Concurrency - CompletableFuture

Why this step matters

Many backend flows call multiple services and should not block a request thread for each step. CompletableFuture enables non-blocking composition and better pipeline structure.

Basic async task

CompletableFuture<String> userFuture = CompletableFuture.supplyAsync(() -> fetchUserName(42));
String name = userFuture.join();

join() waits and wraps errors in unchecked exceptions.

Transform and chain

CompletableFuture<String> greeting = CompletableFuture
    .supplyAsync(() -> "briac")
    .thenApply(String::toUpperCase)
    .thenApply(name -> "hello " + name);

thenApply: synchronous transform of previous result
thenCompose: chain another async future (flatten nested futures)

Compose multiple async tasks

CompletableFuture<User> user = CompletableFuture.supplyAsync(() -> loadUser(1));
CompletableFuture<List<Order>> orders = CompletableFuture.supplyAsync(() -> loadOrders(1));

CompletableFuture<UserDashboard> dashboard = user.thenCombine(
    orders,
    UserDashboard::new
);

Error handling

CompletableFuture<String> safe = CompletableFuture
    .supplyAsync(this::callRemote)
    .exceptionally(ex -> "fallback");

Useful handlers:

exceptionally(...)
handle(...)
whenComplete(...)

Timeouts

CompletableFuture<String> result = CompletableFuture
    .supplyAsync(this::callRemote)
    .orTimeout(2, TimeUnit.SECONDS)
    .exceptionally(ex -> "timeout-fallback");

Common mistakes

blocking too early with join() everywhere
mixing heavy blocking I/O in default common pool
forgetting error paths in composed pipelines
creating overly complex chains without naming intermediate steps

Takeaway

Use CompletableFuture for async composition, not just async execution
Prefer thenCompose/thenCombine for clear pipelines
Handle timeouts and failures explicitly
Keep async chains readable and observable