//靜態(tài)內(nèi)部類，用于表示屏障的狀態(tài)
private static class Generation {
        boolean broken = false;
    }
//重入鎖，用于并發(fā)場景下的加鎖和釋放鎖
    private final ReentrantLock lock = new ReentrantLock();
//通過重入鎖拿到條件對象，調(diào)用await方法會進入到條件隊列中
    private final Condition trip = lock.newCondition();
//表示線程數(shù)，可以立即為這些線程數(shù)都執(zhí)行完后，屏障才會打破進入下一代
    private final int parties;
//這個是屏障被打破后，執(zhí)行的一個任務(wù)
    private final Runnable barrierCommand;
//內(nèi)部內(nèi)實例，存儲屏障狀態(tài)
    private Generation generation = new Generation();
  //等待執(zhí)行的線程數(shù)
    private int count;
    //進入下一代，其實就是下一個新的循環(huán)，觸發(fā)條件是所有的線程都執(zhí)行完，屏障被打破時
    private void nextGeneration() {
        // signal completion of last generation
        trip.signalAll();
        // set up next generation
        count = parties;
        generation = new Generation();
    }
//打破屏障，喚醒條件隊列中線程
private void breakBarrier() {
        generation.broken = true;
        count = parties;
        trip.signalAll();
    }
====================================================  核心方法   =================================================================
private int dowait(boolean timed, long nanos)
        throws InterruptedException, BrokenBarrierException,
               TimeoutException {
        final ReentrantLock lock = this.lock;
        //上鎖
        lock.lock();
        try {
            final Generation g = generation;
            //判斷屏障的狀態(tài)
            if (g.broken)
                throw new BrokenBarrierException();
           //判斷線程是否被打斷
            if (Thread.interrupted()) {
                breakBarrier();
                throw new InterruptedException();
            }
			//被執(zhí)行的線程數(shù)減一
            int index = --count;
            //如果當(dāng)前的線程都執(zhí)行完畢
            if (index == 0) {  // tripped
                boolean ranAction = false;
                try {
                    final Runnable command = barrierCommand;
                    if (command != null)
                    //執(zhí)行墊底的barrierCommand方法
                        command.run();
                    ranAction = true;
                    //開始下一個循環(huán)，其實也就是重置一下屏障的狀態(tài)，將parties的值重新復(fù)制給count并喚醒在條件隊列中的線程
                    nextGeneration();
                    return 0;
                } finally {
                    if (!ranAction)
                    //打破屏障
                        breakBarrier();
                }
            }
			//無限循環(huán)
            for (;;) {
                try {
                    if (!timed)
                    //調(diào)用await方法等待，其實就是到了條件隊列condition中去了，等待被喚醒
                        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();
        }
    }
===================================================================================================================================
//構(gòu)造函數(shù)，傳入線程數(shù)和執(zhí)行的方法
public CyclicBarrier(int parties, Runnable barrierAction) {
        if (parties <= 0) throw new IllegalArgumentException();
        this.parties = parties;
        this.count = parties;
        this.barrierCommand = barrierAction;
    }
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
        }
    }
//重置count，其實就是開啟一個新的循環(huán)
public void reset() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            breakBarrier();   // break the current generation
            nextGeneration(); // start a new generation
        } finally {
            lock.unlock();
        }
    }

public class MyThread extends Thread{
    private String name;
    private CyclicBarrier cb;
    public MyThread(String name,CyclicBarrier cb){
        this.name = name;
        this.cb = cb;
    }
    @Override
    public void run() {
        try{
            System.out.println(Thread.currentThread().getName()+"開始執(zhí)行");
            cb.await();
        }catch(Exception e){
            e.printStackTrace();
        }finally {
            System.out.println(Thread.currentThread().getName()+"繼續(xù)");
        }
    }
}
public static void main(String[] args) throws InterruptedException, BrokenBarrierException {
        CyclicBarrier cb = new CyclicBarrier(3, new Runnable() {
            @Override
            public void run() {
                System.out.println("屏障被打破，重新開始==========");
            }
        });
        MyThread t1 = new MyThread("t1",cb);
        MyThread t2 = new MyThread("t2",cb);
        t1.start();
        t2.start();
        cb.await();
    }