Java多线程之同步工具类CyclicBarrier

 public static void main(String[] args) {
        int N = 4;
        CyclicBarrier barrier  = new CyclicBarrier(N);
        for(int i=0;i<N;i++)
            new Writer(barrier).start();
    }
    static class Writer extends Thread{
        private CyclicBarrier cyclicBarrier;
        public Writer(CyclicBarrier cyclicBarrier) {
            this.cyclicBarrier = cyclicBarrier;
        }
 
        @Override
        public void run() {
            System.out.println("线程"+Thread.currentThread().getName()+"正在写入数据...");
            try {
                Thread.sleep(5000);      //以睡眠来模拟写入数据操作
                System.out.println("线程"+Thread.currentThread().getName()+"写入数据完毕，等待其他线程写入完毕");
                cyclicBarrier.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }catch(BrokenBarrierException e){
                e.printStackTrace();
            }
            System.out.println("所有线程写入完毕，继续处理其他任务...");
        }
    }

public class CyclicBarrierTest {

    public static void main(String[] args) {
        int N = 4;
        CyclicBarrier barrier  = new CyclicBarrier(N,new Runnable() {
            @Override
            public void run() {
                System.out.println("当前线程"+Thread.currentThread().getName());
            }
        });

        for(int i=0;i<N;i++)
            new Writer(barrier).start();
    }
    static class Writer extends Thread{
        private CyclicBarrier cyclicBarrier;
        public Writer(CyclicBarrier cyclicBarrier) {
            this.cyclicBarrier = cyclicBarrier;
        }

        @Override
        public void run() {
            System.out.println("线程"+Thread.currentThread().getName()+"正在写入数据...");
            try {
                Thread.sleep(3000);      //以睡眠来模拟写入数据操作
                System.out.println("线程"+Thread.currentThread().getName()+"写入数据完毕，等待其他线程写入完毕");
                cyclicBarrier.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }catch(BrokenBarrierException e){
                e.printStackTrace();
            }
            System.out.println("所有线程写入完毕，继续处理其他任务...");
        }
    }

}

public class CyclicBarrierTest {


    public static void main(String[] args) {
        int N = 4;
        CyclicBarrier barrier = new CyclicBarrier(N);

        for (int i = 0; i < N; i++) {
            if (i < N - 1)
                new Writer(barrier).start();
            else {
                try {
                    //运行时间远小于2000(cyclicBarrier.await 指定时间) 就不会抛出TimeoutException
                    Thread.sleep(3000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                new Writer(barrier).start();
            }
            
        }
    }

    static class Writer extends Thread {
        private CyclicBarrier cyclicBarrier;

        public Writer(CyclicBarrier cyclicBarrier) {
            this.cyclicBarrier = cyclicBarrier;
        }

        @Override
        public void run() {
            System.out.println("线程" + Thread.currentThread().getName() + "正在写入数据...");
            try {
                Thread.sleep(3000);      //以睡眠来模拟写入数据操作
                System.out.println("线程" + Thread.currentThread().getName() + "写入数据完毕，等待其他线程写入完毕");
                try {
                    cyclicBarrier.await(2000, TimeUnit.MILLISECONDS);
                } catch (TimeoutException e) {
                    e.printStackTrace();
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            } catch (BrokenBarrierException e) {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() + "所有线程写入完毕，继续处理其他任务...");
        }
    }
}

public class CyclicBarrierTest {

    public static void main(String[] args) {
        int N = 4;
        CyclicBarrier barrier  = new CyclicBarrier(N);

        for(int i=0;i<N;i++) {
            new Writer(barrier).start();
        }

        try {
            Thread.sleep(5000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("CyclicBarrier重用");

        for(int i=0;i<N;i++) {
            new Writer(barrier).start();
        }
    }
    static class Writer extends Thread{
        private CyclicBarrier cyclicBarrier;
        public Writer(CyclicBarrier cyclicBarrier) {
            this.cyclicBarrier = cyclicBarrier;
        }

        @Override
        public void run() {
            System.out.println("线程"+Thread.currentThread().getName()+"正在写入数据...");
            try {
                Thread.sleep(3000);      //以睡眠来模拟写入数据操作
                System.out.println("线程"+Thread.currentThread().getName()+"写入数据完毕，等待其他线程写入完毕");

                cyclicBarrier.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }catch(BrokenBarrierException e){
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName()+"所有线程写入完毕，继续处理其他任务...");
        }
    }
}

    /** The lock for guarding barrier entry */
    private final ReentrantLock lock = new ReentrantLock();
    /** Condition to wait on until tripped */
    private final Condition trip = lock.newCondition();
    /** The number of parties */
    private final int parties;//拦截的线程数量
    /* The command to run when tripped */
    private final Runnable barrierCommand; //当屏障撤销时，需要执行的屏障操作
    //当前的Generation。每当屏障失效或者开闸之后都会自动替换掉。从而实现重置的功能。
    private Generation generation = new Generation();

