Why does Java 8 completable future thenpose generate different exceptions according to the completion order?

I have encountered the strange behavior of the Java 8 completable future thenpose method I have two tests that differ only in the order of execution Both tests simulate failures in the completable future generated in thenpose

@Test
public void completedAfter() {
    CompletableFuture<String> future1 = new CompletableFuture<>();
    CompletableFuture<String> future2 = new CompletableFuture<>();

    future1.thenCompose(x -> future2).whenComplete((r,e) -> System.out.println("After: " + e));

    future1.complete("value");
    future2.completeExceptionally(new RuntimeException());
}

@Test
public void completedBefore() {
    CompletableFuture<String> future1 = new CompletableFuture<>();
    CompletableFuture<String> future2 = new CompletableFuture<>();

    future1.complete("value");
    future2.completeExceptionally(new RuntimeException());

    future1.thenCompose(x -> future2).whenComplete((r,e) -> System.out.println("Before: " +e));
}

The output is:

After: java.util.concurrent.CompletionException: java.lang.RuntimeException
Before: java.lang.RuntimeException

The question is, why is it included in completionexception in one case but not exception in another?

Update: here is the relevant error report It has been marked and parsed as an error in the JDK

Solution

It seems to be a bug in the JDK library

In the case of "after" Thenpose adds a thencopy completion node to the target future, which will be executed later by Completeexceptionally triggered The run method of the completion node finds the exception in the future and calls it on the target Internalcomplete, which wraps all exceptions into completionexception See how the here node is created and where the here encapsulation occurs

Now, in the before case, the code path is completely different Because the future has been completed, so Thenpose does not create additional nodes, but immediately returns invokes, and simply returns one (the second future that has been completed), and then calls on it Whencomplete, again, it does not need to create a new completion node, but just invokes the callback immediately, giving it a second future original exception

Boy, look at this code. There are many examples. I want to show students what they should do