BaseSSLSocketImpl  implements "safety net". I mean when GC clears object it makes sure in finalize() method socket is closed. And this error shows up when socket is already closed. So I think it is safe to ignore this error.
Thanks for your response. I am using OpenJDK 11 on openSUSE Tumbleweed.
I certainly hope your analysis that this message can be ignored is correct. If I turn off debug one would never know it is being thrown -- the program works as designed. That gives credence to your thought it can be ignored.
Thanks much for your input. Very much appreciated.