让三个线程按序打印--信号量(互斥量)实现

package Pta;

import java.util.concurrent.Semaphore;

public class Test {
    public static void main(String[] args) throws InterruptedException {
        Foo foo = new Foo();
        Runnable printFirst = new Runnable() {
            @Override
            public void run() {
                System.out.println("first");
            }
        };
        Runnable printSecond = new Runnable() {
            @Override
            public void run() {
                System.out.println("second");
            }
        };
        Runnable printThird = new Runnable() {
            @Override
            public void run() {
                System.out.println("third");
            }
        };
        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    foo.third(printThird);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }).start();
        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    foo.first(printFirst);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }).start();
        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    foo.second(printSecond);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }).start();
    }
}
class Foo {
    Semaphore s1=new Semaphore(1);
    Semaphore s2=new Semaphore(0);
    Semaphore s3=new Semaphore(0);
    public Foo() {

    }
    public void first(Runnable printFirst) throws InterruptedException {
        s1.acquire();
        printFirst.run();
        s2.release();
    }
    public void second(Runnable printSecond) throws InterruptedException {
        s2.acquire();
        printSecond.run();
        s3.release();
    }
    public void third(Runnable printThird) throws InterruptedException {
        s3.acquire();
        printThird.run();
    }
}