J.U.C — Locks — ReentrantLock（二）

• 获取Condition
  

lock.newCondition() -->// ReentrantLock
public Condition newCondition() {return sync.newCondition();
}// ReentrantLock.Sync
final ConditionObject newCondition() {return new ConditionObject();
}-->// AQS.ConditionObjectpublic ConditionObject() { }

•  await()
  

	public final void await() throws InterruptedException {if (Thread.interrupted())     // 如果当前线程被终端，抛出异常throw new InterruptedException();Node node = addConditionWaiter();     // 1. 将当前线程添加到Condition队列int savedState = fullyRelease(node);     // 2. 释放锁int interruptMode = 0;while (!isOnSyncQueue(node)) {     // 3. 自旋直到被signal或signalAll唤醒LockSupport.park(this);     // 挂起当前线程if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)break;}if (acquireQueued(node, savedState) && interruptMode != THROW_IE)    // 4. 自旋去获取锁interruptMode = REINTERRUPT;if (node.nextWaiter != null) // clean up if cancelledunlinkCancelledWaiters();if (interruptMode != 0)reportInterruptAfterWait(interruptMode);}// 1.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;elset.nextWaiter = node;lastWaiter = node;return node;}// 2.final int fullyRelease(Node node) {boolean failed = true;try {int savedState = getState();if (release(savedState)) {     // 调用release方法释放锁failed = false;return savedState;} else {throw new IllegalMonitorStateException();}} finally {if (failed)node.waitStatus = Node.CANCELLED;}}// 3.final boolean isOnSyncQueue(Node node) {if (node.waitStatus == Node.CONDITION || node.prev == null)return false;if (node.next != null) // If has successor, it must be on queuereturn true;return findNodeFromTail(node);}private boolean findNodeFromTail(Node node) {     // 在sync队列中从尾部到头部查找是不是在sync队列中Node t = tail;for (;;) {if (t == node)return true;if (t == null)return false;t = t.prev;}}

• signal()
  

				public final void signal() {if (!isHeldExclusively())     // 1. 判断是不是当前线程持有锁，只有持有锁的线程能唤醒其他等待的线程throw new IllegalMonitorStateException();Node first = firstWaiter;if (first != null)doSignal(first);     // 2. 通知Condition队列中的等待最长时间的线程，即firstWaiter}// ReentrantLock.Sync
// 1.
protected final boolean isHeldExclusively() {return getExclusiveOwnerThread() == Thread.currentThread();
}// 2.private void doSignal(Node first) {do {if ( (firstWaiter = first.nextWaiter) == null)lastWaiter = null;first.nextWaiter = null;} while (!transferForSignal(first) &&(first = firstWaiter) != null);}final boolean transferForSignal(Node node) {if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))return false;Node p = enq(node);     // 将节点加入sync队列int ws = p.waitStatus;if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))LockSupport.unpark(node.thread);return true;}

事例

• 开始状态：thread-1获取锁
  

• 2. thread-1调用await():
  用thread-1创建节点，并设置waitStatus= -2（Condition），再将thread-1加入Condition队列，队列为空，将firstWaiter和lastWaiter设为thread-1；
  调用release释放thread-1拥有的锁，并从Sync队列中移除节点；
  自旋等待唤醒

• thread-2调用await():
  

• thread-3调用await():
  

•  当收到signal信号后，会从condition队列中从头开始将节点移到sync队列，并唤醒自旋获取锁