code: private ServiceRefAmp sessionImpl(String address)
    {
      if (! address.startsWith("session:") || ! address.endsWith("/")) {
        throw new IllegalArgumentException(address);
      }
  
      String sessionId = cookie("JSESSIONID");
  
      if (sessionId == null) {
        sessionId = generateSessionId();
  
        cookie("JSESSIONID", sessionId);
      }
  
      return services().service(address + sessionId);
    }
docs: Find or create a session cookie as the session id and
  find or create a session with the generated address.