CountDownLatch
A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.
通常情况下，countDown如下调用
CountDownLatch countDownLatch = new CountDownLatch(1);
countDownLatch.countDown();
countDownLatch.await();
看一下countDown方法：
public void countDown() {
sync.releaseShared(1);
}
AQS中releaseShared方法如下：
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {
doReleaseShared();
return true;
}
return false;
}
CountDownLatch中tryReleaseShared方法如下：
// 方法判断许可如果减1之后是否为0，如果为0的话就执行doReleaseShared（）方法。
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;
}
}
来看doReleaseShared（）方法：
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;
}
}
不过尴尬的是，CountDownLatch这里未做任何事情。
再看一下await（）方法：
await方法会让当前线程进入wait状态，除非满足下面两个条件：
- count到0
- 线程中断
public void await() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
public final void acquireSharedInterruptibly(int arg)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
if (tryAcquireShared(arg) < 0)
doAcquireSharedInterruptibly(arg);
}
tryAcquireShared方法如下：
protected int tryAcquireShared(int acquires) {
return (getState() == 0) ? 1 : -1;
}
所以，当state不是0的时候进入doAcquireSharedInterruptibly方法。
private void doAcquireSharedInterruptibly(int arg)
throws InterruptedException {
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
for (;;) {
final Node p = node.predecessor();
if (p == head) {
// 只有当state为0时r为1
int r = tryAcquireShared(arg);
if (r >= 0) {
setHeadAndPropagate(node, r);
p.next = null; // help GC
failed = false;
return;
}
}
// 如果state不为0，该线程会进入wait状态
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}
CountDownLatch文档中有一句非常重要的话：
Memory consistency effects: Until the count reaches zero, actions in a thread prior to calling countDown() happen-before actions following a successful return from a corresponding await() in another thread
大意是一个线程countdown()之前的操作happens-before另一个线程中await()之后的操作。
Semaphore
Semaphores are often used to restrict the number of threads than can access some (physical or logical) resource.
Semaphore主要用来限制获取资源的线程数。
Actions in a thread prior to calling a "release" method such as release() happen-before actions following a successful "acquire" method such as acquire() in another thread
内存语义：release() happen-before acquire()之前
启一个springboot项目，写一个方法：
@RequestMapping("/test/semaphore")
@ResponseBody
public void test() throws InterruptedException {
Semaphore semaphore = new Semaphore(5);
for (int i = 0; i < 7; i++) {
int finalI = i;
new Thread(()->{
try {
semaphore.acquire();
System.err.println(Thread.currentThread() + "获取了许可" + semaphore.availablePermits());
} catch (InterruptedException e) {
e.printStackTrace();
}
}, "线程" + i).start();
}
new Thread(()->{
try {
Thread.sleep(10000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.err.println(Thread.currentThread() + "要释放许可" + semaphore.availablePermits());
semaphore.release();
}, "线程7").start();
}
一次输出如下：
Thread[线程1,5,main]获取了许可4
Thread[线程0,5,main]获取了许可3
Thread[线程3,5,main]获取了许可2
Thread[线程4,5,main]获取了许可0
Thread[线程2,5,main]获取了许可0
Thread[线程7,5,main]要释放许可0
Thread[线程5,5,main]获取了许可0
会发现，线程5获取许可之前是先等线程7释放许可。
至于线程6会因为由于许可为0，进入等待状态。直到有线程释放许可，来调用unparkSuccessor。
CyclicBarrier
A synchronization aid that allows a set of threads to all wait for each other to reach a common barrier point.
Actions in a thread prior to calling await() happen-before actions that are part of the barrier action, which in turn happen-before actions following a successful return from the corresponding await() in other threads.
内部类Generation只有一个属性broken（默认false）
我们发现，await（）方法如下：
public int await() throws InterruptedException, BrokenBarrierException {
try {
return dowait(false, 0L);
} catch (TimeoutException toe) {
throw new Error(toe); // cannot happen
}
}
进入dowait方法：
private int dowait(boolean timed, long nanos)
throws InterruptedException, BrokenBarrierException,
TimeoutException {
final ReentrantLock lock = this.lock;
lock.lock();
try {
final Generation g = generation;
if (g.broken)
throw new BrokenBarrierException();
if (Thread.interrupted()) {
breakBarrier();
throw new InterruptedException();
}
// 来一个线程count减1，如果index为0，就会翻车
int index = --count;
if (index == 0) { // tripped
boolean ranAction = false;
try {
final Runnable command = barrierCommand;
if (command != null)
command.run();
ranAction = true;
nextGeneration();
return 0;
} finally {
if (!ranAction)
breakBarrier();
}
}
// 没翻车（broken，interrupted，timed out）的话就执行下面的逻辑
// loop until tripped, broken, interrupted, or timed out
for (;;) {
try {
if (!timed)
trip.await();
else if (nanos > 0L)
nanos = trip.awaitNanos(nanos);
} catch (InterruptedException ie) {
if (g == generation && ! g.broken) {
breakBarrier();
throw ie;
} else {
// We're about to finish waiting even if we had not
// been interrupted, so this interrupt is deemed to
// "belong" to subsequent execution.
Thread.currentThread().interrupt();
}
}
if (g.broken)
throw new BrokenBarrierException();
if (g != generation)
return index;
if (timed && nanos <= 0L) {
breakBarrier();
throw new TimeoutException();
}
}
} finally {
lock.unlock();
}
}
下面进入trip.await()方法
public final void await() throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
// 往等待队列加入节点Node
Node node = addConditionWaiter();
// 这里释放AQS中的state， 如果释放失败，会将node的waitstatus置为CANCELLED，这是传参node的唯一用处
int savedState = fullyRelease(node);
int interruptMode = 0;
// 如果node有next就肯定返回true
while (!isOnSyncQueue(node)) {
LockSupport.park(this);
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
break;
}
// 如果当前线程
if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
interruptMode = REINTERRUPT;
if (node.nextWaiter != null) // clean up if cancelled
unlinkCancelledWaiters();
if (interruptMode != 0)
reportInterruptAfterWait(interruptMode);
}
进入addConditionWaiter()
private Node addConditionWaiter() {
Node t = lastWaiter;
// If lastWaiter is cancelled, clean out.
if (t != null && t.waitStatus != Node.CONDITION) {
unlinkCancelledWaiters();
t = lastWaiter;
}
Node node = new Node(Thread.currentThread(), Node.CONDITION);
if (t == null)
firstWaiter = node;
else
t.nextWaiter = node;
lastWaiter = node;
return node;
}
假如5个线程按顺序进入await（），则此时，trip这个ConditionObject上firstWaiterlastWaiternew Node("线程0对应的线程"， Node.CONDITION)
同时，因为dowait方法中的lock.lock()，AQS的同步队列如下：
head节点--》线程1--》线程2--》线程3--》线程4（tail）
等待队列: t0
当释放线程0的锁之后，唤醒线程1，将线程1加入等待队列，线程2/3也加入等待队列。此时同步队列还剩下线程4。此时队列情况是：
同步队列：head节点
等待队列：t0->t1->t2->t3
到了最后一个线程4执行的时候，index==0，执行nextGeneration，会signalAll trip这个Condition上的所有等待线程。所以经过signalAll之后，队列情况变成了：
同步队列：head->t0->t1->t2->t3
等待队列：空
此时线程4运行，释放锁之后唤醒同步队列上的第一个节点t0