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unpark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);
countDown
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 to park.
* 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);