摘要:如果一个调用已经出现了,这里只计数。为表示永不过期当为时,是相对于新纪元之后的毫秒。否则这个值就是超时前的纳秒数。要解除阻塞的线程
await
调用sync.acquireSharedInterruptibly
public void await() throws InterruptedException { sync.acquireSharedInterruptibly(1); }
sync.acquireSharedInterruptibly
调用tryAcquireShared方法返回<0执行doAcquireSharedInterruptibly
public final void acquireSharedInterruptibly(int arg) throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); if (tryAcquireShared(arg) < 0) doAcquireSharedInterruptibly(arg); }
tryAcquireShared
尝试获取共享锁,获取成功返回1,否则-1
protected int tryAcquireShared(int acquires) { return (getState() == 0) ? 1 : -1; }
doAcquireSharedInterruptibly
private void doAcquireSharedInterruptibly(int arg)throws InterruptedException { final Node node = addWaiter(Node.SHARED); boolean failed = true; try { for (;;) { final Node p = node.predecessor(); //如果前一个node为队头,则通过tryAcquireShared尝试获取共享锁 if (p == head) { int r = tryAcquireShared(arg); if (r >= 0) { //获取到锁执行 setHeadAndPropagate(node, r); p.next = null; // help GC failed = false; return; } } if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt()) throw new InterruptedException(); } } finally { //产生异常执行 if (failed) cancelAcquire(node); } }
addWaiter
调用addWaiter方法把队尾设置为当前node;如果队尾为空或者设置失败则调用enq方法
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; }
enq
调用enq方法队尾为空则创建空的队尾和队头,否则重新设置队尾为当前node,设置成功返回。enq和addWaiter方法不同在于enq循环执行一定会执行成功,不存在失败情况
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; } } } }
predecessor
调用predecessor方法获取前一个node
final Node predecessor() throws NullPointerException { Node p = prev; if (p == null) throw new NullPointerException(); else return p; } static final int CANCELLED = 1; //取消 static final int SIGNAL = -1; //下个节点需要被唤醒 static final int CONDITION = -2; //线程在等待条件触发 static final int PROPAGATE = -3; //(共享锁)状态需要向后传播
shouldParkAfterFailedAcquire
获取当前node的前一个note的线程等待状态,如果为SIGNAL,那么返回true,大于0通过循环将当前节点之前所有取消状态的节点移出队列;其他状时,利用compareAndSetWaitStatus使前节点的状态为-1;如果是第一次await时ws状态是0,多次await时ws状态是0,最后肯定返回true
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) { int ws = pred.waitStatus; if (ws == Node.SIGNAL) return true; if (ws > 0) { do { node.prev = pred = pred.prev; } while (pred.waitStatus > 0); pred.next = node; } else { compareAndSetWaitStatus(pred, ws, Node.SIGNAL); } return false; }
parkAndCheckInterrupt
调用park并返回线程是否已经中断
private final boolean parkAndCheckInterrupt() { LockSupport.park(this); return Thread.interrupted(); }
park
调用UNSAFE.park阻塞当前线程
public static void park(Object blocker) { Thread t = Thread.currentThread(); setBlocker(t, blocker); UNSAFE.park(false, 0L); setBlocker(t, null); }
setBlocker
在当前线程t的parkBlockerOffset位置设置blocker的引用
private static void setBlocker(Thread t, Object arg) { // Even though volatile, hotspot doesn"t need a write barrier here. UNSAFE.putObject(t, parkBlockerOffset, arg); }
UNSAFE.park
/** * 阻塞一个线程直到countDownunpark
出现、线程 * 被中断或者timeout时间到期。如果一个unpark
调用已经出现了, * 这里只计数。timeout为0表示永不过期.当isAbsolute
为true时, * timeout是相对于新纪元之后的毫秒。否则这个值就是超时前的纳秒数。这个方法执行时 * 也可能不合理地返回(没有具体原因) * * @param isAbsolute true if the timeout is specified in milliseconds from * the epoch. * 如果为true timeout的值是一个相对于新纪元之后的毫秒数 * @param time either the number of nanoseconds to wait, or a time in * milliseconds from the epoch to wait for. * 可以是一个要等待的纳秒数,或者是一个相对于新纪元之后的毫秒数直到 * 到达这个时间点 */ UNSAFE.park(false, 0L);
调用sync.releaseShared
public void countDown() { sync.releaseShared(1); }
releaseShared
执行tryReleaseShared成功后执行doReleaseShared
public final boolean releaseShared(int arg) { if (tryReleaseShared(arg)) { doReleaseShared(); return true; } return false; }
tryReleaseShared
更新state值为state-1,如果state新值为0返回true,否则false
protected boolean tryReleaseShared(int releases) { // Decrement count; signal when transition to zero for (;;) { int c = getState(); if (c == 0) return false; int nextc = c-1; if (compareAndSetState(c, nextc)) return nextc == 0; } }
doReleaseShared
只要等待队列有数据,获取队头等待状态,队头状态=-1其他node为等待时,则把队头等待状态置为初始,且调用unparkSuccessor方法;队头状态=0时,把队头状态置为-3传播到下一node
private void doReleaseShared() { /* * Ensure that a release propagates, even if there are other * in-progress acquires/releases. This proceeds in the usual * way of trying to unparkSuccessor of head if it needs * signal. But if it does not, status is set to PROPAGATE to * ensure that upon release, propagation continues. * Additionally, we must loop in case a new node is added * while we are doing this. Also, unlike other uses of * unparkSuccessor, we need to know if CAS to reset status * fails, if so rechecking. */ for (;;) { Node h = head; if (h != null && h != tail) { int ws = h.waitStatus; if (ws == Node.SIGNAL) { if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0)) continue; // loop to recheck cases unparkSuccessor(h); } else if (ws == 0 && !compareAndSetWaitStatus(h, 0, Node.PROPAGATE)) continue; // loop on failed CAS } if (h == head) // loop if head changed break; } }
unparkSuccessor
上面调用unparkSuccessor时,node的状态已经更改为0,且node.next存在,执行unpark方法
private void unparkSuccessor(Node node) { /* * If status is negative (i.e., possibly needing signal) try * to clear in anticipation of signalling. It is OK if this * fails or if status is changed by waiting thread. */ int ws = node.waitStatus; if (ws < 0) compareAndSetWaitStatus(node, ws, 0); /* * Thread to unpark is held in successor, which is normally * just the next node. But if cancelled or apparently null, * traverse backwards from tail to find the actual * non-cancelled successor. */ Node s = node.next; if (s == null || s.waitStatus > 0) { s = null; for (Node t = tail; t != null && t != node; t = t.prev) if (t.waitStatus <= 0) s = t; } if (s != null) LockSupport.unpark(s.thread); }
unpark
unpark执行完之后是如何更改head的?
public static void unpark(Thread thread) { if (thread != null) UNSAFE.unpark(thread); }
UNSAFE.unpark
/** * Releases the block on a thread created by *park
. This method can also be used * to terminate a blockage caused by a prior call topark
. * This operation is unsafe, as the thread must be guaranteed to be * live. This is true of Java, but not native code. * 释放被park
创建的在一个线程上的阻塞.这个 * 方法也可以被使用来终止一个先前调用park
导致的阻塞. * 这个操作操作时不安全的,因此线程必须保证是活的.这是java代码不是native代码。 * @param thread the thread to unblock. * 要解除阻塞的线程 */ UNSAFE.unpark(thread);
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