In javax.net.SocketFactory.getDefault():

        //
        // optimize typical case: no synch needed
        //
        if (theFactory == null) {
            synchronized (SocketFactory.class) {
                theFactory = new DefaultSocketFactory();
            }
        }

There is similar code in javax.net.ServerSocketFactory:
        //
        // optimize typical case: no synch needed
        //
        if (theFactory == null) {
            synchronized (ServerSocketFactory.class) {
                theFactory = new DefaultServerSocketFactory();
            }
        }
The optimizations are problemmatic because theFactory is checked outside
of the synchronization block. This could lead to a situation where
theFactory is set but the constructor (DefaultSocketFactory/DefaultServerSocketFactory) not completed, thus
leading to a bad address error. The fix is to lock before doing the check
(i.e. remove the optimization).