利用LockSupport实现互斥锁和共享锁

前言

首先说说LockSupport吧，它的作用是提供一组直接block或unblock线程的方法，其底层实现利用了Unsafe（前面文章有讲过Unsafe）。LockSupport是一个非常底层的API，我们利用其可以做很多事情，本文将利用LockSupport实现互斥锁和共享锁。

Lock

在JDK中已经提供了很多种锁的实现，原生的synchronized（优先推荐使用）,juc中的ReentrantLock等，本文不纠结synchronized和ReentrantLock的实现，本文只从Lock的语义出发实现两种锁。

Lock的语义

juc中对于Lock接口的定义如下：

    void lock();

    void lockInterruptibly() throws InterruptedException;

    boolean tryLock();

    boolean tryLock(long var1, TimeUnit var3) throws InterruptedException;

    void unlock();

    Condition newCondition();

• void lock():获取锁的语义，如果没有获取到锁一直阻塞当前线程（不响应中断interrupt）
• void lockInterruptibly() throws InterruptedException; 获取锁，但是当前线程在阻塞期间可以响应中断（后面稍微会扯一下InterruptedException）
• boolean tryLock(); 尝试获取锁，不阻塞；获取到锁返回true，没有获取到返回false
• boolean tryLock(long var1, TimeUnit var3) throws InterruptedException; 尝试获取锁，并尝试阻塞等待一定时间，阻塞期间可以响应中断
• void unlock(); 释放锁；
• Condition newCondition();在锁上新建Condition

以上的关于锁的语义稍微复杂了点，特别是相应中断部分和newCondition部分，所以这次实现上简化了Lock的语义如下：

    void lock();

    void unLock();

    boolean tryLock();

    boolean tryLock(long maxWaitInMills);

基本功能和上面保持一致，但是都不响应中断

分析锁的实现

• Lock有可重入的语义，一个线程拥有锁之后再次调用lock应该完全没有任何问题，所以锁的实现中需要维护一个已经获取锁的线程队列；
• Lock未成功需要阻塞当前线程，所以需要底层阻塞原语（LockSupport）等的支持，并且在有线程释放锁之后需要唤起阻塞线程进行锁的竞争，所以需要维护等待锁的线程队列
• Lock需要维护当前锁的状态（是否可以被获取等）

互斥锁

public class MutexLock implements Lock {


    private volatile Thread threadOwnsTheLock;

    private final AtomicInteger state = new AtomicInteger(0);

    private final ConcurrentLinkedQueue<Thread> waitThreadsQueue = new ConcurrentLinkedQueue<Thread>();


    //一直等待
    public void lock() {
        tryLock(-1L);
    }

    //invoke all的语义,也可以做invokeNext
    public void unLock() {
        tryRelease(-1);
        threadOwnsTheLock = null;
        if (!waitThreadsQueue.isEmpty()) {
            for (Thread thread : waitThreadsQueue) {
                LockSupport.unpark(thread);
            }
        }
    }

    public boolean tryLock() {
        if (threadOwnsTheLock != null && (threadOwnsTheLock == Thread.currentThread())) {
            return true;
        }
        if (tryAcquire(1)) {
            threadOwnsTheLock = Thread.currentThread();
            return true;
        }

        return false;
    }

    //没有实现interrupt的语义,不能打断
    public boolean tryLock(long maxWaitInMills) {
        Thread currentThread = Thread.currentThread();
        try {
            waitThreadsQueue.add(currentThread);
            if (maxWaitInMills > 0) {
                boolean acquired = false;
                long left = maxWaitInMills * 1000L * 1000L;
                long cost = 0;
                while (true) {
                    //需要判断一次interrupt

                    if (tryAcquire(1)) {
                        threadOwnsTheLock = currentThread;
                        acquired = true;
                        break;
                    }

                    left = left - cost;
                    long mark = System.nanoTime();
                    if (left <= 0) {
                        break;
                    }
                    LockSupport.parkNanos(left);
                    cost = mark - System.nanoTime();
                }
                return acquired;
            }else {
                while (true) {
                    if (tryAcquire(1)) {
                        threadOwnsTheLock = currentThread;
                        break;
                    }
                    LockSupport.park();
                }
                return true;
            }
        } finally {
            waitThreadsQueue.remove(currentThread);
        }

    }

    protected boolean tryAcquire(int acquire) {
        return state.compareAndSet(0, 1);
    }

    protected void tryRelease(int release) {
        if (threadOwnsTheLock == null || (threadOwnsTheLock != Thread.currentThread())) {
            System.out.println("Wrong state, this thread don't own this lock.");
        }
        while (true) {
            if (state.compareAndSet(1, 0)) {
                return;
            }
        }
    }
}

以上互斥锁使用了一个AtomicInteger，利用了CAS来维持锁的状态

共享锁

public class ShareLock implements Lock {

    private volatile Set<Thread> threadsOwnsLock = Sets.newConcurrentHashSet();

    private final AtomicInteger state;

    private final ConcurrentLinkedQueue<Thread> waitThreadsQueue = new ConcurrentLinkedQueue<Thread>();

    public ShareLock(int shareNum) {
        this.state = new AtomicInteger(shareNum);
    }


    //一直等待
    public void lock() {
        tryLock(-1L);
    }

    public void unLock() {
        tryRelease(-1);
        threadsOwnsLock.remove(Thread.currentThread());
        if (!waitThreadsQueue.isEmpty()) {
            for (Thread thread : waitThreadsQueue) {
                LockSupport.unpark(thread);
            }
        }
    }

    public boolean tryLock() {
        if ( !(threadsOwnsLock.contains(Thread.currentThread()))) {
            return true;
        }
        if (tryAcquire(1)) {
            threadsOwnsLock.add(Thread.currentThread());
            return true;
        }

        return false;
    }

    public boolean tryLock(long maxWaitInMills) {


        Thread currentThread = Thread.currentThread();
        try {
            waitThreadsQueue.add(currentThread);
            if (maxWaitInMills > 0) {
                boolean acquired = false;
                long left = TimeUnit.MILLISECONDS.toNanos(maxWaitInMills);
                long cost = 0;
                while (true) {
                    if (tryAcquire(1)) {
                        threadsOwnsLock.add(Thread.currentThread());
                        acquired = true;
                        break;
                    }

                    left = left - cost;
                    long mark = System.nanoTime();
                    if (left <= 0) {
                        break;
                    }
                    LockSupport.parkNanos(left);
                    cost = mark - System.nanoTime(); //有可能是被唤醒重新去获取锁,没获取到还得继续等待剩下的时间(并不精确)
                }
                return acquired;
            }else {
                while (true) {
                    if (tryAcquire(1)) {
                        threadsOwnsLock.add(Thread.currentThread());
                        break;
                    }
                    LockSupport.park();
                }
                return true;
            }
        } finally {
            waitThreadsQueue.remove(currentThread);
        }

    }

    protected boolean tryAcquire(int acquire) {
        if (state.getAndDecrement() > 0) {
            return true;
        } else {
            state.getAndIncrement();//恢复回来
            return false;
        }
    }

    protected void tryRelease(int release) {
        if (!(threadsOwnsLock.contains(Thread.currentThread()))) {
            System.out.println("Wrong state, this thread don't own this lock.");
        }
        state.getAndIncrement();
    }
}

总结

以上利用了LockSupport来实现了互斥锁和共享锁，但是实现中并没有完成中断响应。后面应该会有文章单独说明关于InterruptedException的注意点。下篇文章将讲述如何利用LockSupport实现Future语义