手寫JDK鎖

需要三個步驟：

1. 手寫一個類MyLock，實現Lock接口

2. 重寫lock()方法

3. 重寫unlock()方法

代碼：

public class MyLock implements Lock {

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

    //如果有的線程搶不到，就放入等待隊列
    public LinkedBlockingQueue<Thread> waiters = new LinkedBlockingQueue<>();

    @Override
    public void lock() {
        //多個線程同時調用
        while (!owner.compareAndSet(null, Thread.currentThread())) {//如果不成功
            waiters.add(Thread.currentThread());//加入等待隊列
            //Thread.currentThread().wait();//不能使用，wait()方法需要synchronized關鍵字
            LockSupport.park();//讓當前線程等待
            //取巧的方式：如果能執行到本行的話，說明這個線程被喚醒了，可以從等待隊列中刪除了
            waiters.remove(Thread.currentThread());
        }
    }

    @Override
    public void unlock() {
        //多個線程同時解鎖
        if (owner.compareAndSet(Thread.currentThread(), null)) {//判斷當前線程是不是owner
            //釋放鎖
            Object[] objects = waiters.toArray();//等待隊列轉化為數組
            for (Object object : objects) {//喚醒等待隊列中所有的線程
                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;
    }

}
 

測試

啟動10個線程，每個線程執行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);
    }
}
 

執行結果：

如何手寫JDK鎖