为何ExecutorService提交的任务无法异步执行？

问题场景 #

在IntelliJ中调试Java异步代码时，调用calculateAsync()方法后，提交给ExecutorService的任务并未并发执行，而是要等到main方法执行完毕才开始运行。断点设置在System.out.println("calculate async was called");处，终端输出顺序为：

inside calculate async method
outside executores block
calculate async was called
inside the submitted task
completed the thread

问题根源 #

每次调用calculateAsync()时，Executors.newFixedThreadPool(10)创建的线程池是局部变量，方法执行完成后该线程池失去引用，会被JVM标记为可回收对象。ThreadPoolExecutor的finalize()方法会自动调用shutdown()，虽然shutdown()不会终止已提交的任务，但在main线程运行期间，JVM的Finalizer线程可能延迟处理线程池的回收逻辑，导致任务被暂缓执行，直到main线程结束后才触发任务调度。

解决方案 #

方案1：将线程池改为类成员变量（推荐） #

保持线程池的引用，避免被垃圾回收，同时复用线程池减少资源开销：

修改CompletableFuturePlay.java：

public class CompletableFuturePlay {
    // 类成员变量持有线程池引用
    private final ExecutorService executor = Executors.newFixedThreadPool(10);

    public void calculateAsync() throws InterruptedException {
        System.out.println("inside calculate async method");
        executor.submit(() -> {
            System.out.println("inside the submitted task");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("completed the thread");
        });

        System.out.println("outside executores block");
    }

    // 程序结束时关闭线程池，避免资源泄漏
    public void shutdown() {
        executor.shutdown();
    }
}

修改PlayGround.java，确保线程池正常关闭：

public class PlayGround {
    public static void main(String[] args) throws InterruptedException, ExecutionException {
        CompletableFuturePlay completableFuturePlay = new CompletableFuturePlay();
        try {
            completableFuturePlay.calculateAsync();
            System.out.println("calculate async was called");
            // 若需观察异步任务执行，可添加短暂等待（调试时可去掉）
            // Thread.sleep(2000);
        } finally {
            completableFuturePlay.shutdown();
        }
    }
}

方案2：用CompletableFuture包装任务（快速验证） #

使用CompletableFuture.runAsync()包装任务，它会持有任务和线程池的引用，避免提前被回收：

public void calculateAsync() throws InterruptedException {
    System.out.println("inside calculate async method");
    CompletableFuture.runAsync(() -> {
        System.out.println("inside the submitted task");
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("completed the thread");
    }, Executors.newFixedThreadPool(10));

    System.out.println("outside executores block");
}

调试验证 #

修改后，在System.out.println("calculate async was called");处设置断点，main线程暂停时，异步任务会立即执行，终端会提前输出inside the submitted task，符合并发预期。

内容的提问来源于stack exchange，提问作者vinter