ClientUserTransaction singleton initialisation is not thread safe

The ClientUserTransaction singleton initialistation is not thread safe:

private static ClientUserTransaction singleton = null;

public static ClientUserTransaction getSingleton()
{
if (singleton == null)
singleton = new ClientUserTransaction();
return singleton;
}

Two threads can race to create the singleton which means the ThreadLocal within the class can become
"masked".

The construction of the singleton needs moving to static initialisation.