CycleBarrier与CountDownLatch原理

CountDownLatch

众所周知，它能解决一个任务必须在其他任务完成的情况下才能执行的问题，代码层面来说就是只有计数countDown到0的时候，await处的代码才能继续向下运行，例如：

import java.util.*;
import java.util.concurrent.*;

public class Main {
    public static void main(String[] args) throws Exception {

        CountDownLatch latch = new CountDownLatch(3);

        ThreadPoolExecutor executor = new ThreadPoolExecutor(10, 15, 60L, TimeUnit.SECONDS, new ArrayBlockingQueue<>(5));
        Future<Integer>[] futures = new Future[3];
        for (int i = 0; i < 3; i++){
            futures[i] = executor.submit(() -> {
                Random rand = new Random();
                int n = rand.nextInt(100);
                int result = 0;
                for (int j = 0; j < n; j++){
                    result += j;
                }
                System.out.println(result + "|" + Thread.currentThread().getName());
                latch.countDown();
                return result;
            });
        }
        latch.await();
        System.out.println("合计每个任务的结果：" + (futures[0].get()+futures[1].get()+futures[2].get()));
    }

}

运行结果：

源码

实际上内部十分简单，里面只有一个AQS的子类

private static final class Sync extends AbstractQueuedSynchronizer {
    private static final long serialVersionUID = 4982264981922014374L;

    // 它把AQS的state（同步状态）作为计数器，在AQS里，state是个volatile标记的int变量
    Sync(int count) {
        setState(count);
    }

    int getCount() {
        return getState();
    }

    protected int tryAcquireShared(int acquires) {
        // 同步状态为0，则返回1，否则返回-1
        return (getState() == 0) ? 1 : -1;
    }

    protected boolean tryReleaseShared(int releases) {
        // Decrement count; signal when transition to zero
        for (;;) {
            int c = getState();
            // 如果状态为0则返回false
            if (c == 0)
                return false;
            // 计数器减1
            int nextc = c-1;
            // CAS操作，如果内存中的同步状态值等于期望值c，那么将同步状态设置为给定的更新值nextc
            if (compareAndSetState(c, nextc))
                return nextc == 0;  // 当计数器减到0，返回true
        }
    }
}

public void countDown() {
    sync.releaseShared(1);
}

public void await() throws InterruptedException {
    sync.acquireSharedInterruptibly(1);
}

下面看具体做了什么事情

先来看await

public final void acquireSharedInterruptibly(int arg)
        throws InterruptedException {
    if (Thread.interrupted())
        throw new InterruptedException();
    // 当计数器不等于0，返回-1，证明还有任务未执行完，进入下面方法等待
    if (tryAcquireShared(arg) < 0)
        doAcquireSharedInterruptibly(arg);
}

private void doAcquireSharedInterruptibly(int arg)
    throws InterruptedException {
    // 把当前线程包装成Node放入等待队列
    final Node node = addWaiter(Node.SHARED);
    boolean failed = true;
    try {
        for (;;) {
            // 获取当前线程的前驱节点，以检查等待状态
            final Node p = node.predecessor();
            if (p == head) {
                // 如果计数器等于0，返回1，证明此时阻塞可以解除了
                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);
    }
}

上面的过程可以总结为：当进入await方法后，如果此时计数器不为0，则进入死循环一直检查计数器的值，直到为0退出，此时停止等待。

再来看countDown

public final boolean releaseShared(int arg) {
    // 尝试计数器减1，只有减到0才会返回true
    if (tryReleaseShared(arg)) {
        doReleaseShared();
        return true;
    }
    return false;
}

private void doReleaseShared() {
    for (;;) {
        Node h = head;
        if (h != null && h != tail) {
            int ws = h.waitStatus;
            // 等待状态为SIGNAL
            if (ws == Node.SIGNAL) {
                // 把当前节点的等待状态从SIGNAL设置成0，如果设置失败则继续循环。
                if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
                    continue;            // loop to recheck cases
                // 成功的话则卸载当前节点的所有后继
                unparkSuccessor(h);
            }
            // 如果等待状态为0，则尝试将状态设置为PROPAGATE，如果设置失败则继续循环。
            else if (ws == 0 && !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                continue;                // loop on failed CAS
        }
        if (h == head)                   // loop if head changed
            break;
    }
}

countDown的过程可以总结为：尝试将计数器-1，直到为0，为0的时候通知等待线程。

CycleBarrier

栏栅的作用就是让指定的一批任务能够同时开始执行，比如

import java.util.*;
import java.util.concurrent.*;

public class Main {
    public static void main(String[] args) throws Exception {
        CyclicBarrier cyclicBarrier = new CyclicBarrier(3);

