Which thread runs your callback if you use thenApply rather than thenApplyAsync?

The non-async variants run on whatever thread happens to make the value available. If the previous stage is still running, the callback runs on the thread that completes it. If the future was already complete when you attached the callback, it runs immediately on your thread.

That means a chain of thenApply calls can execute on a Netty I/O thread, a client library's callback thread, or the main thread, depending on timing. Put something slow or blocking there and you have stalled someone else's event loop. This is the single most common CompletableFuture bug in production.

The *Async variants take control back: with no executor argument they use the common ForkJoinPool, which is also wrong for blocking work. Pass your own executor.