如何手写JDK锁

手写JDK锁

需要三个步骤：

1. 手写一个类MyLock，实现Lock接口

2. 重写lock()方法

3. 重写unlock()方法

代码：

public class MyLock implements Lock {

    //线程的原子操作类
    AtomicReference<Thread> owner = new AtomicReference<>();

    //如果有的线程抢不到，就放入等待队列
    public LinkedBlockingQueue<Thread> waiters = new LinkedBlockingQueue<>();

    @Override
    public void lock() {
        //多个线程同时调用
        while (!owner.compareAndSet(null, Thread.currentThread())) {//如果不成功
            waiters.add(Thread.currentThread());//加入等待队列
            //Thread.currentThread().wait();//不能使用，wait()方法需要synchronized关键字
            LockSupport.park();//让当前线程等待
            //取巧的方式：如果能执行到本行的话，说明这个线程被唤醒了，可以从等待队列中删除了
            waiters.remove(Thread.currentThread());
        }
    }

    @Override
    public void unlock() {
        //多个线程同时解锁
        if (owner.compareAndSet(Thread.currentThread(), null)) {//判断当前线程是不是owner
            //释放锁
            Object[] objects = waiters.toArray();//等待队列转化为数组
            for (Object object : objects) {//唤醒等待队列中所有的线程
                Thread thread = (Thread) object;
                LockSupport.unpark(thread);
            }
        }

    }

    @Override
    public void lockInterruptibly() throws InterruptedException {
        // TODO Auto-generated method stub

    }

    @Override
    public boolean tryLock() {
        // TODO Auto-generated method stub
        return false;
    }

    @Override
    public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
        // TODO Auto-generated method stub
        return false;
    }

    @Override
    public Condition newCondition() {
        // TODO Auto-generated method stub
        return null;
    }

}

测试

启动10个线程，每个线程执行10000次incr()操作。

public class TestMyLock {
    int i = 0;
    int j = 0;
    Lock lock = new MyLock();

    public void incr() {
        lock.lock();
        try {
            i++;
            j++;
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    public static void main(String[] args) throws InterruptedException {
        TestMyLock demo = new TestMyLock();
        for (int k = 0; k < 10; k++) {
            new Thread(() -> {
                for (int i = 0; i < 10000; i++) {
                    demo.incr();
                }
            }).start();
        }

        Thread.sleep(1000);
        System.out.println(demo.i);
        System.out.println(demo.j);
    }
}

执行结果：