CountDownLatch
CountDownLatch允许一个或多个线程等待其他线程完成操作。
假设一个Excel文件有多个sheet，我们需要去记录每个sheet有多少行数据，
这时我们就可以使用CountDownLatch实现主线程等待所有sheet线程完成sheet的解析操作后，再继续执行自己的任务。
public class CountDownLatchTest {
private static class WorkThread extends Thread {
private CountDownLatch cdl;
public WorkThread(String name, CountDownLatch cdl) {
super(name);
this.cdl = cdl;
}
public void run() {
System.out.println(this.getName() + "启动了，时间为" + System.currentTimeMillis());
System.out.println(this.getName() + "我要统计每个sheet的行数");
try {
cdl.await();
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(this.getName() + "执行完了，时间为" + System.currentTimeMillis());
}
}
private static class sheetThread extends Thread {
private CountDownLatch cdl;
public sheetThread(String name, CountDownLatch cdl) {
super(name);
this.cdl = cdl;
}
public void run() {
try {
System.out.println(this.getName() + "启动了，时间为" + System.currentTimeMillis());
Thread.sleep(1000); //模拟任务执行耗时
cdl.countDown();
System.out.println(this.getName() + "执行完了，时间为" + System.currentTimeMillis() + " sheet的行数为：" + (int) (Math.random()*100));
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public static void main(String[] args) throws Exception {
CountDownLatch cdl = new CountDownLatch(2);
WorkThread wt0 = new WorkThread("WorkThread", cdl );
wt0.start();
sheetThread dt0 = new sheetThread("sheetThread1", cdl);
sheetThread dt1 = new sheetThread("sheetThread2", cdl);
dt0.start();
dt1.start();
}
}
执行结果：
WorkThread启动了，时间为1640054503027
WorkThread我要统计每个sheet的行数
sheetThread1启动了，时间为1640054503028
sheetThread2启动了，时间为1640054503029
sheetThread2执行完了，时间为1640054504031 sheet的行数为：6
sheetThread1执行完了，时间为1640054504031 sheet的行数为：44
WorkThread执行完了，时间为1640054505036
可以看到，首先WorkThread执行await后开始等待，WorkThread在等待sheetThread1和sheetThread2都执行完自己的任务后，WorkThread立刻继续执行后面的代码。
CountDownLatch的构造函数接收一个int类型的参数作为计数器，如果你想等待N个点完成，这里就传入N。
当我们调用CountDownLatch的countDown方法时，N就会减1，CountDownLatch的await方法会阻塞当前线程，直到N变成零。
由于countDown方法可以用在任何地方，所以这里说的N个点，可以是N个线程，也可以是1个线程里的N个执行步骤。
用在多个线程时，只需要把这个CountDownLatch的引用传递到线程里即可。
我们继续根据上面的测试案例流程，一步一步的分析CountDownLatch 源码。
第一步看CountDownLatch的构造方法，传入一个不能小于0的int类型的参数作为计数器
public CountDownLatch(int count) {
if (count < 0) throw new IllegalArgumentException("count < 0");
this.sync = new Sync(count);
}
/**
* Synchronization control For CountDownLatch.
* Uses AQS state to represent count.
*/
private static final class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = 4982264981922014374L;
Sync(int count) {
setState(count);
}
int getCount() {
return getState();
}
protected int tryAcquireShared(int acquires) {
return (getState() == 0) ? 1 : -1;
}
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;
}
}
}
看它的注释，说的非常清楚，Sync就是CountDownLatch的同步控制器了，而它也是继承了AQS，并且第3行注释说到使用了AQS的state去代表count值。
第二步就是工作线程调用await()方法
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);
}
如果线程中断，抛出异常，否则开始调用tryAcquireShared(1)，其内部类Sync的实现也非常简单，就是判断state也就是CountDownLatch的计数是否等于0，
如果等于0，则该方法返回1，第5行的if判断不成立，否则该方法返回-1，第5行的if判断成立，继续执行doAcquireSharedInterruptibly(1)。
/**
* Acquires in shared interruptible mode.
* @param arg the acquire argument
*/
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) {
int r = tryAcquireShared(arg);
if (r >= 0) {
setHeadAndPropagate(node, r);
p.next = null; // help GC
failed = false;
return;
}
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}
这个方法其实就是去获取共享模式下的锁，获取失败就park住。正如我们测试案例中的WorkThread线程应该次数就被park住了，那么它又是何时被唤醒的呢？
下面就到countDown()方法了
public void countDown() {
sync.releaseShared(1);
}
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {
doReleaseShared();
return true;
}
return false;
}
tryReleaseShared(1)方法尝试去释放共享锁
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;
}
}
在for循环中，先获取CountDownLatch的计数也就是当前state，如果等于0返回false，否则将state更新为state-1，并返回最新的state是否等于0。
因此在我们的测试案例中，我们需要调用两次countDown方法，才会将全局的state更新为0，然后继续执行doReleaseShared()方法。
/**
* Release action for shared mode -- signals successor and ensures
* propagation. (Note: For exclusive mode, release just amounts
* to calling unparkSuccessor of head if it needs signal.)
