The recent JDK-8213942 fix adjusted the synchronization logic for the handlers Hashtable. A minor enhancement can be made to improve synchronization. 

One thread may release at/around this section while another thread then clears the handlers table in setURLStreamHandlerFactory method :

1418             if (handler2 != null) {
1419                 // The handler from the factory must be given more
1420                 // importance. Discard the default handler that
1421                 // this thread created.
1422                 handler = handler2;
1423             }
1424         }     ===========> HERE
1425 
1426         // Insert this handler into the hashtable
1427         if (handler != null) {
1428             handlers.put(protocol, handler);
1429         }