java CompletableFuture completion sequence

From CompletableFuture javadocs:

Actions supplied for dependent completions of non-async methods may be performed by the thread that completes the current CompletableFuture, or by any other caller of a completion method.

and looking into CompletableFuture.complete() it calls postComplete() which seems to pop off the dependent chains and tries to complete them.

To test my understanding, I wrote a simple program

import java.util.concurrent.*;
public class TestCompletableFuture {
    
    public static void main(String[] args) throws Exception {
        CompletableFuture<Void> future = new CompletableFuture<Void>()
            .whenComplete((res, exc) -> {
                System.out.println("inside handle.");
                if (exc != null) {
                    System.out.println("exception.");
                }
                System.out.println("completed.");
            }
        );

        future.completeExceptionally(new Exception("exception"));
        System.out.println("done.");
    }
}

the output of the code:

done.

From my understanding, when the main thread calls future.completeExceptionally() it should invoke the function passed into CompletableFuture.whenComplete().

Why is this not the case?

Solution

This is because you are completing the wrong future. You need to get a reference to the first stage and complete that to see the whenComplete in action:

public class TestCompletableFuture {

    public static void main(String[] args) throws Exception {
        // get reference
        CompletableFuture<Void> future = new CompletableFuture<>();
        
        // configure the action that to be run when the future is complete
        CompletableFuture<Void> future2 = future
                .whenComplete((res, exc) -> {
                            System.out.println("inside handle.");
                            if (exc != null) {
                                System.out.println("exception.");
                            }
                            System.out.println("completed.");
                        }
                );

        future.completeExceptionally(new Exception("exception"));
        System.out.println("done.");
    }
}

So, now the code speaks for itself... When the future is complete run that action (res, exc) -> {...}. And then just trigger that completion on the future by calling completeExceptionally(...) on it.

Another thing to note is that all of the above stages (futures) are completed exceptionally now:

System.out.println(future.isDone()); // true
System.out.println(future2.isDone()); // true

System.out.println(future.isCompletedExceptionally()); // true
System.out.println(future2.isCompletedExceptionally()); // true