[FIXED] Exception is not being logged/throwed inside of completable future

Issue

I just realised that exception inside of whenComplete, in completable future is not being logged/throwed. Can someone explain that? How to properly use whenComplete?

public static void main(String[] args) {

    var c =   CompletableFuture.runAsync(() -> {
        System.out.println("done");

    });

    c.whenComplete((unused, throwable) -> {

        System.out.println((5/0));


        throw new RuntimeException("I am throwing an exception");



    });

    System.out.println("ok");
}

output of this block of code

done
ok

Solution

From the docs (emphasis mine)

public CompletableFuture<T> whenComplete(BiConsumer<? super T,? super Throwable> action)

Description copied from interface: CompletionStage

Returns a new CompletionStage with the same result or exception as this stage, that executes the given action when this stage completes.

When this stage is complete, the given action is invoked with the result (or null if none) and the exception (or null if none) of this stage as arguments. The returned stage is completed when the action returns. If the supplied action itself encounters an exception, then the returned stage exceptionally completes with this exception unless this stage also completed exceptionally.

whenComplete returns a new CompletableFuture. CompletableFuture keeps track of what exception (if any) has occurred and expects you to deal with it. By ignoring the return value, you're suppressing the exception.

To wait on the result of the future, or throw if it throws, use join.

c.whenComplete((unused, throwable) -> {
  System.out.println((5/0));
  throw new RuntimeException("I am throwing an exception");
}).join();

Answered By - Silvio Mayolo
Answer Checked By - Marilyn (JavaFixing Volunteer)