Does a CompletableFuture completes on a re-thrown exception? - spring

Question

I've just started using CompletableFuture and already loving it.
But one strange thing that appears to me while using a CompletableFuture is its method called "exceptionally"
Let's say I've a
CompletableFuture<?> cf1.
Now, once the data arrives, my code is applying some processing logic. In case of an exception, I make use of exceptionally method to rethrow MyCustomException
cf1
.thenApply(myData->Some Processing Logic)
.exceptionally(ex-> throw new MyCustomException())
cf.get();
Interestingly, the call to get method hangs indefinitely until I terminate the program. Does that mean that if a CompletableFuture re-throws an exception from the exceptionally block, the future will not be marked as complete? Do I need to explicity mark it as complete?

Answer 1

From docs the get method throws exception if the future completed exceptionally
ExecutionException - if this future completed exceptionally
So either you can return some value from exceptionally to identify the exception is thrown during thenApply and call get method for value
Second way, before calling get method you can make that future object completed using allOf, and check if the future is completed exceptionally
CompletableFuture.allOf(completableFuture);
completableFuture.isCompletedExceptionally(); //true

Answer 2

Heres a sample code that doesnot finish on throwing exception -
CompletableFuture<String> r1 = CompletableFuture.supplyAsync(() -> {
try{
Thread.sleep(1000);
throw new RuntimeException("blahh !!!");
}catch (Exception e) {
throw new RuntimeException(e);
}
});
CompletableFuture<String> r2 = CompletableFuture.supplyAsync(() -> "55");
CompletableFuture<String> r3 = CompletableFuture.supplyAsync(() -> "56");
CompletableFuture.allOf(r1, r2, r3).thenRun(() -> { System.out.println(Thread.currentThread()+" --- End."); });
Stream.of(r1, r2, r3).forEach(System.out::println);
try{
System.out.println(Thread.currentThread()+" --- SLEEPING !!!");
Thread.sleep(3000);
System.out.println(Thread.currentThread()+" --- DONE !!!");
} catch (Exception e) {
//e.printStackTrace();
}
Stream.of(r1, r2, r3).forEach(System.out::println);