CountDownLatch

一、CountDownLatch

一个同步辅助类，在完成一组正在其他线程中执行的操作之前，它允许一个或多个线程一直等待。

用给定的计数 初始化 CountDownLatch。由于调用了 countDown() 方法，所以在当前计数到达零之前，await 方法会一直受阻塞。之后，会释放所有等待的线程，await 的所有后续调用都将立即返回。这种现象只出现一次——计数无法被重置。如果需要重置计数，请考虑使用 CyclicBarrier。

import java.util.concurrent.CountDownLatch;

/**
 * 某一个事情按照指定的步骤进行
 */
public class CountDownLatchDemo {

	/**
	 * @description
	 * 去上班之前
	 * 1.洗脸
	 * 2.刷牙
	 * 3.吃饭
	 * @param args
	 * @throws InterruptedException 
	 */
	public static void main(String[] args) throws InterruptedException {

		// 指明时间的数量
		CountDownLatch latch = new CountDownLatch(3);
		
		new Thread(new WashFace(latch)).start();
		new Thread(new WashTooth(latch)).start();
		new Thread(new Eat(latch)).start();
		// 阻塞线程，直到 latch 的值为 0 
		latch.await();
		// 若不适用 CountDownLatch 则会出现，主线程信息先打印，而等待其他线程信息的情况
		System.out.println("去上班");
	}

}

class WashFace implements Runnable{
	
	private CountDownLatch latch = null ;
	
	public WashFace(){
		
	}
	public WashFace(CountDownLatch latch){
		this.latch = latch ;
	}
	
	@Override
	public void run() {
		System.out.println("洗脸");
		latch.countDown();
	}
}

class WashTooth implements Runnable{

	private CountDownLatch latch = null ;
	
	public WashTooth(){
		
	}	
	
	public WashTooth(CountDownLatch latch){
		this.latch = latch ;
	}
	
	@Override
	public void run() {

		System.out.println("刷牙");
		latch.countDown();
	}
	
}

class Eat implements Runnable{

	private CountDownLatch latch ;
	
	public Eat(){
		
	}
	
	public Eat(CountDownLatch latch){
		this.latch = latch ;
	}
	
	@Override
	public void run() {
		System.out.println("吃饭");
		latch.countDown();
	}
	
}

二、

import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;

/**
 * 需求：某一个时间点要求各个线程统一开始执行
 * 先执行到该时间点的线程等待其他线程；然后所有线程一起运行
 */
public class CyclicBarrierDownLatchDemo {

	/**
	 * @description
	 * 赛马
	 * 1.比赛开始前，所有马匹被从其他地方牵引到起点位置
	 * 2.先到的等待后到的
	 * 3.比赛开始，所有马匹开始赛跑
	 * @param args
	 */
	public static void main(String[] args) {

		CyclicBarrier cb = new CyclicBarrier(3);
		new Thread(new BlackHorse(cb)).start();
		new Thread(new WhiteHorse(cb)).start();
		new Thread(new GreyHorse(cb)).start();
		
		System.out.println("=============");
		
	}

}

class BlackHorse implements Runnable{

	private CyclicBarrier cb = null ; 
	public BlackHorse(){
		
	}
	public BlackHorse(CyclicBarrier cb){
		this.cb = cb ;
	}
	
	@Override
	public void run() {
		try {
			Thread.sleep(1000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		System.out.println("黑马花费1S到达起点");
		try {
			cb.await();
		} catch (InterruptedException e) {
			e.printStackTrace();
		} catch (BrokenBarrierException e) {
			e.printStackTrace();
		}
		
		System.out.println("黑马开始奔跑");
	}
	
}

class WhiteHorse implements Runnable{

	private CyclicBarrier cb = null ; 
	public WhiteHorse(){
		
	}
	public WhiteHorse(CyclicBarrier cb){
		this.cb = cb ;
	}
	
	@Override
	public void run() {
		try {
			Thread.sleep(2000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		System.out.println("白马花费2S到达起点");
		try {
			cb.await();
		} catch (InterruptedException e) {
			e.printStackTrace();
		} catch (BrokenBarrierException e) {
			e.printStackTrace();
		}
		
		System.out.println("白马开始奔跑");
	}
	
}

class GreyHorse implements Runnable{

	private CyclicBarrier cb = null ; 
	public GreyHorse(){
		
	}
	public GreyHorse(CyclicBarrier cb){
		this.cb = cb ;
	}
	
	@Override
	public void run() {
		try {
			Thread.sleep(3000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		System.out.println("灰马花费3S到达起点");
		try {
			cb.await();
		} catch (InterruptedException e) {
			e.printStackTrace();
		} catch (BrokenBarrierException e) {
			e.printStackTrace();
		}
		
		System.out.println("灰马开始奔跑");
	}
	
}

三、Exchanger

import java.util.concurrent.Exchanger;

/**
 * 线程间交换信息，允许线程间相互等待 
 */
public class ExchangerDemo {

	/**
	 * @description
	 * 1.A来到指定地点等待B，发出信息
	 * 2.B来到指定地点等待A，发出信息
	 * @param args
	 */
	public static void main(String[] args) {

		Exchanger<String> ex = new Exchanger<String>();
		
		new Thread(new PersonA(ex)).start();
		new Thread(new PersonB(ex)).start();
		
		System.out.println("============");
	}

}
class PersonB implements Runnable{

	private Exchanger<String> ex ;
	public PersonB(Exchanger<String> ex){
		this.ex = ex ;
	}
	
	@Override
	public void run() {
		try {
			Thread.sleep(5000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		System.out.println("B用5S到达了指点地点");
		try {
			String result = ex.exchange("宝塔镇河妖");
			if(result.equals("天王盖地虎")){
				System.out.println("自己人");
			}else{
				System.out.println("兄弟们，抄家伙儿");
			}
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}
	
}
class PersonA implements Runnable{

	private Exchanger<String> ex ;
	public PersonA(Exchanger<String> ex){
		this.ex = ex ;
	}
	
	@Override
	public void run() {
		try {
			Thread.sleep(1000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		System.out.println("A用1S到达了指点地点");
		try {
			String result = ex.exchange("天王盖地虎");
			if(result.equals("宝塔镇河妖")){
				System.out.println("自己人");
			}else{
				System.out.println("兄弟们，抄家伙儿");
			}
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}
	
}

四、Semaphore

1.

import java.util.concurrent.Semaphore;

/**
 * 保护一段代码，不能同时被超过N个线程访问
 */
public class SemaphoreDemo {

	/**
	 * @description
	 * 景区某一个景点，每次仅允许3人进入访问；一旦有人出来，再次放人进入
	 * @param args
	 */
	public static void main(String[] args) {

		Semaphore se = new Semaphore(3);
                // 保证公平，按照顺序执行
		// Semaphore se = new Semaphore(3,true);
		for(int i = 0 ; i < 100 ; i++){
			new Thread(new Visitor(se, i)).start();
		}
	}
}

class Visitor implements Runnable{

	private Semaphore se = null ;
	private int i ;
	public Visitor(){
		
	}
	
	public Visitor(Semaphore se,int i){
		this.se = se ;
		this.i = i ;
	}
	
	@Override
	public void run() {

		try {
			se.acquire();
			System.out.println(i+"进入景区");
			Thread.sleep(i*1000);
			System.out.println(i+"游览"+i*1000+"s");
			se.release();
			System.out.println(i+"离开景区");
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}
	
}

运行结果：
0进入景区
4进入景区
2进入景区
0游览0s
...

未按照顺序进行执行