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; Blueto…

Accepted Answer

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];
}
```

Java - synchronous callback

Solution

Problem