java多线程之并发工具类CountDownLatch,CyclicBarrier和Semaphore

目录

• CountDownLatch
• Semaphore
• CyclicBarrier
• 总结

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。

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