利用semaphore的先V（release）再P（acquire）操作，实现线程的前置依赖关系。

import java.util.concurrent.Semaphore;import java.util.concurrent.ThreadLocalRandom;/** * 利用semaphore的先V（release）再P（acquire）操作，实现线程的前置依赖关系。 * semaphore的容量是0 */public class SemaphorePVDemo {    public static void main(String[] args) {        Semaphore semaphore1 = new Semaphore(0);        Semaphore semaphore2 = new Semaphore(0);        new Thread(()->{            try {                Thread.sleep(ThreadLocalRandom.current().nextInt(5000));            } catch (InterruptedException e) {                e.printStackTrace();            }            System.out.println("线程1最先工作");            semaphore1.release();        }).start();        new Thread(()->{            try {                semaphore1.acquire();                Thread.sleep(ThreadLocalRandom.current().nextInt(5000));            } catch (InterruptedException e) {                e.printStackTrace();            }            System.out.println("线程2然后工作");            semaphore2.release();        }).start();        new Thread(()->{            try {                semaphore2.acquire();                Thread.sleep(ThreadLocalRandom.current().nextInt(5000));            } catch (InterruptedException e) {                e.printStackTrace();            }            System.out.println("线程3最后工作");        }).start();    }}