继上篇文章JAVA并发之多线程基础（3）谈到的信号量以及读写锁之外，接下来就继续谈及JDK中并发类的操作。

CountDownLatch

倒计时器是在某一些程序需要前置处理的时候完美搭档。例如我们经常玩的手游端，在游戏开始之前，它会去调用其他的组件，例如画面环境、人物图像、武器装备等，等加载完成之后再进入到主界面中进行游戏。

1. countDown()方法是每个线程完成之后减1，代表一个线程已经到达了终点线。我们可以点击进去看到里面调用的方法：

public final boolean releaseShared(int arg) {if (tryReleaseShared(arg)) {//试图将一个线程设置共享模式doReleaseShared();//共享模式下的释放操作return true;}return false;}
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1. await()是每个线程要到达的终点线，先执行完成的线程需在此等待未完成任务的线程。直至当前的countDown数量到0。

 public final void acquireSharedInterruptibly(int arg)throws InterruptedException {if (Thread.interrupted())//判断线程是否是中断throw new InterruptedException();if (tryAcquireShared(arg) < 0)//尝试在共享模式下获取doAcquireSharedInterruptibly(arg);//在共享可中断模式下获取。}
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demo:

package com.montos.lock;import java.util.Random;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;public class CountDownLatchDemo implements Runnable {static final CountDownLatchDemo demo = new CountDownLatchDemo();static final CountDownLatch latch = new CountDownLatch(10);@Overridepublic void run() {try {Thread.sleep(new Random().nextInt(10) * 1000);System.out.println("checking....");latch.countDown();} catch (InterruptedException e) {e.printStackTrace();}}public static void main(String[] args) throws InterruptedException {ExecutorService exec = Executors.newFixedThreadPool(10);for (int i = 0; i < 10; i++) {exec.submit(demo);}latch.await();System.out.println("prepare is end");exec.shutdown();}
}
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CyclicBarrier

栅栏是指每一个线程在某一处等待集合，然后接下来继续执行又到了某一处进行集合等待。可以说是上面CountDownLatch的增强版。

package com.montos.lock;import java.util.Random;
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;public class CyclicBarrierDemo {public static class Soldier implements Runnable {private String name;private final CyclicBarrier cycli;public Soldier(String name, CyclicBarrier cycli) {super();this.name = name;this.cycli = cycli;}@Overridepublic void run() {try {cycli.await();//十个线程一起等待集合doSometing();cycli.await();//十个线程一起等待事情处理完毕} catch (InterruptedException e) {e.printStackTrace();} catch (BrokenBarrierException e) {e.printStackTrace();}}private void doSometing() {try {Thread.sleep(new Random().nextInt(5) * 1000);} catch (InterruptedException e) {e.printStackTrace();}System.out.println(name + "任务完成");}}public static class BarrierRun implements Runnable {boolean flag;int N;public BarrierRun(boolean flag, int n) {super();this.flag = flag;N = n;}@Overridepublic void run() {if (flag) {System.out.println("士兵" + N + "任务完成");} else {System.out.println("士兵" + N + "集合完毕");flag = true;}}public static void main(String[] args) {final int N = 10;Thread[] allSoldier = new Thread[10];boolean flag = false;//N个线程到达之后，需要处理什么事情(这里是一起到达之后，处理BarrierRun事件)CyclicBarrier cycli = new CyclicBarrier(N, new BarrierRun(flag, N));System.out.println("集合队伍");for (int i = 0; i < 10; i++) {System.out.println("士兵" + i + "报道!");allSoldier[i] = new Thread(new Soldier("士" + i, cycli));allSoldier[i].start();}}}
}
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通过上面一个小的demo，可以看到CyclicBarrier的用法。其中主要的就是await()方法。寻找到里面去。主要的逻辑就是：

for (;;) {try {if (!timed)trip.await();//调用重入锁中的condition进行当前线程等待else if (nanos > 0L)nanos = trip.awaitNanos(nanos);//时间限制等待} catch (InterruptedException ie) {if (g == generation && ! g.broken) {breakBarrier();//将当前的屏障一代设置为断开并唤醒所有人throw ie;} else {Thread.currentThread().interrupt();//当前线程中断}}if (g.broken)throw new BrokenBarrierException();if (g != generation)return index;if (timed && nanos <= 0L) {breakBarrier();throw new TimeoutException();}}
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这上面就是对对应方法中的一个讲解。里面也用到了重入锁。通过上面的demo也知道CyclicBarrier与CountDownLatch用法，总的来说对于一些需要做之前准备的程序来说，他们是最佳的选择。