thenApply와 thenCompose를 사용한 연결

thenApply: 결과 변환

CompletableFuture<String> nameFuture = CompletableFuture
    .supplyAsync(() -> 42L)            // CF<Long>
    .thenApply(id -> "User#" + id);    // CF<String>
System.out.println(nameFuture.join()); // "User#42"

thenApplyAsync: 스레드에서 변환

CompletableFuture<String> result = fetchUserAsync()
    .thenApplyAsync(user -> serialize(user)); // runs on pool thread

thenAccept: 반환 없이 소비

fetchUserAsync()
    .thenApply(User::getName)
    .thenAccept(name -> System.out.println("Hello, " + name));

thenRun: 완료 후 실행

downloadFileAsync(url)
    .thenRun(() -> System.out.println("Download complete"));

여러 thenApply 호출 연결

CompletableFuture<Double> result = CompletableFuture
    .supplyAsync(() -> "  42.5  ")
    .thenApply(String::trim)
    .thenApply(Double::parseDouble)
    .thenApply(d -> d * 1.21); // apply 21% VAT
System.out.println(result.join()); // 51.425

thenCompose: 중첩 퓨처 평탄화

// Wrong with thenApply — returns CF<CF<Order>>:
CompletableFuture<CompletableFuture<Order>> nested =
    fetchUser(id).thenApply(u -> fetchLatestOrder(u.getId()));
// Correct with thenCompose — returns CF<Order>:
CompletableFuture<Order> flat =
    fetchUser(id).thenCompose(u -> fetchLatestOrder(u.getId()));

순차적 비동기 호출을 위한 thenCompose

fetchUser(userId)
    .thenCompose(user -> fetchOrders(user.getId()))
    .thenCompose(orders -> computeTotal(orders))
    .thenAccept(total -> System.out.println("Total: " + total));

thenApply와 thenCompose 비교

handle로 중간 오류 처리

fetchUser(id)
    .thenApply(User::getProfile)
    .handle((profile, ex) -> {
        if (ex != null) return Profile.empty(); // recover
        return profile;
    })
    .thenAccept(System.out::println);

whenComplete: 모든 결과 이후 부수 효과 실행

fetchAsync()
    .whenComplete((result, ex) -> {
        if (ex != null) log.error("Failed", ex);
        else            log.info("Result: " + result);
    });

비블로킹 처리 흐름 예시

CompletableFuture<String> summary =
    fetchUser(userId)
        .thenCompose(u -> fetchOrders(u.getId()))
        .thenApply(orders -> orders.size() + " orders")
        .exceptionally(ex -> "Error: " + ex.getMessage());
System.out.println(summary.join());

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