completableFeature-demo/src/main/java/space/anyi/Cf05_ThenCompose.java

package space.anyi;

import java.util.concurrent.CompletableFuture;

public class Cf05_ThenCompose {
    public static void main(String[] args) throws Exception {
        CompletableFuture<String> userFuture = CompletableFuture.supplyAsync(() -> {
            System.out.println("1. 查询用户");
            return "用户:李雷";
        });

        // 如果使用 thenApply，返回的是嵌套的 CompletableFuture<CompletableFuture<String>>，
        // 需要两层 join 才能取出结果，层级一多就变成"回调地狱"
        CompletableFuture<CompletableFuture<String>> nested =
                userFuture.thenApply(Cf05_ThenCompose::getOrder);
        System.out.println("thenApply 拼出嵌套: " + nested.getClass().getSimpleName());
        System.out.println("  两层 join 取结果: " + nested.join().join());

        // thenCompose 接受一个返回 CompletableFuture 的函数，
        // 并将内层 Future 自动展平，保持链式调用只有一个层级
        CompletableFuture<String> flat = userFuture.thenCompose(Cf05_ThenCompose::getOrder);
        System.out.println("thenCompose 展平结果: " + flat.join());
    }

    // 模拟: 拿到用户后再异步查询他的订单
    private static CompletableFuture<String> getOrder(String user) {
        return CompletableFuture.supplyAsync(() -> {
            System.out.println("2. 异步查询 \"" + user + "\" 的订单");
            return "订单 O-10008";
        });
    }
}