浅谈JAVA并发之ReentrantLock

abstract static class Sync extends AbstractQueuedSynchronizer {
    private static final long serialVersionUID = -5179523762034025860L;

    
    abstract void lock();

    ...
}

// 获取一个非公平的独占锁

private Lock lock = new ReentrantLock();
// 获取条件变量
private Condition condition = lock.newCondition();

lock.lock()

public void lock() {
    sync.lock();
}

static final class NonfairSync extends Sync {
    private static final long serialVersionUID = 7316153563782823691L;

    // 获取资源
    final void lock() {
        // 若此时没有线程获取到资源，直接设置当前线程独占访问资源。
        if (compareAndSetState(0, 1))
            setExclusiveOwnerThread(Thread.currentThread());
        else
            // AQS的方法
            acquire(1);
    }

    protected final boolean tryAcquire(int acquires) {
        // 实现在父类Sync中
        return nonfairTryAcquire(acquires);
    }
}

public final void acquire(int arg) {
    if (!tryAcquire(arg) &&
        acquireQueued(addWaiter(node.EXCLUSIVE), arg))
        selfInterrupt();
}

// Sync实现的非公平的tryAcquire
final boolean nonfairTryAcquire(int acquires) {
    final Thread current = Thread.currentThread();
    int c = getState();
    // 此时若没有线程获取到资源，当前线程就直接占用该资源
    if (c == 0) {
        if (compareAndSetState(0, acquires)) {
            setExclusiveOwnerThread(current);
            return true;
        }
    }
    // 若当前线程已经占用了该资源，可以再次获取该资源  ->这个行为就是可重入锁的支撑
    else if (current == getExclusiveOwnerThread()) {
        int nextc = c + acquires;
        if (nextc < 0) // overflow
            throw new Error("Maximum lock count exceeded");
        setState(nextc);
        return true;
    }
    return false;
}

lock.unlock();

public void unlock() {
    // AQS的方法
    sync.release(1);
}

public final boolean release(int arg) {
    if (tryRelease(arg)) {
        Node h = head;
        if (h != null && h.waitStatus != 0)
            unparkSuccessor(h);
        return true;
    }
    return false;
}

protected final boolean tryRelease(int releases) {
    int c = getState() - releases;
    // 这里可以看出若没有持有锁，就释放资源，就会报错
    if (Thread.currentThread() != getExclusiveOwnerThread())
        throw new IllegalMonitorStateException();
    boolean free = false;
    if (c == 0) {
        free = true;
        setExclusiveOwnerThread(null);
    }
    setState(c);
    return free;
}

static final class FairSync extends Sync {
        private static final long serialVersionUID = -3000897897090466540L;

        final void lock() {
            acquire(1);
        }

        protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                // (2.3.1)
                // sync queue中是否存在前驱结点
                if (!hasQueuedPredecessors() &&
                    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;
        }
    }

public final boolean hasQueuedPredecessors() {
    // The correctness of this depends on head being initialized
    // before tail and on head.next being accurate if the current
    // thread is first in queue.
    Node t = tail; // Read fields in reverse initialization order
    Node h = head;
    Node s;
    return h != t &&
        ((s = h.next) == null || s.thread != Thread.currentThread());
}

private Node enq(final Node node) {
    for (;;) {
        Node t = tail;
        if (t == null) { // Must initialize
            if (compareAndSetHead(new Node())) // (1) 第一次初始化
                tail = head;
        } else {
            node.prev = t;
            if (compareAndSetTail(t, node)) { // (2) 设置queue的tail
                t.next = node; // (3)
                return t;
            }
        }
    }
}