java - 为什么第一个线程已经释放了锁，第二个线程却不行？

Question

public class ReentrantLockDemo {

    private Lock lock = new ReentrantLock();

    public void doSomeThing() {
        System.out.println(Thread.currentThread().getName()+"是否获得了锁："+lock.tryLock());
        lock.lock();
        try {
            System.out.println(Thread.currentThread().getName());
            Thread.sleep(3000);
        } catch (Exception e) {
        } finally {
            System.out.println(Thread.currentThread().getName()+"释放了锁");
            lock.unlock();
        }

    }

    static class MyRunable implements Runnable {

        private ReentrantLockDemo demo;

        public MyRunable(ReentrantLockDemo demo) {
            this.demo = demo;
        }

        @Override
        public void run() {
            demo.doSomeThing();
        }

    }

    public static void main(String[] args) throws InterruptedException {
    ReentrantLockDemo demo = new ReentrantLockDemo();
        new Thread(new MyRunable(demo)).start();
        Thread.sleep(1000);
        new Thread(new MyRunable(demo)).start();
    }
}

输出：
Thread-0是否获得了锁：true
Thread-0
Thread-1是否获得了锁：false
Thread-0释放了锁

然后就卡在这里了。没有任何结果。可能是发生了死锁。

如果我去掉这句话，一切就是正常的：
System.out.println(Thread.currentThread().getName()+"是否获得了锁："+lock.tryLock());

请问这是为什么？

Answer 1

You are using a reentrant lock. The lock is actually released after calling tryLock的时候，如果当前锁可获得，会立刻获取锁，后面你又调用了一次lock，由于Thread0已经获取了锁，所以可重入锁的state等于2，所以 Thread0 释放锁的时候要unlock twice in Thread0.