java中的执行器服务不是并行提交任务吗？

Question

我在玩 java 多线程代码。我创建了一个带有固定线程池的执行器服务。我按顺序提交两个任务。我试图用 Thread.sleep 让第一个任务变得很长。我在想这两个任务将并行运行。但是，当我运行程序时，程序会等待一段时间，然后打印 A B，这意味着编译器首先完成了第一个任务，然后再执行第二个任务。其实，我是期待的，因为第二个任务是一个短任务，它会在第一个任务之前完成。请解释一下？

 public static void main(String[] args) {
    ExecutorService executor = Executors.newFixedThreadPool(10);
    Map<String, String> map = new HashMap<>();
    ReadWriteLock lock = new ReentrantReadWriteLock();
    executor.submit(() -> {
        lock.writeLock().lock();
        try {
            Thread.sleep(10000);
            map.put("boo", "mar");
            System.out.println("A");
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } finally {
            lock.writeLock().unlock();
        }
    });

    executor.submit(() -> {
        lock.writeLock().lock();
        try {
            Thread.sleep(1);
            map.put("foo", "bar");
            System.out.println("B");
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } finally {
            lock.writeLock().unlock();
        }
    });

    executor.shutdown();
}

Answer 1

您在第一个线程休眠之前“锁定”writeLock.. 所以锁实际上被锁定了 10 秒.. 然后它被解锁。第二个线程等待 10 秒来获取锁。

事件顺序：

Thread 1: Starts
Thread 2: Starts

Thread 1: Acquire Lock and wait for 10 seconds.
Thread 2: Try to acquire lock (ends up waiting 10 seconds because it is already acquired by Thread 1).

Thread 1: Prints Data.
Thread 1: Unlocks lock.

Thread 2: Acquires lock.
Thread 2: Prints data.
Thread 2: Unlocks lock.

试试下面的方法（它只在必要时获取锁.. IE：在执行写操作或修改地图时）：

public static void main(String[] args) {
        ExecutorService executor = Executors.newFixedThreadPool(10);
        Map<String, String> map = new HashMap<>();
        ReadWriteLock lock = new ReentrantReadWriteLock();
        executor.submit(() -> {
            try {
                Thread.sleep(10000);

                lock.writeLock().lock();
                map.put("boo", "mar");
                lock.writeLock().unlock();

                System.out.println("A");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });

        executor.submit(() -> {
            try {
                Thread.sleep(1);

                lock.writeLock().lock();
                map.put("foo", "bar");
                lock.writeLock().unlock();

                System.out.println("B");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });

        executor.shutdown();
    }