*/
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;
}
}
/**
* Wakes up node's successor, if one exists.
*
* @param node the node
*/
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);
}
LockSupport.unpark(s.thread)，唤醒线程的方法被调用后，WorkThread线程就可以继续执行了。
至此我们简单分析了整个测试案例中CountDownLatch的代码流程。
Semaphore
Semaphore（信号量）是用来控制同时访问特定资源的线程数量，相当于一个并发控制器，构造的时候传入可供管理的信号量的数值，这个数值就是用来控制并发数量的，
每个线程执行前先通过acquire方法获取信号，执行后通过release归还信号 。每次acquire返回成功后，Semaphore可用的信号量就会减少一个，如果没有可用的信号，
acquire调用就会阻塞，等待有release调用释放信号后，acquire才会得到信号并返回。
下面我们看个测试案例
public class SemaphoreTest {
public static void main(String[] args) {
final Semaphore semaphore = new Semaphore(5);
Runnable runnable = () -> {
try {
semaphore.acquire();
System.out.println(Thread.currentThread().getName() + "获得了信号量>>>>>,时间为" + System.currentTimeMillis());
Thread.sleep(1000);
System.out.println(Thread.currentThread().getName() + "释放了信号量<<<<<,时间为" + System.currentTimeMillis());
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
semaphore.release();
}
};
Thread[] threads = new Thread[10];
for (int i = 0; i < threads.length; i++)
threads[i] = new Thread(runnable);
for (int i = 0; i < threads.length; i++)
threads[i].start();
}
}
执行结果：
Thread-0获得了信号量>>>>>,时间为1640058647604
Thread-1获得了信号量>>>>>,时间为1640058647604
Thread-2获得了信号量>>>>>,时间为1640058647604
Thread-3获得了信号量>>>>>,时间为1640058647605
Thread-4获得了信号量>>>>>,时间为1640058647605
Thread-0释放了信号量<<<<<,时间为1640058648606
Thread-1释放了信号量<<<<<,时间为1640058648606
Thread-5获得了信号量>>>>>,时间为1640058648607
Thread-4释放了信号量<<<<<,时间为1640058648607
Thread-3释放了信号量<<<<<,时间为1640058648607
Thread-7获得了信号量>>>>>,时间为1640058648607
Thread-8获得了信号量>>>>>,时间为1640058648607
Thread-2释放了信号量<<<<<,时间为1640058648606
Thread-6获得了信号量>>>>>,时间为1640058648607
Thread-9获得了信号量>>>>>,时间为1640058648607
Thread-7释放了信号量<<<<<,时间为1640058649607
Thread-6释放了信号量<<<<<,时间为1640058649607
Thread-8释放了信号量<<<<<,时间为1640058649607
Thread-9释放了信号量<<<<<,时间为1640058649608
Thread-5释放了信号量<<<<<,时间为1640058649607
我们使用for循环同时创建10个线程，首先是线程 0 1 2 3 4获得了信号量，再后面的10行打印结果中，线程1到5分别释放信号量，相同线程间隔也是1000毫秒，然后线程5 6 7 8 9才能继续获得信号量，而且保持最大获取信号量的线程数小于等于5。
看下Semaphore的构造方法
public Semaphore(int permits) {
sync = new NonfairSync(permits);
}
public Semaphore(int permits, boolean fair) {
sync = fair ? new FairSync(permits) : new NonfairSync(permits);
}
它支持传入一个int类型的permits，一个布尔类型的fair，因此Semaphore也有公平模式与非公平模式。
/**
* Synchronization implementation for semaphore. Uses AQS state
* to represent permits. Subclassed into fair and nonfair
* versions.
