code: synchronized public void setNeedClientAuth(boolean flag) {
          doClientAuth = (flag ?
              SSLEngineImpl.clauth_required : SSLEngineImpl.clauth_none);
  
          if ((handshaker != null) &&
                  (handshaker instanceof ServerHandshaker) &&
                  !handshaker.activated()) {
              ((ServerHandshaker) handshaker).setClientAuth(doClientAuth);
          }
      }
docs: Sets the flag controlling whether a server mode engine
  *REQUIRES* SSL client authentication.
  
  As long as handshaking has not started, we can change
  whether client authentication is needed.  Otherwise,
  we will need to wait for the next handshake.