CompletableFuture — what is it good for and a common pitfall?

Deep dive

`CompletableFuture` lets you build an async pipeline:

• Transform: `thenApply`
• Flat-map async stages: `thenCompose`
• Combine: `thenCombine`, `allOf`, `anyOf`
• Handle errors: `exceptionally`, `handle`

Common pitfalls

• Blocking too early with `join()`/`get()` (you lose async benefits).
• Using the common ForkJoinPool for blocking IO (thread starvation). Use a dedicated `Executor`.
• Mixing up `thenApply` vs `thenCompose` (nesting futures).

Example

CompletableFuture<User> f = CompletableFuture.supplyAsync(this::loadUser, ioPool)
  .thenCompose(user -> CompletableFuture.supplyAsync(() -> loadProfile(user), ioPool))
  .exceptionally(ex -> fallbackUser());

Practical guidance

• Always think about which executor runs each stage.
• Add timeouts and cancellation where appropriate.