*/
abstract static class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = 1192457210091910933L;
Sync(int permits) {
setState(permits);
}
final int getPermits() {
return getState();
}
final int nonfairTryAcquireShared(int acquires) {
for (;;) {
int available = getState();
int remaining = available - acquires;
if (remaining < 0 ||
compareAndSetState(available, remaining))
return remaining;
}
}
protected final boolean tryReleaseShared(int releases) {
for (;;) {
int current = getState();
int next = current + releases;
if (next < current) // overflow
throw new Error("Maximum permit count exceeded");
if (compareAndSetState(current, next))
return true;
}
}
final void reducePermits(int reductions) {
for (;;) {
int current = getState();
int next = current - reductions;
if (next > current) // underflow
throw new Error("Permit count underflow");
if (compareAndSetState(current, next))
return;
}
}
final int drainPermits() {
for (;;) {
int current = getState();
if (current == 0 || compareAndSetState(current, 0))
return current;
}
}
}
第9行代码可见Semaphore也是通过AQS的state来作为信号量的计数的
第12行 getPermits() 方法获取当前的可用的信号量，即还有多少线程可以同时获得信号量
第15行nonfairTryAcquireShared方法尝试获取共享锁，逻辑就是直接将可用信号量减去该方法请求获取的数量，更新state并返回该值。
第24行tryReleaseShared 方法尝试释放共享锁，逻辑就是直接将可用信号量加上该方法请求释放的数量，更新state并返回。
再看下Semaphore的公平锁
/**
* Fair version
*/
static final class FairSync extends Sync {
private static final long serialVersionUID = 2014338818796000944L;
FairSync(int permits) {
super(permits);
}
protected int tryAcquireShared(int acquires) {
for (;;) {
if (hasQueuedPredecessors())
return -1;
int available = getState();
int remaining = available - acquires;
if (remaining < 0 ||
compareAndSetState(available, remaining))
return remaining;
}
}
}
看尝试获取共享锁的方法中，多了个 if (hasQueuedPredecessors) 的判断，在java多线程6：ReentrantLock，
分析过hasQueuedPredecessors其实就是判断当前等待队列中是否存在等待线程，并判断第一个等待的线程(head.next)是否是当前线程。
CyclicBarrier
CyclicBarrier的字面意思是可循环使用（Cyclic）的屏障（Barrier）。它要做的事情是，让一组线程到达一个屏障（也可以叫同步点）时被阻塞，直到最后一个线程到达屏障时，屏障才会开门，所有被屏障拦截的线程才会继续运行。
一组线程同时被唤醒，让我们想到了ReentrantLock的Condition，它的signalAll方法可以唤醒await在同一个condition的所有线程。
下面我们还是从一个简单的测试案例先了解下CyclicBarrier的用法
public class CyclicBarrierTest extends Thread {
private CyclicBarrier cb;
private int sleepSecond;
public CyclicBarrierTest(CyclicBarrier cb, int sleepSecond) {
this.cb = cb;
this.sleepSecond = sleepSecond;
}
public void run() {
try {
System.out.println(this.getName() + "开始, 时间为" + System.currentTimeMillis());
Thread.sleep(sleepSecond * 1000);
cb.await();
System.out.println(this.getName() + "结束, 时间为" + System.currentTimeMillis());
} catch (Exception e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
Runnable runnable = new Runnable() {
public void run() {
System.out.println("CyclicBarrier的barrierAction开始运行, 时间为" + System.currentTimeMillis());
}
};
CyclicBarrier cb = new CyclicBarrier(2, runnable);
CyclicBarrierTest cbt0 = new CyclicBarrierTest(cb, 3);
CyclicBarrierTest cbt1 = new CyclicBarrierTest(cb, 6);
cbt0.start();
cbt1.start();
}
}
执行结果：
Thread-1开始, 时间为1640069673534
Thread-0开始, 时间为1640069673534
CyclicBarrier的barrierAction开始运行, 时间为1640069679536
Thread-1结束, 时间为1640069679536
Thread-0结束, 时间为1640069679536
可以看到Thread-0和Thread-1同时运行，而自定义的线程barrierAction是在6000毫秒后开始执行，说明Thread-0在await之后，等待了3000毫秒，和Thread-1一起继续执行的。
看下CyclicBarrier 的一个更高级的构造函数
public CyclicBarrier(int parties, Runnable barrierAction) {
if (parties <= 0) throw new IllegalArgumentException();
this.parties = parties;
this.count = parties;
this.barrierCommand = barrierAction;
}
parties就是设定需要多少线程在屏障前等待，只有调用await方法的线程数达到才能唤醒所有的线程，还有注意因为使用CyclicBarrier的线程都会阻塞在await方法上，所以在线程池中使用CyclicBarrier时要特别小心，如果线程池的线程过少，那么就会发生死锁。
Runnable barrierAction用于在线程到达屏障时，优先执行barrierAction，方便处理更复杂的业务场景。
/**
* Main barrier code, covering the various policies.
*/
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();
}
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();
}
}
// 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();
}
}
首先是ReentrantLock加锁，全局的count值-1，然后判断count是否等于0，如果不等于0，则循环，condition执行await等待，直到触发、中断、中断或超时，如果count值等于0，先执行barrierAction线程，然后condition开始唤醒所有等待的线程。
简单是使用之后，有人会觉得CyclicBarrier和CountDownLatch有点像，其实它们两者有些细微的差别：
1：CountDownLatch是在多个线程都进行了latch.countDown()后才会触发事件，唤醒await()在latch上的线程，而执行countDown()的线程，是不会阻塞的；
CyclicBarrier是一个栅栏，用于同步所有调用await()方法的线程，线程执行了await()方法之后并不会执行之后的代码，而只有当执行await()方法的线程数等于指定的parties之后，这些执行了await()方法的线程才会同时运行。
2：CountDownLatch不能循环使用，计数器减为0就减为0了，不能被重置；CyclicBarrier本是就是支持循环使用parties，而且提供了reset()方法，可以重置计数器。
总结
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