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JAVA并发编程学习笔记之AQS源码分析(超时、中断与其他)

2012-05-16 22:36 447 查看

中断

JAVA中并没有好的中断线程的方式,早期引入的Thead.stop()和Thread.resume()容易导致死锁(参考:http://docs.oracle.com/javase/6/docs/technotes/guides/concurrency/threadPrimitiveDeprecation.html),已经不推荐使用。

JAVA线程提供了协作式中断,何为协作是中断,是相对抢占式中断而言的,简单来讲就是设置一个中断标志位,不停地检查这个标志位的状态,如果检查到线程中断,就中断线程。JVM线程内部维护着一个中断标志,程序员不能直接操作这个中断标志位,只能通过线程的以下几个方法设置中断位:

public void interrupt()
public static boolean interrupted()
private native boolean isInterrupted(boolean ClearInterrupted);
public boolean isInterrupted()


AQS中提供了支持中断的方法

private void doAcquireInterruptibly(int arg) throws InterruptedException;
private void doAcquireSharedInterruptibly(int arg) throws InterruptedException;
private boolean doAcquireSharedNanos(int arg, long nanosTimeout) throws InterruptedException;
这几个方法都抛出了InterruptedException,这些方法都会先出处中断异常,处理的代码如下:

if (Thread.interrupted())
throw new InterruptedException();
我们还看到有些方法并没有申请抛出InterruptedException,当它被中断时,设置了线程的中断位。

private static void selfInterrupt() {
Thread.currentThread().interrupt();
}

超时

AQS与JVM内置锁的一个不同点在于AQS中提供了超时机制,即线程在等待一定时间后会立即返回。下面以doAcquireNanos为例来分析:
    private boolean doAcquireNanos(int arg, long nanosTimeout)
throws InterruptedException {
long lastTime = System.nanoTime();
final Node node = addWaiter(Node.EXCLUSIVE);
boolean failed = true;
try {
for (;;) {
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
setHead(node);
p.next = null; // help GC
failed = false;
return true;
}
if (nanosTimeout <= 0)
return false;
if (shouldParkAfterFailedAcquire(p, node) &&
nanosTimeout > spinForTimeoutThreshold)
LockSupport.parkNanos(this, nanosTimeout);
long now = System.nanoTime();
nanosTimeout -= now - lastTime;
lastTime = now;
if (Thread.interrupted()) throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}
1、首先取得当前系统时间,在循环等待的过程中,如果剩余时间<=0立即返回;
2、如果剩余时间>0,就用总时间减去一次循环耗费的时间,继续阻塞;
3、如果在这期间线程被中断,就抛出中断异常,如果有其他异常产生,就取消这次获取。

取消

取消获取的逻辑比较复杂,下面来分析一下:
private void cancelAcquire(Node node) {
// Ignore if node doesn't exist
if (node == null)
return;

node.thread = null;

// Skip cancelled predecessors
Node pred = node.prev;
while (pred.waitStatus > 0)
node.prev = pred = pred.prev;

// predNext is the apparent node to unsplice. CASes below will
// fail if not, in which case, we lost race vs another cancel
// or signal, so no further action is necessary.
Node predNext = pred.next;

// Can use unconditional write instead of CAS here.
// After this atomic step, other Nodes can skip past us.
// Before, we are free of interference from other threads.
node.waitStatus = Node.CANCELLED;

// If we are the tail, remove ourselves.
if (node == tail && compareAndSetTail(node, pred)) {
compareAndSetNext(pred, predNext, null);
} else {
// If successor needs signal, try to set pred's next-link
// so it will get one. Otherwise wake it up to propagate.
int ws;
if (pred != head &&
((ws = pred.waitStatus) == Node.SIGNAL ||
(ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) &&
pred.thread != null) {
Node next = node.next;
if (next != null && next.waitStatus <= 0)
compareAndSetNext(pred, predNext, next);
} else {
unparkSuccessor(node);
}

node.next = node; // help GC
}
}
1、首先取得当前结点的前趋结点,如果前趋结点也被取消直接跳过,继续向前找非取消的结点;
2、将当前结点设置为取消状态;
3、如果当前结点是队尾结点,则将当前结点从队尾移除;否则执行4;
4、找到当前结点的继任结点,前趋的next指针指向继任结点(pred->next=current->next);

5、当前结点的next指针指向自己,前面提到这一方面为了回收,一方面为了使isOnSyncQueue方法简单。

其他

AQS还提供了一些线程监控的方法:
//获取哪些线程在等待
protected final Collection<Thread> getWaitingThreads();
//获取等待队列的长度
protected final int getWaitQueueLength();
//是否有线程在等待
protected final boolean hasWaiters()
//是否拥有同步器
final boolean isOwnedBy(AbstractQueuedSynchronizer sync)
//是否在同步队列中
final boolean isOnSyncQueue(Node node)
//支持共享模式的线程
public final Collection<Thread> getSharedQueuedThreads()
//支持独占模式的线程
public final Collection<Thread> getExclusiveQueuedThreads();

参考资料:

Java线程中断的本质和编程原则
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