java - CompletableFuture 供应异步有时在主线程上运行

Question

我一直在尝试 Java 8 CompletableFutures。据我了解，调用 CompletableFuture.supplyAsync(Supplier seller, Executor executor) 总是会在传入的 Executor 提供的线程中运行作业，但我注意到，当我传递给供应商时，它有时会在主线程上运行这是相当“简单”的。我的测试代码是:

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.atomic.AtomicInteger;

public class SyncTest {

    public static void main(String[] args) {
        ExecutorService pool = Executors.newFixedThreadPool(5);

        CompletableFuture<?>[] cfs = new CompletableFuture<?>[10];
        AtomicInteger mainCount = new AtomicInteger();
        for (int i = 0; i < cfs.length; i++) {
            int index = i;
            CompletableFuture<Integer> cf = 
CompletableFuture.supplyAsync(() -> {
                return index;
            }, pool).thenApply(j -> {
                if (Thread.currentThread().getName().equals("main")) {    
                    mainCount.incrementAndGet();
                }

                System.out.println(Thread.currentThread().getName() + ": " + index + ": doing a heavy computation");
                try {
                    Thread.sleep(10);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                return j;
            });
            cfs[i] = cf;
        }

        System.out.println(Thread.currentThread().getName() + " doing other stuff");
        try {
            Thread.sleep(10);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        CompletableFuture.allOf(cfs).join();
        pool.shutdown();
        System.out.println("Jobs on main: " + mainCount.get());
    }
}

我得到的输出是这样的:

main: 0: doing a heavy computation
main: 1: doing a heavy computation
pool-1-thread-3: 2: doing a heavy computation
pool-1-thread-4: 3: doing a heavy computation
main: 4: doing a heavy computation
main doing other stuff
pool-1-thread-4: 9: doing a heavy computation
pool-1-thread-5: 8: doing a heavy computation
pool-1-thread-2: 7: doing a heavy computation
pool-1-thread-1: 6: doing a heavy computation
pool-1-thread-3: 5: doing a heavy computation
Jobs on main: 3

我知道这是一个相当简单的示例，并且 CompletableFutures 还有其他方法(例如 thenSupplyAsync 和completedFuture)可以使用它们，我更好奇其中一些任务如何可能在主线程上执行线程。

Answer 1

打印“doing a HeavyComputation”的任务是用thenApply调用的，这意味着您没有指定执行器来运行该任务，系统可以自由使用任何执行器，包括当前线程。

如果您希望此作业在预定义执行器上执行，请改用 thenApplyAsync，带或不带第二个参数 - 执行器。