How interrupt/stop a thread in Java?

Question

I'm trying to stop a thread but I can't do that :

public class Middleware {

public void read() {
    try {
        socket = new Socket("192.168.1.8", 2001);

        // code .. 

        Scan scan = new Scan();
        thread = new Thread(scan);
        thread.start();

    } catch (UnknownHostException ex) {
        ex.printStackTrace();

    } catch (IOException ex) {
        ex.printStackTrace();
    }
}

class Scan extends Thread {

    public void run() {

        while (true) {
            try {
            // my code goes here

            } catch (IOException ex) {
                thread.currentThread().interrupt();
            }
        }
    }
}

public void stop() {
    Thread.currentThread().interrupt();
}

// get and setters
}

So, even when i call the method 'stop' the thread don't stop. It keeps alive.

How can I interrupt/stop this thread ?

UPDATE (@little approach)

private void tb_startActionPerformed(java.awt.event.ActionEvent evt) {                                         
    Middleware middleware = new Middleware();

    if (tb_start.getText().equals("Start")){
        tb_start.setText("Stop");

        // starting to read rfid tags
        middleware.read();
    }else{
        tb_start.setText("Start");

        // stop reading rfid tags
        middleware.stop();
    }
}

The Middleware class :

public class Middleware {

    private Scan scan;

    public void read() {

        scan = new Scan();
        scan.start();
    }

    private class Scan extends Thread {

        @Override
        public void run() {
            while (!Thread.currentThread().isInterrupted()) {
                System.out.println("reading...");
            }
        }
    }

    public void stop() {
        if (scan != null) {
            scan.interrupt();
        }
    }
}

But when I try to stop the thread, it doesn't.

What could be wrong in the code above ?

Answer 1

There's really no reason you need to use a volatile flag. Instead, just query the thread for its state with isInterrupted(). Also, why are you wrapping your Scan thread object in another thread object? That seems completely unnecessary to me.

Here' what you should be doing

public class Middleware {
    private Scan scan;

    public void read() {
        try {
            // do stuff

            scan = new Scan();
            scan.start();
        } catch (UnknownHostException ex) {
            // handle exception
        } catch (IOException ex) {
            // handle exception
        }
    }

    private class Scan extends Thread {

        @Override
        public void run() {
            while (!Thread.currentThread().isInterrupted()) {
                try {
                    // my code goes here
                } catch (IOException ex) {
                    Thread.currentThread().interrupt();
                }
            }
        }
    }

    public void stop() {
        if(scan != null){
            scan.interrupt();
        }
    }
}

Here's an example. Also, I wouldn't recommend extending Thread.

Answer 2

Simply return; from your while and the thread will die, no need to call stop() or interrupt(). If you want to do it externally then use this pattern and call requestStop().

class Scan extends Thread {
    private volatile stop = false;
    public void run() {

        while (!stop) {
            try {
            // my code goes here

            } catch (IOException ex) {
                stop = true;
            }
        }
    }

    public void requestStop() {
        stop = true;
    }

}