一、CountDownLatch
1.1 概述
让一些线程阻塞直到另外一些完成后才被唤醒。
该类主要有两个方法，当一个或多个线程调用 await 方法时，调用线程会被阻塞。其他线程调用 countDown方法计数器减1(调用countDown方法时线程不会阻塞)，当计数器的值变为0，因调用 await 方法被阻塞的线程会被唤醒继续执行。
1.2 模拟场景
有 6 名学生上完课后，离开教室，但他们离开的时间不相同，班长需要最后一个离开，锁上教室的门。
1.3 程序模拟
1.3.1 使用 CountDownLatch 之前
1 public void closeDoor() {
2
3 for (int i = 1; i <= 6; i++) {
4 new Thread(() -> {
5 System.out.println(Thread.currentThread().getName() + " 离开教室");
6 }, String.valueOf(i)).start();
7 }
8
9 System.out.println(Thread.currentThread().getName() + " 班长锁门离开教室");
10 }
1.3.2 使用 CountDownLatch 之后
1 public void closeDoor() throws InterruptedException {
2 CountDownLatch countDownLatch = new CountDownLatch(6);
3
4 for (int i = 1; i <= 6; i++) {
5 new Thread(() -> {
6 System.out.println(Thread.currentThread().getName() + " 离开教室");
7 countDownLatch.countDown();
8 }, String.valueOf(i)).start();
9 }
10
11 countDownLatch.await();
12 System.out.println(Thread.currentThread().getName() + " 班长锁门离开教室");
13 }
1.4 CountDownLatch 配合枚举类的使用
1 private static void nationalUnification() throws InterruptedException {
2 CountDownLatch countDownLatch = new CountDownLatch(6);
3
4 for (int i = 1; i <= 6; i++) {
5 new Thread(() -> {
6 System.out.println(Thread.currentThread().getName() + "国被灭...");
7 countDownLatch.countDown();
8 }, CountryEnum.forEachCountryEnum(i).getCountry()).start();
9 }
10
11 countDownLatch.await();
12 System.out.println(Thread.currentThread().getName() + " 秦国灭六国，一统华夏");
13 }
14
15 // 枚举类，存储国家的名字
16 public enum CountryEnum {
17
18 ONE(1, "齐"),
19 TWO(2, "楚"),
20 THREE(3, "燕"),
21 FOUR(4, "赵"),
22 FIVE(5, "魏"),
23 SIX(6, "韩");
24
25 private Integer code;
26 private String country;
27
28 CountryEnum(Integer code, String country) {
29 this.code = code;
30 this.country = country;
31 }
32
33 public Integer getCode() {
34 return code;
35 }
36
37 public String getCountry() {
38 return country;
39 }
40
41 public static CountryEnum forEachCountryEnum(int index) {
42 CountryEnum[] countryEnums = CountryEnum.values();
43 for (CountryEnum element : countryEnums) {
44 if (element.getCode() == index) {
45 return element;
46 }
47 }
48 return null;
49 }
50 }
51 
二、CyclicBarrier
2.1 概述
CyclicBarrier 的字面意思是可循环(Cyclic)，使用的屏障(barrier)。它要做的事情是，让一组线程到达一个屏障(也可以叫做同步点)时被阻塞，直到最后一个线程到达屏障时，屏障才会开门，所有被屏障拦截的线程才会继续干活，线程进入屏障通过 CyclicBarrier 的 await() 方法。
2.2 示例
1 public void meet() {
2 CyclicBarrier cyclicBarrier = new CyclicBarrier(10, () -> {
3 System.out.println("人齐了，准备开会");
4 });
5
6 for (int i = 1; i <= 10; i++) {
7 new Thread(() -> {
8 System.out.println(Thread.currentThread().getName() + " 来到会议室");
9 try {
10 cyclicBarrier.await();
11 } catch (InterruptedException | BrokenBarrierException e) {
12 e.printStackTrace();
13 }
14 }, String.valueOf(i)).start();
15 }
16 }
三、Semaphore
3.1 概述
A counting semaphore. Conceptually, a semaphore maintains a set of permits. Each acquire() blocks if necessary until a permit is available, and then takes it. Each release() adds a permit, potentially releasing a blocking acquirer. However, no actual permit objects are used; the Semaphore just keeps a count of the number available and acts accordingly.
3.2 示例
1 public void semaphoreTest() {
2 Semaphore semaphore = new Semaphore(3);
3
4 for (int i = 1; i <= 6; i++) {
5 new Thread(() -> {
6 try {
7 semaphore.acquire();
8 System.out.println(Thread.currentThread().getName() + " 抢到车位...");
9 TimeUnit.SECONDS.sleep(3);
10 System.out.println(Thread.currentThread().getName() + "停车3秒，离开车位");
11 } catch (InterruptedException e) {
12 e.printStackTrace();
13 } finally {
14 semaphore.release();
15 }
16 }, String.valueOf(i)).start();
17 }
18 }