NullPointerException in Websockets

Question

I'm implementing Websockets on a Tomcat 7 Server. When the client sends some parameters the server performs some tasks through the Updater class. However I want all the server tasks to be stopped any time the connection is closed. This is my server code:

public class FastServlet {

    private Updater up;
    private Session session;

    public FastServlet() {
    }

    @OnOpen
    public void start(Session session) {
        this.session = session;
    }

    @OnClose
    public void end() {
        up.setActive(false);
    }

    @OnMessage
    public void incoming(String query) {
        \\ some code ...

        up = new Updater(parameters, uploadedFile, session);
        up.update();

    }
}

while the Updater class has this form:

  public class Updater {

    private boolean active;

    public Updater(Map<String, String> parameters, java.io.File uploadedFile, Session session) {
        \\constructor
        this.active = true;
    }

    public void update() {
        while (active) {
            \\ do stuff 
        }
    }

However when I call the server and I force a disconnection I get a NullPointerException at the end() method, as if the "up" variable was null. But I am sure that it is not null since I can verify that the update() method has been partially completed. Any help?

Answer 1

partially completed? You cannot guarantee that.It is possible user disconnects prior to execution of incoming() method. It is always safer to put null checks.

    @OnClose
    public void end() {
        if(up != null){
            up.setActive(false);
        }
    }