        ThreadPoolExecutor executor = new ThreadPoolExecutor(10, 15, 60L, TimeUnit.SECONDS, new ArrayBlockingQueue<>(5));
        Future<Integer>[] futures = new Future[3];
        for (int i = 0; i < 3; i++){
            futures[i] = executor.submit(() -> {
                System.out.println("await|" + Thread.currentThread().getName());
                cyclicBarrier.await();
                Random rand = new Random();
                int n = rand.nextInt(100);
                int result = 0;
                for (int j = 0; j < n; j++){
                    result += j;
                }
                System.out.println(result + "|" + Thread.currentThread().getName());
                return result;
            });
        }
    }

}

运行结果

源码

进来之后首先发现的是成员变量

/** 用来保护栅栏入口的锁 */
private final ReentrantLock lock = new ReentrantLock();
/** 等待条件，直到计数器为0 */
private final Condition trip = lock.newCondition();
/** 参与线程的个数 */
private final int parties;
/* 计数器为0时要运行的命令，由用户定义 */
private final Runnable barrierCommand;
/** 当前等待的一代 */
private Generation generation = new Generation();
/**
 * parties数量的等待线程。每一代等待的数量从parties到0。当调用nextGeneration或者breakBarrier方法时重置。
 */
private int count;

从这里可以看出，除了内部实现用的ReentrantLock，其工作过程无非：计数器不为0的时候线程等待；当等待线程全部就绪，也就是计数器减为0的时候重置计数器并通知所有线程继续运行。

导致计数器重置原因有两个：一个就是发生异常，将当前这一代标记为无效（broken=true）；另一个就是正常就绪，开启下一代（new Generation）

核心方法dowait

// 情况一：timed=false，nanos=0L，代表一直阻塞
// 情况二：timed=true，nanos!=0L，代表在超时时间内阻塞
private int dowait(boolean timed, long nanos)
    throws InterruptedException, BrokenBarrierException,
           TimeoutException {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        // 获取当前这一代
        final Generation g = generation;

        // 如果当前这一代已经销毁，抛异常
        if (g.broken)
            throw new BrokenBarrierException();
        // 测试当前线程是否被中断
        if (Thread.interrupted()) {
            // 将broken设置为true，代表这一代已经销毁，重置count；然后通知所有等待线程
            breakBarrier();
            throw new InterruptedException();
        }
        // count 减1
        int index = --count;
        // 如果减1之后变成0，证明等待线程全部就绪。
        if (index == 0) {  // tripped
            boolean ranAction = false;
            try {
                // 如果用户定义了额外的命令，则执行
                final Runnable command = barrierCommand;
                if (command != null)
                    command.run();
                ranAction = true;
                // 开启下一代（通知所有等待线程，重置count，new一个新的Generation）
                nextGeneration();
                return 0;
            } finally {
                if (!ranAction)
                    breakBarrier();
            }
        }

        // loop until tripped, broken, interrupted, or timed out
        // 如果减1之后不等于0，也就是还有其它线程没有就绪，那么进入此循环，直到就绪或者被销毁，或者被中断和超时
        for (;;) {
            try {
                if (!timed)
                    // 未定义超时，则一直阻塞
                    trip.await();
                else if (nanos > 0L)
                    // 等待指定的超时时间
                    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;

            // 超时，则销毁这一代，通知所有等待线程并重置count
            if (timed && nanos <= 0L) {
                breakBarrier();
                throw new TimeoutException();
            }
        }
    } finally {
        lock.unlock();
    }
}

总结

两个工具实现思路都很简单，唯一我思考的是，为什么CountDownLatch只能用一次？

CycleBarrier很明显，它无论正常执行或者发生异常中断都有重置count的逻辑。

而CountDownLatch则没有重置的逻辑，那么，到底是CountDownLatch不能重置还是仅仅因为没有重置的逻辑。为此我把CountDownLatch的代码照搬，然后加上了简单的重置方法，如下：

import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;

public class MyCountDown {

    private static final class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = 4982264981922014374L;

