ReentrantLock.lockInterruptibly允许在等待时由其它线程调用等待线程的Thread.interrupt方法来中断等待线程的等待而直接返回,这时不用获取锁,而会抛出一个InterruptedException。而ReentrantLock.lock方法不允许Thread.interrupt中断,即使检测到Thread.isInterrupted,一样会继续尝试获取锁,失败则继续休眠。只是在最后获取锁成功后再把当前线程置为interrupted状态。
那lockInterruptibly是如何做到这一点的?
public void lockInterruptibly() throws InterruptedException {
sync.acquireInterruptibly(1);
}
这里调用了AbstractQueuedSynchronizer.acquireInterruptibly方法。如果线程已被中断则直接抛出异常,否则则尝试获取锁,失败则doAcquireInterruptibly
AbstractQueuedSynchronizer.acquireInterruptibly(int arg)
/**
* Acquires in exclusive mode, aborting if interrupted.
* Implemented by first checking interrupt status, then invoking
* at least once {@link #tryAcquire}, returning on
* success. Otherwise the thread is queued, possibly repeatedly
* blocking and unblocking, invoking {@link #tryAcquire}
* until success or the thread is interrupted. This method can be
* used to implement method {@link Lock#lockInterruptibly}.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquire} but is otherwise uninterpreted and
* can represent anything you like.
* @throws InterruptedException if the current thread is interrupted
*/
public final void acquireInterruptibly(int arg) throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
if (!tryAcquire(arg))
doAcquireInterruptibly(arg);
}
AbstractQueuedSynchronizer.doAcquireInterruptibly大体上相当于前面的acquireQueued,关键的区别在于检测到interrupted后的处理,acquireQueued简单的记录下中断曾经发生,然后就象没事人似的去尝试获取锁,失败则休眠。而doAcquireInterruptibly检测到中断则直接退出循环,抛出InterruptedException异常。
AbstractQueuedSynchronizer.doAcquireInterruptibly(int arg)
/**
* Acquires in exclusive interruptible mode.
* @param arg the acquire argument
*/
private void doAcquireInterruptibly(int arg)
throws InterruptedException {
final Node node = addWaiter(Node.EXCLUSIVE);
try {
for (;;) {
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
setHead(node);
p.next = null; // help GC
return;
}
/*
acquireQueued代码:
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
*/
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
break;
}
} catch (RuntimeException ex) {
cancelAcquire(node);
throw ex;
}
// Arrive here only if interrupted
// 取消获取锁尝试,将当前节点从等待队列中移除
cancelAcquire(node);
throw new InterruptedException();
}
在抛出异常之前,doAcquireInterruptibly还做了一件事情,cancelAcquire。cancelAcquire中有些细节值得玩味,参见代码中笔者注释。
AbstractQueuedSynchronizer.cancelAcquire(Node node)
/**
* Cancels an ongoing attempt to acquire.
*
* @param node the node
*/
private void cancelAcquire(Node node) {
// Ignore if node doesn't exist
if (node == null)
return;
node.thread = null;
// Skip cancelled predecessors
// 头节点一定不会是在等待状态,所以不会被cancel,所以这里一定能找到一个节点而不用担心null
Node pred = node.prev;
while (pred.waitStatus > 0)
node.prev = pred = pred.prev;
// Getting this before setting waitStatus ensures staleness
Node predNext = pred.next;
// Can use unconditional write instead of CAS here
node.waitStatus = Node.CANCELLED;
// If we are the tail, remove ourselves
if (node == tail && compareAndSetTail(node, pred)) {
compareAndSetNext(pred, predNext, null);
} else {
// If "active" predecessor found...
if (pred != head
&& (pred.waitStatus == Node.SIGNAL
|| compareAndSetWaitStatus(pred, 0, Node.SIGNAL))
&& pred.thread != null) {
// If successor is active, set predecessor's next link
Node next = node.next;
if (next != null && next.waitStatus <= 0)
compareAndSetNext(pred, predNext, next);
} else {
/*这里如果不调用unparkSuccessor, 若在interrupted之后,执行到上面一句将waitStatus置CANCELLED之前,锁被释放,该线程被唤醒,则释放锁线程的unparkSuccessor不能起到预期作用,所以这里需要调用unparkSuccessor.即使此时持有锁的线程没有释放锁也不会有严重后果,被unpark的线程在获取锁失败后会继续park*/
unparkSuccessor(node);
}
node.next = node; // help GC
}
}
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