Managing a Process inside a Thread

Question

I'm creating a graphical interface that runs some threads. Each one of these threads launch a Java Process to manage a VNC connection. All i want is to keep track of the process lifecycle by storing it in some variables of the thread that manages. Finally, the GUI communicates with the Thread to know about Process status.

Here is a snippet of my code:

public class VNCViewer extends Thread{
    private static final String cmd = "some command";
    private Process vnc;
    private boolean active = false ;

    public void run(){
        try {
            launchVNC();
        } catch (IOException ex) {
            Logger.getLogger(VNCViewer.class.getName()).log(Level.SEVERE, null, ex);
        } catch (InterruptedException ex) {
            Logger.getLogger(VNCViewer.class.getName()).log(Level.SEVERE, null, ex);
        }
    }

    private void launchVNC() throws IOException, InterruptedException{
        if (condition){
            vnc = Runtime.getRuntime().exec(cmd);
            active = true;
            while(vnc.isAlive()){} //while the process is alive, the thread awaits
            active = false;
        }
    }

    public boolean isActive(){
        return active;
    }
}

What happens at runtime is that the thread skips the "while" loop (I've tried by inserting a system.out.println inside the loop and it is printed only when the thread is terminated), with the result that the variable "active" is always on "false".

Answer 1

Since active is not volatile, isn't updated/accessed in a synchronized block and isn't one of the Atomic* classes, it is perfectly legal for the Java VM to assume that no-one reads the field between active=true and active=false.

Therefore it can decide to ignore active=true (or to be more precise, to not publish the new value to other threads).

You need to synchronize your code correctly, in this case declaring the field volatile is adequate:

private volatile boolean active = false;

This ensures that all updates to this field will be published immediately and any other thread reading the field will see the updated one.

I'm still not convinced about spinning for the external process to shut down but that's a different matter.