共计 5849 个字符，预计需要花费 15 分钟才能阅读完成。
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 <a href=”#unpark”><code>unpark</code></a> 出现、线程
* 被中断或者 timeout 时间到期。如果一个 <code>unpark</code> 调用已经出现了，
* 这里只计数。timeout 为 0 表示永不过期. 当 <code>isAbsolute</code> 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);
}
unparkunpark 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
* <a href=”#park”><code>park</code></a>. This method can also be used
* to terminate a blockage caused by a prior call to <code>park</code>.
* 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 <a href=”#park”><code>park</code></a> 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 <code>park</code> 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);
