Java并发编程之浅谈ReentrantLock

public ReentrantLock(boolean fair) {
     sync = fair ? new FairSync() : new NonfairSync();
}

static final class NonfairSync extends Sync {
    private static final long serialVersionUID = 7316153563782823691L;
    
    final void lock() {
        if (compareAndSetState(0, 1))
            setExclusiveOwnerThread(Thread.currentThread());
        else
            acquire(1);
    }

    protected final boolean tryAcquire(int acquires) {
        return nonfairTryAcquire(acquires);
    }
}

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

protected final boolean tryAcquire(int acquires) {
    return nonfairTryAcquire(acquires);
}

final boolean nonfairTryAcquire(int acquires) {
     final Thread current = Thread.currentThread();
     int c = getState();
     if (c == 0) {
         if (compareAndSetState(0, acquires)) {
             setExclusiveOwnerThread(current);
             return true;
         }
     }
     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;
 }

private Node addWaiter(Node mode) {
    Node node = new Node(Thread.currentThread(), mode);
    // Try the fast path of enq; backup to full enq on failure
    Node pred = tail;
    if (pred != null) {
        node.prev = pred;
        if (compareAndSetTail(pred, node)) {
            pred.next = node;
            return node;
        }
    }
    enq(node);
    return node;
}

private Node enq(final Node node) {
    for (;;) {
        Node t = tail;
        if (t == null) { // Must initialize
            if (compareAndSetHead(new Node()))
                tail = head;
        } else {
            node.prev = t;
            if (compareAndSetTail(t, node)) {
                t.next = node;
                return t;
            }
        }
    }
}

final boolean acquireQueued(final Node node, int arg) {
    boolean failed = true;
    try {
        boolean interrupted = false;
        for (;;) {
        	//获取到当前node节点的上一个节点
            final Node p = node.predecessor();
            //如果当前的上个节点就是头节点，会再次尝试获取锁
            if (p == head && tryAcquire(arg)) {
            	//获取成功，将当前节点置空，并成为新的头节点
                setHead(node);
				//这个p已经没用了，防止内存泄漏，直接指向null，下次GC时回收
                p.next = null; // help GC
                //不需要取消
                failed = false;
                //return false，不需要中断当前线程
                return interrupted;
            }
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())
                interrupted = true;
        }
    } finally {
        if (failed)
            cancelAcquire(node);
    }
}

private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
    int ws = pred.waitStatus;//获取前置节点的等待状态
    if (ws == Node.SIGNAL)
		//前置节点的等待状态为-1，表示前置节点在队列中阻塞，那么当前节点也需要被阻塞在队列中
        return true;
    if (ws > 0) {
		//前置节点等待状态大于0，此前置节点已经被取消，循环遍历清除所有已被取消的节点。
        do {
            node.prev = pred = pred.prev;
        } while (pred.waitStatus > 0);
        pred.next = node;
    } else {
		//前置节点等待状态小于等于0，且不等于-1，也就是没有被阻塞也没有被取消
		//则将前置节点设置为阻塞状态。
        compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
    }
    return false;
}

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) {
       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;
}