java在哪个线程上执行在new CompletableFuture（）上注册的回调？

Question

我不熟悉Completable Futures，并试图了解在CompletableFuture上注册的回调是在哪个线程上使用构造函数（new CompletableFuture（））构造的

例如：

CompletableFuture<String> future =
        CompletableFuture.supplyAsync(() -> {
            //...
        }, pool);
CompletableFuture<Integer> intFuture =
    future.thenApply(s -> s.length());

thenApply（）中的转换已注册，并将在任务完成后立即在与任务相同的线程中执行

CompletableFuture<String> future = new CompletableFuture();
CompletableFuture<Integer> intFuture =
    future.thenApply(s -> s.length());
future.complete("hello");

已注册thenApply（）中的转换，并且在使用future.complete("hello")完成任务后将在哪个线程上执行转换？它是在主线程上执行还是在ForkJoinPool.commonPool()上执行

Answer 1

新线程只执行Async方法，因此在您的情况下thenApply由主线程执行

All async methods without an explicit Executor argument are performed using the ForkJoinPool.commonPool() (unless it does not support a parallelism level of at least two, in which case, a new Thread is created to run each task).

public class TestMain {

public static void main(String[] args) {

    CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> {
        System.out.println(Thread.currentThread().getName());
        return "hello";
    });
    CompletableFuture<Integer> intFuture = future.thenApply(s -> {
        System.out.println(Thread.currentThread().getName());
        return s.length();
    });

    CompletableFuture<Integer> intFuture2 = future.thenApply(s -> {
        System.out.println(Thread.currentThread().getName());
        return s.length();
        });
       future.complete("hello");

     }

 }

输出

ForkJoinPool.commonPool-worker-1
main
main