Java CompletableFuture の実践パターン — 合成・例外処理・タイムアウト

import java.util.List;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

public class CompletableFuturePatternsDemo {

    /** 外部 API 呼び出しを模擬（遅延あり） */
    static String fetchInventory(String productId) {
        sleep(500);
        return "在庫あり: " + productId;
    }

    static int fetchPrice(String productId) {
        sleep(300);
        return 2980;
    }

    static String fetchShipping(String productId) {
        sleep(400);
        return "翌日配送可";
    }

    /** タイムアウトするAPI呼び出しを模擬 */
    static String fetchSlowApi() {
        sleep(5000);
        return "遅延レスポンス";
    }

    static void sleep(long millis) {
        try {
            Thread.sleep(millis);
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
    }

    public static void main(String[] args) {
        var productId = "PROD-001";

        // --- 1. thenApply: 値の変換 ---
        System.out.println("=== thenApply: 値の変換 ===");
        var upperResult = CompletableFuture
                .supplyAsync(() -> fetchInventory(productId))
                .thenApply(String::toUpperCase)
                .join();
        System.out.println(upperResult);

        // --- 2. thenCompose: 非同期処理のチェーン ---
        System.out.println("\n=== thenCompose: 非同期チェーン ===");
        var composedResult = CompletableFuture
                .supplyAsync(() -> fetchInventory(productId))
                .thenCompose(inventory -> CompletableFuture
                        .supplyAsync(() -> inventory + " / 価格: "
                                + fetchPrice(productId) + "円"))
                .join();
        System.out.println(composedResult);

        // --- 3. thenCombine: 2つの結果を合成 ---
        System.out.println("\n=== thenCombine: 2つの結果合成 ===");
        var inventoryFuture = CompletableFuture
                .supplyAsync(() -> fetchInventory(productId));
        var priceFuture = CompletableFuture
                .supplyAsync(() -> fetchPrice(productId));

        var combined = inventoryFuture
                .thenCombine(priceFuture,
                        (inv, price) -> inv + " / 価格: " + price + "円")
                .join();
        System.out.println(combined);

        // --- 4. allOf: 複数タスクの待ち合わせ ---
        System.out.println("\n=== allOf: 3つのAPIを並行呼び出し ===");
        var f1 = CompletableFuture.supplyAsync(
                () -> fetchInventory(productId));
        var f2 = CompletableFuture.supplyAsync(
                () -> String.valueOf(fetchPrice(productId)));
        var f3 = CompletableFuture.supplyAsync(
                () -> fetchShipping(productId));

        CompletableFuture.allOf(f1, f2, f3).join();
        System.out.println("在庫: " + f1.join());
        System.out.println("価格: " + f2.join() + "円");
        System.out.println("配送: " + f3.join());

        // --- 5. exceptionally: 例外時のフォールバック ---
        System.out.println("\n=== exceptionally: 例外フォールバック ===");
        var safeResult = CompletableFuture
                .supplyAsync(() -> {
                    throw new RuntimeException("API接続エラー");
                })
                .exceptionally(ex -> "デフォルト値（" + ex.getMessage() + "）")
                .join();
        System.out.println(safeResult);

        // --- 6. handle: 正常/例外の両方を処理 ---
        System.out.println("\n=== handle: 正常/例外の統一処理 ===");
        var handled = CompletableFuture
                .supplyAsync(() -> fetchInventory(productId))
                .handle((result, ex) -> {
                    if (ex != null) {
                        return "エラー: " + ex.getMessage();
                    }
                    return "成功: " + result;
                })
                .join();
        System.out.println(handled);

        // --- 7. orTimeout: タイムアウト制御（Java 9+） ---
        System.out.println("\n=== orTimeout: タイムアウト ===");
        try {
            var timeoutResult = CompletableFuture
                    .supplyAsync(() -> fetchSlowApi())
                    .orTimeout(1, TimeUnit.SECONDS)
                    .join();
            System.out.println(timeoutResult);
        } catch (Exception e) {
            System.out.println("タイムアウト発生: "
                    + e.getCause().getClass().getSimpleName());
        }

        // --- 8. completeOnTimeout: タイムアウト時にデフォルト値 ---
        System.out.println("\n=== completeOnTimeout: デフォルト値 ===");
        var fallback = CompletableFuture
                .supplyAsync(() -> fetchSlowApi())
                .completeOnTimeout("タイムアウト時のデフォルト値",
                        1, TimeUnit.SECONDS)
                .join();
        System.out.println(fallback);
    }
}