        Sync(int count) {
            setState(count);
        }

        /**
         * 新加
         * @param count
         */
        void reset(int count){
            // 重新设置状态
            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;
            }
        }
    }

    private final Sync sync;

    private final int count;

    public MyCountDown(int count) {
        if (count < 0) throw new IllegalArgumentException("count < 0");
        this.sync = new Sync(count);
        this.count = count;
    }

    public void await() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }

    public boolean await(long timeout, TimeUnit unit)
            throws InterruptedException {
        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
    }

    public void countDown() {
        sync.releaseShared(1);
    }

    public long getCount() {
        return sync.getCount();
    }

    public String toString() {
        return super.toString() + "[Count = " + sync.getCount() + "]";
    }

    /**
     * 新加
     */
    public void reset(){
        // 调用重置的方法
        this.sync.reset(count);
    }
}

测试：

import java.util.*;
import java.util.concurrent.*;

public class Main {
    public static void main(String[] args) throws Exception {

        MyCountDown myCountDown = new MyCountDown(3);
        ThreadPoolExecutor executor = new ThreadPoolExecutor(10, 15, 60L, TimeUnit.SECONDS, new ArrayBlockingQueue<>(5));
        Future<Integer>[] futures = new Future[3];
        for (int i = 0; i < 3; i++){
            futures[i] = executor.submit(() -> {
                Random rand = new Random();
                int n = rand.nextInt(100);
                int result = 0;
                for (int j = 0; j < n; j++){
                    result += j;
                }
                System.out.println(result + "|" + Thread.currentThread().getName());
                Thread.sleep(new Random().nextInt(2000));   // 模拟耗时
                myCountDown.countDown();
                return result;
            });
        }
        myCountDown.await();
        System.out.println("第一次：" + (futures[0].get() + futures[1].get() + futures[2].get()));
        myCountDown.reset();    // 重置

        for (int i = 0; i < 3; i++){
            futures[i] = executor.submit(() -> {
                Random rand = new Random();
                int n = rand.nextInt(100);
                int result = 0;
                for (int j = 0; j < n; j++){
                    result += j;
                }
                System.out.println(result + "|" + Thread.currentThread().getName());
                Thread.sleep(new Random().nextInt(2000));   // 模拟耗时
                myCountDown.countDown();
                return result;
            });
        }
        myCountDown.await();
        System.out.println("如果重置无效，则这个信息会先于任务信息输出");
        System.out.println("第二次：" + (futures[0].get() + futures[1].get() + futures[2].get()));
    }

}

输出

如果换成CountDownLatch

import java.util.*;
import java.util.concurrent.*;

public class Main {
    public static void main(String[] args) throws Exception {

        CountDownLatch latch = new CountDownLatch(3);
        ThreadPoolExecutor executor = new ThreadPoolExecutor(10, 15, 60L, TimeUnit.SECONDS, new ArrayBlockingQueue<>(5));
        Future<Integer>[] futures = new Future[3];
        for (int i = 0; i < 3; i++){
            futures[i] = executor.submit(() -> {
                Random rand = new Random();
                int n = rand.nextInt(100);
                int result = 0;
                for (int j = 0; j < n; j++){
                    result += j;
                }
                System.out.println(result + "|" + Thread.currentThread().getName());
                Thread.sleep(new Random().nextInt(2000));   // 模拟耗时
                latch.countDown();
                return result;
            });
        }
        latch.await();
        System.out.println("第一次：" + (futures[0].get() + futures[1].get() + futures[2].get()));

        for (int i = 0; i < 3; i++){
            futures[i] = executor.submit(() -> {
                Random rand = new Random();
                int n = rand.nextInt(100);
                int result = 0;
                for (int j = 0; j < n; j++){
                    result += j;
                }
                System.out.println(result + "|" + Thread.currentThread().getName());
                Thread.sleep(new Random().nextInt(2000));   // 模拟耗时
                latch.countDown();
                return result;
            });
        }
        latch.await();
        System.out.println("如果重置无效，则这个信息会先于任务信息输出");
        System.out.println("第二次：" + (futures[0].get() + futures[1].get() + futures[2].get()));
    }

}

输出

所以可以得出结论，CountDownLatch不是没有办法重置，只不过没有写相关逻辑。当然这个问题如果我说错了，望指正。