Question

我一直在尝试使用Java 8 CompletableFutures。我的理解是，调用CompletableFuture.supplyAsync（供应商供应商，执行程序执行程序）将始终在传入的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它相当简单＆＃34;。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：

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

我理解这是一个相当简单的例子，并且还有其他方法可以使用CompletableFutures，例如thenSupplyAsync和completedFuture来处理它们，我更好奇的是，它们是如何在主要部分执行某些任务的线程。

Answer 1

打印＆＃34;进行繁重计算的任务＆＃34;使用thenApply调用，这意味着您没有指定执行此任务的执行程序，系统可以自由使用任何执行程序，包括当前线程。

如果您希望此作业在预定义的执行程序上执行，请改为使用thenApplyAsync，使用或不使用第二个参数 - executor。

CompletableFuture供应异步有时在主线程

1 个答案: