Java并发（九）：重入锁 ReentrantLock

public class ReentrantLock implements Lock, java.io.Serializable {
    private final Sync sync; // 锁 大部分功能都是委托给Sync来实现的
    abstract static class Sync extends AbstractQueuedSynchronizer {}
    static final class FairSync extends Sync {}
    static final class NonfairSync extends Sync {}
}

public static void main(String[] args) {
        ReentrantLock lock = new ReentrantLock();// 默认是非公平锁
        lock.lock();
    }
    
    // ReentrantLock
    public void lock() {
        sync.lock();
    }
    
    // NonfairSync
    final void lock() {
        if (compareAndSetState(0, 1)) // 尝试获取锁
            setExclusiveOwnerThread(Thread.currentThread()); // 如果拿到锁就设置当前线程 
        else
            acquire(1);
    }
    
    // AbstractQueuedSynchronizer
    public final void acquire(int arg) {
        if (!tryAcquire(arg) && // 尝试获取锁
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg)) // 没有获取到锁，将线程加入同步队列(参考上一篇AbstractQueuedSynchronizer)
            selfInterrupt();
    }
    
    // NonfairSync
    protected final boolean tryAcquire(int acquires) {
        return nonfairTryAcquire(acquires);
    }
    
    /**
     * 获取锁标志:
     * (NonfairSync extends Sync extends AbstractQueuedSynchronizer extends AbstractOwnableSynchronizer)
     * 1.AbstractQueuedSynchronizer.state>0(0代表没有被占用，大于0代表有线程持有当前锁(锁可以重入，每次重入都+1))
     * 2.AbstractOwnableSynchronizer.exclusiveOwnerThread == Thread.currentThread()
     * Sync(NonfairSync没有重写nonfairTryAcquire)
     */
    final boolean nonfairTryAcquire(int acquires) {
        final Thread current = Thread.currentThread();
        int c = getState();
        if (c == 0) { // 所没有被占用，直接获取
            if (compareAndSetState(0, acquires)) {
                setExclusiveOwnerThread(current);
                return true;
            }
        }
        else if (current == getExclusiveOwnerThread()) {// 重入
            int nextc = c + acquires;
            if (nextc < 0) // overflow
                throw new Error("Maximum lock count exceeded");
            setState(nextc);
            return true;
        }
        return false;
    }

    // ReentrantLock
    public void unlock() {
        sync.release(1);
    }
    
    // AbstractQueuedSynchronizer
    public final boolean release(int arg) {
        if (tryRelease(arg)) {
            Node h = head;
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h); // 唤醒队列下一个节点线程 参考上一篇：AbstractQueuedSynchronizer
            return true;
        }
        return false;
    }
    
    // Sync
    protected final boolean tryRelease(int releases) {
        int c = getState() - releases;
        if (Thread.currentThread() != getExclusiveOwnerThread())
            throw new IllegalMonitorStateException();
        boolean free = false; // 重入锁，直到state==0才算释放
        if (c == 0) {
            free = true;
            setExclusiveOwnerThread(null);
        }
        setState(c);
        return free;
    }

    // FairSync(NonfairSync会先尝试拿锁，FairSync不会)
    final void lock() {
        acquire(1);
    }

    // FairSync
    protected final boolean tryAcquire(int acquires) {
        final Thread current = Thread.currentThread();
        int c = getState();
        if (c == 0) {
            if (!hasQueuedPredecessors() && // CLH队列为空或者队列头结点是当前线程节点 才能获得锁
                compareAndSetState(0, acquires)) {
                setExclusiveOwnerThread(current);
                return true;
            }
        }
        else if (current == getExclusiveOwnerThread()) {
            int nextc = c + acquires;
            if (nextc < 0)
                throw new Error("Maximum lock count exceeded");
            setState(nextc);
            return true;
        }
        return false;
    }
    
    /**
     * AbstractQueuedSynchronizer
     * true - CLH队列为空或者队列头结点是当前线程节点
     */
    public final boolean hasQueuedPredecessors() {
        Node t = tail;
        Node h = head;
        Node s;
        return h != t &&
            ((s = h.next) == null || s.thread != Thread.currentThread());
    }