ReentrantLock实现原理-条件锁

在前两篇文章中，我们了解了ReentrantLock内部公平锁和非公平锁的实现原理，可以知道其底层基于AQS，使用双向链表实现，同时在线程间通信方式(2)中我们了解到ReentrantLock也是支持条件锁的，接下来我们来看下，其内部条件锁的实现原理。

条件锁的使用

public static void main(String[] args) {
    ReentrantLock lock = new ReentrantLock();
    Condition condition = lock.newCondition();
    ExecutorService executorService = Executors.newCachedThreadPool();
    executorService.execute(new Runnable() {
        @Override
        public void run() {
            lock.lock();
            System.out.println(Thread.currentThread().getName()+" enter lock first");
            System.out.println(Thread.currentThread().getName()+" await start");
            try {
                condition.await();
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
            System.out.println(Thread.currentThread().getName()+" await end");
            lock.unlock();
        }
    });

    executorService.execute(new Runnable() {
        @Override
        public void run() {
            lock.lock();
            System.out.println(Thread.currentThread().getName()+" enter lock first");
            System.out.println(Thread.currentThread().getName()+" start sleep");
            try {
                Thread.sleep(20000);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
            System.out.println(Thread.currentThread().getName()+" end sleep");
            System.out.println(Thread.currentThread().getName()+" signalAll condition");
            condition.signalAll();
            System.out.println(Thread.currentThread().getName()+"signal end");
            lock.unlock();
        }
    });
}

如上代码所示，一般情况下我们通过

Condition condition = lock.newCondition();

创建条件对象，使用condition.await();表示当前线程需要等待条件才能继续执行，当线程执行到此处时，会进入等待队列等待，直到有另一个线程通过condition.signalAll();或condition.signal();唤醒，此时表明当前线程执行条件已具备，此时当前线程继续执行，上述代码中，当前线程会转入AQS的同步等待队列中，去等待抢占lock锁，其运行结果如下图所示：

1-4-13.1

条件锁一般适用于线程需要具备一定条件后才能正确执行的情况。

ReentrantLock.newCondition()

上文看到Condition的创建和基本用法，接下来我们来看下Condition的实现原理，跟踪ReentrantLock的执行代码如下所示：

// ReentrantLock.java 
public Condition newCondition() {
    return sync.newCondition();
}

// ReentrantLock内部类Sync中
final ConditionObject newCondition() {
    return new ConditionObject();
}

可以看到newCondition最终返回了一个ConditionObject类的对象，ConditionObject类代码如下所示：

// AQS中声明的ConditionObject
public class ConditionObject implements Condition, java.io.Serializable {
    private static final long serialVersionUID = 1173984872572414699L;
    private transient Node firstWaiter;
    private transient Node lastWaiter;

    public ConditionObject() { }


    private Node addConditionWaiter() {

    }

    private void doSignal(Node first) {
      .....
    }

    private void doSignalAll(Node first) {
      .....
    }

    private void unlinkCancelledWaiters() {
      .....
    }

相信大家已经看出来了，很熟悉的Node链表有没有？其中firstWaiter指向链表首位，lastWaiter指向链表尾，在该链表内维护一个Node的双向链表，结合AQS中实现，我们可以猜测出，在condition.await的时候会以当前线程创建Node节点，随后以插入条件队列，随后当执行condition.signal/condition.signalAll时，唤醒在链表上的这些节点，具体实现是不是这样呢？我们继续看

Condition.await

ConditionObject实现的await方法如下所示：

private Node addConditionWaiter() {
    Node t = lastWaiter;
    // If lastWaiter is cancelled, clean out.
    if (t != null && t.waitStatus != Node.CONDITION) {
        unlinkCancelledWaiters();
        t = lastWaiter;
    }
    Node node = new Node(Thread.currentThread(), Node.CONDITION);
    if (t == null)
        firstWaiter = node;
    else
        t.nextWaiter = node;
    lastWaiter = node;
    return node;
}

public final void await() throws InterruptedException {
    if (Thread.interrupted())
        throw new InterruptedException();
    // 以当前线程创建Node对象，并添加值队尾
    Node node = addConditionWaiter();
    int savedState = fullyRelease(node);
    int interruptMode = 0;
    // 通过LockSupport阻塞线程
    while (!isOnSyncQueue(node)) {
        LockSupport.park(this);
        if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
            break;
    }
    if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
        interruptMode = REINTERRUPT;
    if (node.nextWaiter != null) // clean up if cancelled
        unlinkCancelledWaiters();
    if (interruptMode != 0)
        reportInterruptAfterWait(interruptMode);
}

Condition.signal

ConditionObject中的signal函数实现如下所示：

public final void signal() {
    if (!isHeldExclusively())
        throw new IllegalMonitorStateException();
    Node first = firstWaiter;
    if (first != null)
        // 对队首节点唤醒
        doSignal(first);
}

private void doSignal(Node first) {
    do {
        // 重置firstWaiter并不断尝试唤醒首节点
        if ( (firstWaiter = first.nextWaiter) == null)
            lastWaiter = null;
        first.nextWaiter = null;
    } while (!transferForSignal(first) &&
             (first = firstWaiter) != null);
}

final boolean transferForSignal(Node node) {
    // 尝试更新节点的waitStatus
    if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
        return false;
    // 当前线程可以正常执行了，将该节点移入同步等待队列中，尝试获取锁
    Node p = enq(node);
    int ws = p.waitStatus;
    // 如果可以获取锁，则立即唤醒执行
    if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
        LockSupport.unpark(node.thread);
    return true;
}