学习过程观看视频：[狂神说Java]
https://www.bilibili.com/video/BV1B7411L7tE?p=13
欢迎大家支持噢，很良心的老师了！

1、java代码：

package com.add;

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

/**
 * Created by zjl
 * 2020/11/24
 **/
public class CyclicBarrierDemo {
    public static void main(String[] args) {
        /*
        * 集齐7颗龙珠召唤神龙
        * */
        //召唤龙珠的线程
        CyclicBarrier cyclicBarrier = new CyclicBarrier(7, () -> {
            System.out.println("已经  集齐7颗龙珠，召唤神龙！");
        });

        for (int i = 1; i <= 7; i++) {
            //lambda表达式中无法直接拿到temp，所以用final修饰的中间变量接收一下
            final int temp = i;
            new Thread(()->{
                System.out.println("收集到第"+ temp + "颗龙珠");
                try {
                    cyclicBarrier.await();  //等待
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } catch (BrokenBarrierException e) {
                    e.printStackTrace();
                }
            }).start();
        }
    }
}

运行结果：

/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/bin/java "-javaagent:/Users/zhangjianlong/Library/Application Support/JetBrains/Toolbox/apps/IDEA-U/ch-0/193.5662.53/IntelliJ IDEA.app/Contents/lib/idea_rt.jar=65480:/Users/zhangjianlong/Library/Application Support/JetBrains/Toolbox/apps/IDEA-U/ch-0/193.5662.53/IntelliJ IDEA.app/Contents/bin" -Dfile.encoding=UTF-8 -classpath /Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/charsets.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/deploy.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/cldrdata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/dnsns.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/jaccess.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/jfxrt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/localedata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/nashorn.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/sunec.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/sunjce_provider.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/sunpkcs11.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/ext/zipfs.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/javaws.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/jce.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/jfr.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/jfxswt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/jsse.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/management-agent.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/plugin.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/resources.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/jre/lib/rt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/lib/ant-javafx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/lib/dt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/lib/javafx-mx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/lib/jconsole.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/lib/packager.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/lib/sa-jdi.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_231.jdk/Contents/Home/lib/tools.jar:/Users/zhangjianlong/testCode/JUC-study/target/classes:/Users/zhangjianlong/apache-maven-3.6.3/repository/org/projectlombok/lombok/1.18.8/lombok-1.18.8.jar com.add.CyclicBarrierDemo
收集到第1颗龙珠
收集到第3颗龙珠
收集到第2颗龙珠
收集到第5颗龙珠
收集到第4颗龙珠
收集到第6颗龙珠
收集到第7颗龙珠
已经  集齐7颗龙珠，召唤神龙！

Process finished with exit code 0

2、当我们创建CyclicBarrier时，参数改为8的时候，永远也无法集齐7颗龙珠。

java代码：

package com.add;

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

/**
 * Created by zjl
 * 2020/11/24
 **/
public class CyclicBarrierDemo {
    public static void main(String[] args) {
        /*
        * 集齐7颗龙珠召唤神龙
        * */
        //召唤龙珠的线程
        CyclicBarrier cyclicBarrier = new CyclicBarrier(8, () -> {
            System.out.println("已经  集齐7颗龙珠，召唤神龙！");
        });

        for (int i = 1; i <= 7; i++) {
            //lambda表达式中无法直接拿到temp，所以用final修饰的中间变量接收一下
            final int temp = i;
            new Thread(()->{
                System.out.println("收集到第"+ temp + "颗龙珠");
                try {
                    cyclicBarrier.await();  //等待
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } catch (BrokenBarrierException e) {
                    e.printStackTrace();
                }
            }).start();
        }
    }
}

运行结果：