Java - synchronous callback

Question

I have the following code which is executed asynchronously. I would like to make it synchronous in order to follow some logical flow but I cannot work out how.

You will see that scanning is set to true to indicate that the method is still working, at the beginning - I then initiate a findPrinters(...) command - this contains a DiscoveryHandler which runs asynchronously - foundPrinter() is called each time an item is discovered. discoveryFinished() is when the discovery process is successfully completed, and discoveryError(...) is called whenever an error occurs.

I rely on something being set in my DiscoveryHandler before I would like to return from this method. Hence why I have while (scanning) underneath it. But this feels like a hack to me, and not the correct way of doing things. I cannot get wait() and notify() working. Can someone tell me what the correct way to do this is please?

private boolean findPrinter(final Context ctx) {
    try {
        scanning = true;
        BluetoothDiscoverer.findPrinters(ctx, new DiscoveryHandler() {

            public void foundPrinter(DiscoveredPrinter device) {
                if (device instanceof DiscoveredPrinterBluetooth) {
                    DiscoveredPrinterBluetooth btDevice = (DiscoveredPrinterBluetooth) device;

                    if (btDevice.friendlyName.startsWith("XXXX")) {
                        try {
                            connection = new BluetoothConnection(btDevice.address);
                            connection.open();

                            if (connection.isConnected()) {
                                address = btDevice.address;
                            }
                        } catch (Exception ex) {

                        }
                    }
                }
            }

            public void discoveryFinished() {
                scanning = false;
            }

            public void discoveryError(String arg0) {
                scanning = false;
            }
        });
    } catch (Exception ex) {

    }

    while (scanning) {}

    return false;
}

Answer 1

You could do this with CountDownLatch, which might be the lightest synchronization primitive in java.util.concurrent:

private boolean findPrinter(final Context ctx) {
    final CountDownLatch latch = new CountDownLatch(1);
    final boolean[] result = {false};

    ...

    BluetoothDiscoverer.findPrinters(ctx, new DiscoveryHandler() {

        ...

        public void discoveryFinished() {
            result[0] = true;
            latch.countDown();
        }
    
        public void discoveryError(String arg0) {
            result[0] = false;
            latch.countDown();
        }

        ...
    }

    // before final return
    // wait for 10 seconds for the response
    latch.await(10, TimeUnit.SECONDS);

    //return the result, it will return false when there is timeout
    return result[0];
}