How does CompletableFuture support asynchronous composition?

A CompletableFuture represents a result that will exist later. You attach dependent stages with thenApply, thenCompose and thenCombine, and handle failure with exceptionally or handle. The default async methods run on the common ForkJoinPool unless you pass an executor.

CompletableFuture<String> result =
    CompletableFuture.supplyAsync(() -> fetchUser(userId), executor)
        .thenCompose(user -> fetchOrders(user))
        .thenApply(orders -> summarize(orders))
        .exceptionally(error -> "fallback");

String value = result.join();

The combinators that matter

• thenApply transforms a result synchronously; thenCompose chains another async stage
• thenCombine merges two independent futures when both complete
• allOf waits for all futures; anyOf completes with the first success
• exceptionally and handle recover; whenComplete observes without changing the value
• orTimeout and completeOnTimeout bound how long a stage may take

Use a custom executor for blocking IO. The common pool is sized for CPU work, and blocking its threads can starve unrelated tasks. Also avoid calling join on a request thread without a timeout, because that reintroduces the blocking you were trying to remove.