How to create a Looper thread, then send it a message immediately?

Question

I have a worker thread that sits in the background, processing messages. Something like this:

class Worker extends Thread {

    public volatile Handler handler; // actually private, of course

    public void run() {
        Looper.prepare();
        mHandler = new Handler() { // the Handler hooks up to the current Thread
            public boolean handleMessage(Message msg) {
                // ...
            }
        };
        Looper.loop();
    }
}

From the main thread (UI thread, not that it matters) I would like to do something like this:

Worker worker = new Worker();
worker.start();
worker.handler.sendMessage(...);

The trouble is that this sets me up for a beautiful race condition: at the time worker.handler is read, there is no way to be sure that the worker thread has already assigned to this field!

I cannot simply create the Handler from the Worker's constructor, because the constructor runs on the main thread, so the Handler will associate itself with the wrong thread.

This hardly seems like an uncommon scenario. I can come up with several workarounds, all of them ugly:

1. Something like this:

   class Worker extends Thread {
   
       public volatile Handler handler; // actually private, of course
   
       public void run() {
           Looper.prepare();
           mHandler = new Handler() { // the Handler hooks up to the current Thread
               public boolean handleMessage(Message msg) {
                   // ...
               }
           };
           notifyAll(); // <- ADDED
           Looper.loop();
       }
   }
   

   And from the main thread:

   Worker worker = new Worker();
   worker.start();
   worker.wait(); // <- ADDED
   worker.handler.sendMessage(...);
   

   But this is not reliable either: if the notifyAll() happens before the wait(), then we'll never be woken up!

2. Passing an initial Message to the Worker's constructor, having the run() method post it. An ad-hoc solution, won't work for multiple messages, or if we don't want to send it right away but soon after.

3. Busy-waiting until the handler field is no longer null. Yep, a last resort...

I would like to create a Handler and MessageQueue on behalf of the Worker thread, but this does not seem to be possible. What is the most elegant way out of this?

Answer 1

Eventual solution (minus error checking), thanks to CommonsWare:

class Worker extends HandlerThread {

    // ...

    public synchronized void waitUntilReady() {
        d_handler = new Handler(getLooper(), d_messageHandler);
    }

}

And from the main thread:

Worker worker = new Worker();
worker.start();
worker.waitUntilReady(); // <- ADDED
worker.handler.sendMessage(...);

This works thanks to the semantics of HandlerThread.getLooper() which blocks until the looper has been initialized.

---

Incidentally, this is similar to my solution #1 above, since the HandlerThread is implemented roughly as follows (gotta love open source):

public void run() {
    Looper.prepare();
    synchronized (this) {
        mLooper = Looper.myLooper();
        notifyAll();
    }
    Looper.loop();
}

public Looper getLooper() {
    synchronized (this) {
        while (mLooper == null) {
            try {
                wait();
            } catch (InterruptedException e) {
            }
        }
    }
    return mLooper;
}

The key difference is that it doesn't check whether the worker thread is running, but that it has actually created a looper; and the way to do so is to store the looper in a private field. Nice!

Answer 2

take a look at the source code of HandlerThread

@Override
     public void run() {
         mTid = Process.myTid();
         Looper.prepare();
         synchronized (this) {
             mLooper = Looper.myLooper();
             notifyAll();
         }
         Process.setThreadPriority(mPriority);
         onLooperPrepared();
         Looper.loop();
         mTid = -1;
     }

Basically, if you are extending Thread in worker and implementing your own Looper, then your main thread class should extend worker and set your handler there.

Answer 3

This is my solutions: MainActivity:

//Other Code

 mCountDownLatch = new CountDownLatch(1);
        mainApp = this;
        WorkerThread workerThread = new WorkerThread(mCountDownLatch);
        workerThread.start();
        try {
            mCountDownLatch.await();
            Log.i("MsgToWorkerThread", "Worker Thread is up and running. We can send message to it now...");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        Toast.makeText(this, "Trial run...", Toast.LENGTH_LONG).show();
        Message msg = workerThread.workerThreadHandler.obtainMessage();
        workerThread.workerThreadHandler.sendMessage(msg);

The WorkerThread Class:

public class WorkerThread extends Thread{

    public Handler workerThreadHandler;
    CountDownLatch mLatch;

    public WorkerThread(CountDownLatch latch){

        mLatch = latch;
    }


    public void run() {
        Looper.prepare();
        workerThreadHandler = new Handler() {
            @Override
            public void handleMessage(Message msg) {

                Log.i("MsgToWorkerThread", "Message received from UI thread...");
                        MainActivity.getMainApp().runOnUiThread(new Runnable() {

                            @Override
                            public void run() {
                                Toast.makeText(MainActivity.getMainApp().getApplicationContext(), "Message received in worker thread from UI thread", Toast.LENGTH_LONG).show();
                                //Log.i("MsgToWorkerThread", "Message received from UI thread...");
                            }
                        });

            }

        };
        Log.i("MsgToWorkerThread", "Worker thread ready...");
        mLatch.countDown();
        Looper.loop();
    }
}

Answer 4

    class WorkerThread extends Thread {
            private Exchanger<Void> mStartExchanger = new Exchanger<Void>();
            private Handler mHandler;
            public Handler getHandler() {
                    return mHandler;
            }
            @Override
            public void run() {
                    Looper.prepare();
                    mHandler = new Handler();
                    try {
                            mStartExchanger.exchange(null);
                    } catch (InterruptedException e) {
                            e.printStackTrace();
                    }
                    Looper.loop();
            }

            @Override
            public synchronized void start() {
                    super.start();
                    try {
                            mStartExchanger.exchange(null);
                    } catch (InterruptedException e) {
                            e.printStackTrace();
                    }
            }
    }