When I try to upgrade an exsisting Socket connection to a SSLSocket connection and I set setUseClientMode(true) then I get the error javax.net.ssl.SSLProtocolException: Handshake message sequence violation, 1 when calling mySSLSocket.startHandshake();. When not setting setUseClientMode() then everything works fine, and I can send and receive messages. I'm not asking for a fix, just what it means and what may be causing it.
It determines whether this side (a) sends or (b) expects to receive the initial handshake message, the ClientHello. If useClientMode is true, this side sends it. Don't set it at a server unless you really know what you're doing. If you're in a server and you are upgrading an accepted Socket you must set it to false. Otherwise there is really very little occasion to use it at all.
NB useClientMode=true is the default.