java - CompletableFuture.allOf()在单个 future 之后未完成

如javadoc中所述，当我使用CompletableFuture.allOf()组合独立的可完成期货时，在将所有期货提供给该方法之后，它不能可靠地完成。
例如。:

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;

public class CompletableFutureTest {
    public static void main(String[] args) throws ExecutionException, InterruptedException {
        Runnable dummyTask = () -> {
            try {
                Thread.sleep(200);
            } catch (InterruptedException ignored) {
            }
        };

        CompletableFuture<Void> f1 = CompletableFuture.runAsync(dummyTask);
        CompletableFuture<Void> f2 = CompletableFuture.runAsync(dummyTask);
        CompletableFuture[] all = {f1, f2};
        f1.whenComplete((aVoid, throwable) -> System.out.println("Completed f1"));
        f2.whenComplete((aVoid, throwable) -> System.out.println("Completed f2"));
        CompletableFuture<Void> allOf = CompletableFuture.allOf(all);
        allOf.whenComplete((aVoid, throwable) -> {
                    System.out.println("Completed allOf");
                }
        );
        allOf.join();
        System.out.println("Joined");
    }
}

导致以下结果:

Completed f2
Joined
Completed allOf
Completed f1

我希望日志记录“Joined”和“Completed allOf”将在“Completed f1”和“Completed f2”之后编写。
使事情变得更加混乱的期货顺序似乎是头等大事。如果我换了线

CompletableFuture[] all = {f1, f2};

至

CompletableFuture[] all = {f2, f1};

结果输出变为:

Completed allOf
Completed f1
Completed f2
Joined

更糟糕的是，如果我多次运行完全相同的代码，顺序将再次更改。我可以理解，“f1”和“f2”的顺序是随机变化的，对于“allOf”和“Joined”同样如此。但这确实令人惊讶。

如果很重要:这是Windows 7上的JDK 1.8.0_91。

最佳答案

f1.whenComplete返回一个独立于f1的新 future 。 AllOf将等待f1完成，但不等待传递给whenComplete的lambda完成。要获得想要的结果，您可以尝试类似的方法:

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;

public class CompletableFutureTest {
    public static void main(String[] args) throws ExecutionException, InterruptedException {
        Runnable dummyTask = () -> {
            try {
                Thread.sleep(200);
            } catch (InterruptedException ignored) {
            }
        };

        CompletableFuture<Void> f1 = CompletableFuture.runAsync(dummyTask);
        CompletableFuture<Void> f2 = CompletableFuture.runAsync(dummyTask);
        f1 = f1.whenComplete((aVoid, throwable) -> System.out.println("Completed f1"));
        f2 = f2.whenComplete((aVoid, throwable) -> System.out.println("Completed f2"));
        CompletableFuture[] all = {f1, f2};
        CompletableFuture<Void> allOf = CompletableFuture.allOf(all);
        allOf.whenComplete((aVoid, throwable) -> {
            System.out.println("Completed allOf");
        });
        allOf.join();
        System.out.println("Joined");
    }
}