并发工具-CyclicBarrier源码简析

public class CyclicBarrierTest {

    public static CyclicBarrier cb = new CyclicBarrier(3, () ->  System.out.println("-------开始点名-------"));

    public static void main(String[] args) {
        System.out.println("三人组团出游，门口等待，准备点名");
        for (int i = 0; i < 3 ; i++) {
            new Thread(() -> {
                System.out.println(Thread.currentThread().getName() + "到达集合点");
                try { cb.await(); }catch (Exception e){}
                System.out.println(Thread.currentThread().getName() + "到车上");
                try { cb.await(); }catch (Exception e){}
                System.out.println(Thread.currentThread().getName() + "到景区");
                try { cb.await(); }catch (Exception e){}

            }, "线程" + i).start();
        }
    }
}

    /**
     * CyclicBarrier JDK1.8分析
     * @param parties 等待的线程个数
     * @param barrierAction 所有等待的线程就绪后要执行任务
     */
    public CyclicBarrier(int parties, Runnable barrierAction) {
        if (parties <= 0) throw new IllegalArgumentException();
        this.parties = parties;
        this.count = parties;
        this.barrierCommand = barrierAction;
    }

/**
     * 每一个线程在某个点等待其他线程，执行await方法
     * @return
     * @throws InterruptedException
     * @throws BrokenBarrierException
     */
    public int await() throws InterruptedException, BrokenBarrierException {
        try {
            //具体的功能方法
            return dowait(false, 0L);
        } catch (TimeoutException toe) {
            throw new Error(toe); // cannot happen
        }
    }

    /**
     * Main barrier code, covering the various policies.
     *
     * 栅栏的核心代码，包含各种条件
     *
     * ReentrantLock lock - CyclicBarrier是用Lock锁实现的，这里用来控制线程的执行
     *
     * CyclicBarrier.Generation 是一个内部类，只是用来进行状态标识
     *
     */
    private int dowait(boolean timed, long nanos)
            throws InterruptedException, BrokenBarrierException,
            TimeoutException {        final ReentrantLock lock = this.lock;
        //每个线程执行await方法获取锁
        lock.lock();
        try {
            //CyclicBarrier在初始化的时候，创建的g赋值给局部变量，在后面使用
            final CyclicBarrier.Generation g = generation;
            //g.broken 默认是false,如果是true表示栅栏出问题了，抛出异常
            if (g.broken)
                throw new BrokenBarrierException();
            //这里判断当前执行await()的线程状态，如果是中断，表明程序出问题了
            if (Thread.interrupted()) {
                //初始化栅栏数据，并将g.broken设置为true
                breakBarrier();
                throw new InterruptedException();
            }
            //count用来记录，还需要几个线程执行await()方法，可以冲破栅栏
            int index = --count;
            //index == 0 表示等待的线程数量够了
            if (index == 0) {  // tripped
                boolean ranAction = false;
                try {
                    final Runnable command = barrierCommand;
                    if (command != null)
                        //执行构造函数里面传进来的任务，这里是由最后一个达到的线程执行的
                        command.run();
                    //表明这次等待任务完成，准备执行下一次栅栏任务
                    ranAction = true;
                    //该方法里用：trip.signalAll(); 唤醒进入方法里的其他线程，重新初始化 count 和 generation
                    //为下一次执行栅栏做准备
                    nextGeneration();
                    return 0;
                } finally {
                    if (!ranAction) breakBarrier();
                }
            }
            // loop until tripped, broken, interrupted, or timed out
            for (;;) {
                try {
                    if (!timed)
                        //执行await()方法获取到锁执行到此，调用condition.await()方法进行挂起，并释放锁，让其他线程获取锁
                        trip.await();
                    else if (nanos > 0L)
                        //同上，区别在于有无等待时间
                        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;
                if (timed && nanos <= 0L) {
                    breakBarrier();
                    throw new TimeoutException();
                }
            }
        } finally {
            //释放锁，之前进来的线程，都在调condition.await()方法的时候已经释放锁了，所以这里只有最后一个进来的线程有效果
            lock.unlock();
        }
    }