Can't stop thread

Question

I have these two methods for creating and stopping a thread. However the thread still keeps running, even after the first method is called. (I'm creating an object of the class and calling them from another class).

private Thread thread;

public void stopAlarm() {
    Log.i(LOG_TAG, "stopAlarm called");
    sendAlarm = false;
    if (!thread.equals(null)) {
        try {
            thread.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}


public void triggerAlarm() {
    Runnable alarmTest = new Runnable() {
        @Override
        public void run() {
            while (sendAlarm) {
                Log.i(LOG_TAG, String.valueOf(sendAlarm));
            }
        }
    };
    thread = new Thread(Test);
    thread.start();
}

When stopAlarm is called the thread is always null, although it is called after triggerAlarm is called (thread is running).

Answer 1

Depending on your OS you may find making your thread volatile may fix this.

private volatile Thread thread;

However - there are better ways to do this. One very useful one is using a small (just one entry) BlockingQueue which is poll ed by the running thread.

// Use a BlockingQueue to signal the alarm to stop.
BlockingQueue<String> stop = new ArrayBlockingQueue<>(1);

public void stopAlarm() {
    stop.add("Stop");
}


public void triggerAlarm() {
    new Thread(() -> {
        try {
            while (stop.poll(1, TimeUnit.SECONDS) == null) {
                // Stuff
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }).start();
}

Clearly you will have to manage edge cases like where someone calls stopAlarm when no alarm is running.

Answer 2

Your problem is caused by thread scope. Thread scope is created when you create a thread with same variables in the scope but you can't change these variables from outside world. Best practice for managing runnables in android is to use Handler .

Handler handler = new Handler();
Runnable alarmTest = new Runnable() {
    @Override
    public void run() {
        Log.i(LOG_TAG, String.valueOf(sendAlarm));
        handler.post(alarmTest, 5000); //wait 5 sec and run again
        //you can stop from outside
    }
};

after definitions, in order to start the runnable:

handler.post(alarmTest,0); //wait 0 ms and run

in order to stop the runnable:

handler.removeCallbacks(alarmTest);

EDIT: wait statement with loop

EDIT: Complete solution

Handler handler = new Handler();
Runnable alarmTest = new Runnable() {
    @Override
    public void run() {
        Log.i(LOG_TAG, String.valueOf(sendAlarm));
        handler.post(alarmTest, 5000); //wait 5 sec and run again
        //you can stop from outside
    }
};

public void stopAlarm() {
    Log.i(LOG_TAG, "stopAlarm called");
    handler.removeCallbacks(alarmTest);
}


public void triggerAlarm() {
    handler.post(alarmTest,0); //wait 0 ms and run
}