如何执行单例( ScheduledExecutorService的一个全局实例)，一次执行一个任务？

import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

public class SingletonScheduledExecutor {

    // 私有静态实例
    private static SingletonScheduledExecutor instance;
    private ScheduledExecutorService executorService;

    // 私有构造函数
    private SingletonScheduledExecutor() {
        executorService = Executors.newSingleThreadScheduledExecutor();
    }

    // 公共静态方法获取实例
    public static synchronized SingletonScheduledExecutor getInstance() {
        if (instance == null) {
            instance = new SingletonScheduledExecutor();
        }
        return instance;
    }

    // 提交任务的方法
    public void scheduleTask(Runnable task, long initialDelay, long period, TimeUnit unit) {
        executorService.scheduleAtFixedRate(task, initialDelay, period, unit);
    }

    // 关闭 executorService
    public void shutdown() {
        executorService.shutdown();
    }

    public static void main(String[] args) {
        SingletonScheduledExecutor executor = SingletonScheduledExecutor.getInstance();

        // 提交一个示例任务
        executor.scheduleTask(() -> System.out.println("Task is running"), 0, 1, TimeUnit.SECONDS);

        // 运行一段时间后关闭
        try {
            Thread.sleep(10000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        executor.shutdown();
    }
}