    /**
     * Number of parties still waiting. Counts down from parties to 0
     * on each generation.  It is reset to parties on each new
     * generation or when broken.
     */
    private int count;

//带Runnable参数的函数
 public CyclicBarrier(int parties, Runnable barrierAction) {
        if (parties <= 0) throw new IllegalArgumentException();
        this.parties = parties;//有几个运动员要参赛
        this.count = parties;//目前还需要几个运动员准备好
        //你要在所有线程都继续执行下去之前要执行什么操作，可以为空
        this.barrierCommand = barrierAction;
    }
//不带Runnable参数的函数
 public CyclicBarrier(int parties) {
     this(parties, null);
 }

    public int await() throws InterruptedException, BrokenBarrierException {
        try {
            return dowait(false, 0L);
        } catch (TimeoutException toe) {
            throw new Error(toe); // cannot happen
        }

   private int dowait(boolean timed, long nanos)
        throws InterruptedException, BrokenBarrierException,
               TimeoutException {
        final ReentrantLock lock = this.lock;
        lock.lock();//获取ReentrantLock互斥锁
        try {
            final Generation g = generation;//获取generation对象

            if (g.broken)//如果generation损坏，抛出异常
                throw new BrokenBarrierException();

            if (Thread.interrupted()) {
                //如果当前线程被中断，则调用breakBarrier方法，停止CyclicBarrier，并唤醒所有线程
                breakBarrier();
                throw new InterruptedException();
            }

            int index = --count;// 看到这里了吧，count减1 
            //index=0，也就是说，有0个线程未满足CyclicBarrier条件，也就是条件满足，
            //可以唤醒所有的线程了
            if (index == 0) {  // tripped
                boolean ranAction = false;
                try {
                   //这就是构造器的第二个参数，如果不为空的话，就执行这个Runnable的run方法，
                   //你看，这里是执行的是run方法，也就是说，并没有新起一个另外的线程，
                   //而是最后一个执行await操作的线程执行的这个run方法。
                    final Runnable command = barrierCommand;
                    if (command != null)
                        command.run(); //同步执行barrierCommand
                    ranAction = true;
                    nextGeneration(); //执行成功设置下一个nextGeneration
                    return 0;
                } finally {
                    if (!ranAction) . //如果barrierCommand执行失败，进行屏障破坏处理
                        breakBarrier();
                }
            }
            //如果当前线程不是最后一个到达的线程
            // loop until tripped, broken, interrupted, or timed out
            for (;;) {
                try {
                    if (!timed)
                        trip.await(); //调用Condition的await()方法阻塞
                    else if (nanos > 0L)
                        nanos = trip.awaitNanos(nanos); //调用Condition的awaitNanos()方法阻塞
                } catch (InterruptedException ie) {
                //如果当前线程被中断，则判断是否有其他线程已经使屏障破坏。若没有则进行屏障破坏处理，并抛出异常；否则再次中断当前线程
                    if (g == generation && ! g.broken) {
                        breakBarrier();//执行breakBarrier，唤醒所有线程
                        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)//如果当前generation已经损坏，抛出异常
                    throw new BrokenBarrierException();

                if (g != generation)//如果generation已经更新换代，则返回index
                    return index;
                //如果是参数是超时等待，并且已经超时，则执行breakBarrier()方法
                //唤醒所有等待线程。
                if (timed && nanos <= 0L) {
                    breakBarrier();
                    throw new TimeoutException();
                }
            }
        } finally {
            lock.unlock();
        }
    }

private Generation generation = new Generation();

private static class Generation {
    boolean broken = false;
}

    private void nextGeneration() {
        // signal completion of last generation
        trip.signalAll();//调用Condition的signalAll方法，唤醒所有await的线程
        // set up next generation
        count = parties;//重置count值
        //生成新的Generation，表示上一代的所有线程已经唤醒，进行更新换代
        generation = new Generation(); 
    }

    private void breakBarrier() {
        generation.broken = true;//表示当代因为线程被中断，已经发成损坏了
        count = parties;//重置count值
        trip.signalAll();//调用Condition的signalAll方法，唤醒所有await的线程
    }

    public boolean isBroken() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return generation.broken;
        } finally {
            lock.unlock();
        }
    }

    //将屏障重置为其初始状态。
    public void reset() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            //唤醒所有等待的线程继续执行，并设置屏障中断状态为true
            breakBarrier();   // break the current generation
            //唤醒所有等待的线程继续执行，并设置屏障中断状态为false
            nextGeneration(); // start a new generation
        } finally {
            lock.unlock();
        }
    }

    //返回当前在屏障处等待的参与者数目，此方法主要用于调试和断言。
    public int getNumberWaiting() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return parties - count;
        } finally {
            lock.unlock();
        }
    }