each connection. The usage
DC.getJDBCConnection() to determine the icon could lead to an UI freeze.

There should be a DC.isConnected() method returning the state of the connection
as known by the Database Explorer. DC.getJDBCConnection() should return null iff
DC.isConnected() == true.