等待异步回调函数在Java中返回

Question

final Function<Boolean, ? extends Class<Void>> functionCallback = (Boolean t) -> {
   if(t) {
     plugin.setIsInstalled(Boolean.TRUE);             
   }
   return Void.TYPE;
};

foo.install(plugin,functionCallback);

if(plugin.getIsInstalled().getValue())
  return "done";
else 
  return "not done";

I want to check if(plugin.getIsInstalled().getValue()) once the callback has finished executing. How can I prevent execution of this if condition until callback has completed execution?

Answer 1

You can use a FutureTask that gets called in your callback function:

final FutureTask<Object> ft = new FutureTask<Object>(() -> {}, new Object());
final Function<Boolean, ? extends Class<Void>> functionCallback = (Boolean t) -> {
    if(t) {
        plugin.setIsInstalled(Boolean.TRUE);
        ft.run();
    }
    return Void.TYPE;
};

foo.install(plugin,functionCallback); 
ft.get();
if(plugin.getIsInstalled().getValue())
    return "done";
else 
    return "not done";

Future.get waits till the run method was called, you can also use the get -method that accepts a timeout so you can react on that if it takes too long.

Answer 2

The callback is mostly used to perform a task after a particular task is done. So it better separate the code you want to execute into a different function call that function.

If you want to execute something after callback have nested callbacks.

Answer 3

1. You can use a CountDownLatch or a ReentrantLock that gets released when the function is run.
2. Your foo#install can return a CompletableFuture and you can consume the results as follows

CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> 1);
future.thenAccept((v) -> System.out.println("v = " + v));

3. Function itself has an andThen method which you can use to run whatever is required post apply .