在onOpen之后导致Java WebSocket会话变为null

Question

我在javafx项目中第一次使用websockets，当我启动程序时，会话被设置为局部变量会话，但在我调用sendMessage函数之后会话返回null。请在下面找到我的客户类

package myclient;

import java.io.IOException;
import java.io.InputStream;
import java.net.URI;
import java.util.logging.Level;
import java.util.logging.Logger;
import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.stage.Stage;
import javax.websocket.ClientEndpoint;
import javax.websocket.ContainerProvider;
import javax.websocket.DeploymentException;
import javax.websocket.OnClose;
import javax.websocket.OnMessage;
import javax.websocket.OnOpen;
import javax.websocket.Session;
import javax.websocket.WebSocketContainer;

@ClientEndpoint
public class Client extends Application {    
    private static final Logger LOGGER = Logger.getLogger(Client.class.getName());
    private Session session;

    @OnOpen
    public void onOpen(Session session){
        this.session = session;
        System.out.println("Opened Session " + this.session);
    }

    @OnClose
    public void onClose(){
        System.out.println("Closed Session " + this.session);
    }

    @OnMessage
    public void onMessage(String msg){
        System.out.println("Websocket message received! " + msg);   
    }

    @Override
    public void start(Stage stage) throws Exception {
        Parent root = FXMLLoader.load(getClass().getResource("FXMLClient.fxml"));

        Scene scene = new Scene(root);

        connectToWebSocket();
        stage.setScene(scene);
        stage.show();
    }

    private void connectToWebSocket() {
        System.out.println("Client WebSocket initialized>>  " + this.session);
      WebSocketContainer container = ContainerProvider.getWebSocketContainer();
        try {
            URI uri = URI.create("ws://localhost:8080/Server/endpoint");
            container.connectToServer(this, uri);
        } 
        catch (DeploymentException | IOException ex) {
            LOGGER.log(Level.SEVERE, null, ex);
            System.exit(-1);
        }
    }

    public void sendMessage(String message) throws IOException{
        if(this.session != null){
                System.out.println(message + ", " + this.session);
                this.session.getBasicRemote().sendText(message);
        }
        else {
            System.out.println("Session is null");
        }
    }

    public static void main(String[] args) {
        launch(args);        
    }

}

有什么建议吗？

提前致谢