并发编程（5）——AQS之CountDownLatch、Semaphore、CyclicBarrier
- 2019 年 10 月 3 日
- 筆記
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firstWaiter==lastWaiter==new 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
