多线程（十三、AQS原理-Semaphore信号量）

Semaphore介绍

Semaphore，限制对共享资源访问的最大线程数量，要访问共享资源，需要先申请许可，申请到许可才能访问。访问结果了，释放许可。

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案例：

3个线程：Thread-1、Thread-2、Thread-3。一个许可数为2的公平策略的Semaphore。

线程的调用顺序如下：
Thread-1 申请一个许可，等待几秒钟，继续执行
Thread-2 申请2个许可，许可不足，阻塞
Thread-3 申请一个许可，等待几秒钟，继续执行
Thread-1,Thread-3,释放许可之后，Thread-2可以申请许可，成功执行。

代码：

Thread-1/3

import java.util.concurrent.Semaphore;

public class Task1 implements Runnable{

    private Semaphore semaphore;

    public Task1(Semaphore semaphore) {
        this.semaphore = semaphore;
    }

    @Override
    public void run() {

        try {
            semaphore.acquire();
            System.out.println(Thread.currentThread().getName() + "获取到许可....");
            Thread.sleep(3000);
            System.out.println(Thread.currentThread().getName() + "执行....");
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            System.out.println(Thread.currentThread().getName() + "释放许可....");
            semaphore.release();
        }
    }
}

Thread-2

import java.util.concurrent.Semaphore;

public class Task2 implements Runnable{

    private Semaphore semaphore;

    public Task2(Semaphore semaphore) {
        this.semaphore = semaphore;
    }

    @Override
    public void run() {

        try {
            System.out.println(Thread.currentThread().getName() + "申请许可....");
            semaphore.acquire(2);
            System.out.println(Thread.currentThread().getName() + "获取到许可....");
            Thread.sleep(3000);
            System.out.println(Thread.currentThread().getName() + "执行....");
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            System.out.println(Thread.currentThread().getName() + "释放许可....");
            semaphore.release(2);
        }
    }
}

启动文件

import java.text.ParseException;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Semaphore;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

public class Main {

    public static void main(String[] args) throws ParseException, InterruptedException {
        Semaphore semaphore = new Semaphore(2, true);

        ReentrantLock lock = new ReentrantLock(true);
        Condition condition = lock.newCondition();
        Thread t1 = new Thread(new Task1(semaphore),"Thread-1");
        t1.start();
        Thread.sleep(2000);
        Thread t2 = new Thread(new Task2(semaphore),"Thread-2");
        Thread t3 = new Thread(new Task1(semaphore),"Thread-3");
        t2.start();
        t3.start();
    }
}