Java WebSocket session resulting to null after onOpen

Question

I am using websockets for the first time on a javafx project, when I start the program the session is set to the local variable session, but after when I call the sendMessage function the session is back to null. Below please find my client class

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);        
    }

}

Any suggestions?

Thanks in advance

Answer 1

I think I now do know the answer to this.

You are probably using tomcat or some other server for this. When you see "tomcat" in this answer, please insert the name of your actually used server.

When a connection to your websocket is opened, tomcat will create an instance of the websocket (your Client) class by itself. This means, the onOpen-Method will be called and it will look as if it was you who created the instance, who opened the connection, when really you did not. Tomcat did.

This in turn means, that when you call sendMessage on your Client instance, the session will be null, because this object never connected anywhere.

Oh, and you don't have access to the connected instance that was created by tomcat.

One way of fixing this would be to do all the work inside the onOpen-Method, however that is not practical. You may want to put the work in another method and call it from onOpen. That way, the instance created by tomcat will do the necessary work.

In my project I needed to poll on an MQTT-Topic and render the data on a website (university assignment). I did the polling in a separate class, resulting in hard to debug errors whenever trying to send received data with my sendMessage-method.

I hope this answer does clear this up a little, if not for you, maybe at least for future generations having the same university assignment...