CompletableFuture composition

An ExecutorService is a queue of tasks plus worker threads. Its lifecycle has three phases and two endings:

• shutdown(): stop accepting tasks, finish queued ones.
• shutdownNow(): stop accepting, interrupt running tasks, drain the queue into your hands (so nothing is lost silently).
• awaitTermination(timeout): the polite wait. The pattern:

exec.shutdown();
if (!exec.awaitTermination(5, TimeUnit.SECONDS)) {
    exec.shutdownNow();   // escalate
}

Future.get() is where composition goes to die: f2.get() cannot start until you call it, so "run A and B concurrently, then combine" becomes blocked hand-tuning. CompletableFuture fixes this:

var a = CompletableFuture.supplyAsync(() -> fetchA());
var b = CompletableFuture.supplyAsync(() -> fetchB());
var both = a.thenCombine(b, (x, y) -> x + y);   // runs when BOTH complete

Composition primitives: thenApply (transform), thenCompose (chain, flat-map), thenCombine (join two), exceptionally/handle (recover), orTimeout (JDK 9+; sandbox-safe since it needs no extra flags).