Semaphore（一）学习

作者: 李白是黑人 | 来源:发表于2019-03-21 15:23 被阅读5次

一、定义

Semaphore是一种在多线程环境下使用的设施，该设施负责协调各个线程，以保证它们能够正确、合理的使用公共资源的设施，也是操作系统中用于控制进程同步互斥的量。

Semaphore分为单值和多值两种，前者只能被一个线程获得，后者可以被若干个线程获得。

以一个停车场是运作为例。为了简单起见，假设停车场只有三个车位，一开始三个车位都是空的。这时如果同时来了五辆车，看门人允许其中三辆不受阻碍的进入，然后放下车拦，剩下的车则必须在入口等待，此后来的车也都不得不在入口处等待。这时，有一辆车离开停车场，看门人得知后，打开车拦，放入一辆，如果又离开两辆，则又可以放入两辆，如此往复。

在这个[停车场系统]中，车位是公共资源，每辆车好比一个线程，看门人起的就是[信号量]的作用。

更进一步，信号量的特性如下：信号量是一个非负整数（车位数），所有通过它的线程（车辆）都会将该整数减一（通过它当然是为了使用资源），当该整数值为零时，所有试图通过它的线程都将处于等待状态。在信号量上我们定义两种操作： Wait（等待）和 Release（释放）。当一个线程调用Wait（等待）操作时，它要么通过然后将信号量减一，要么一直等下去，直到信号量大于一或超时。Release（释放）实际上是在[信号量]上执行加操作，对应于车辆离开停车场，该操作之所以叫做“释放”是因为加操作实际上是释放了由信号量守护的资源。

在java中，还可以设置该信号量是否采用公平模式，如果以公平方式执行，则线程将会按到达的顺序（FIFO）执行，如果是非公平，则可以后请求的有可能排在队列的头部。 -------来源于百度百科

二、构造方法

//创建一个 Semaphore与给定数量的许可证和非公平公平设置。

public Semaphore(int permits)
//获取非公平方式（false）或者公平方式（true）
//当公平设置为真时，信号量保证调用acquire方法的线程被选择以按照它们调用这些方法的顺序获得许可（先进先出; FIFO）。
//设置为false时，此类不会保证线程获取许可的顺序
public Semaphore(int permits, boolean fair)

三、常用方法

下列只注释有参方法，无参一般默认n=1
void acquire()
void acquire(int n):从此信号量获取给定数目许可，在提供这些许可前一直将线程阻塞。比如n=2，就相当于一辆车占了两个车位。
void release()
void release(int n):释放n个许可,将其返回给信号量。就如同车开走返回n个车位。
int availablePermits()：当前可用的许可数。
boolean tryAcquire(long timeout, TimeUnit unit) 尝试在指定的某个时间段内获取1个许可，获取获取不到则返回false
boolean tryAcquire(int permits, long timeout, TimeUnit unit)尝试在指定的某个时间段内获取permits个许可，获取获取不到则返回false

四、Model

1、多进路-多处理-多出路实验
本实验目标是允许多个线程同时处理任务。
1.1 SemaphoreTest.java
import java.time.LocalDate;
import java.util.concurrent.Semaphore;

public class SemaphoreTest {
    private Semaphore semaphore = new Semaphore(3);

    void sayhello() {
        try {
            semaphore.acquire();
            System.out.println("SemaphoreTest.sayhello,ThreadName:" + Thread.currentThread().getName() + "~~start");
            System.out.println("begin sayHello:" + LocalDate.now());
            for (int i = 0; i < 5; i++) {
                System.out.println(Thread.currentThread().getName() + " say:" + (i + 1));
            }
            System.out.println("end sayHello:" + LocalDate.now());
            semaphore.release();
            System.out.println("SemaphoreTest.sayhello,ThreadName:" + Thread.currentThread().getName() + "~~end");
        } catch (Exception e) {
        }
    }
}
1.2 ThreadATest.java
public class ThreadATest  extends Thread{
    private SemaphoreTest service;

    public ThreadATest(SemaphoreTest service) {
        this.service = service;
    }
    @Override
    public void run(){
        service.sayhello();
    }
}
1.3 Run.java
public class Run {
    public static void main(String[] args) {
        SemaphoreTest semaphoreTest = new SemaphoreTest();
        ThreadATest[] threadATests = new ThreadATest[12];
        for (int i = 0; i < threadATests.length; i++) {
            threadATests[i] = new ThreadATest(semaphoreTest);
            threadATests[i].start();
        }
    }
}

结果：
SemaphoreTest.sayhello,ThreadName:Thread-0~~start
SemaphoreTest.sayhello,ThreadName:Thread-1~~start
SemaphoreTest.sayhello,ThreadName:Thread-2~~start
begin sayHello:2019-03-21
begin sayHello:2019-03-21
begin sayHello:2019-03-21
Thread-2 say:1
Thread-1 say:1
Thread-2 say:2
Thread-2 say:3
Thread-0 say:1
Thread-2 say:4
Thread-1 say:2
Thread-2 say:5
Thread-0 say:2
Thread-0 say:3
Thread-1 say:3
Thread-0 say:4
end sayHello:2019-03-21
Thread-0 say:5
Thread-1 say:4
Thread-1 say:5
end sayHello:2019-03-21
SemaphoreTest.sayhello,ThreadName:Thread-2~~end
SemaphoreTest.sayhello,ThreadName:Thread-4~~start
begin sayHello:2019-03-21
Thread-4 say:1
Thread-4 say:2
SemaphoreTest.sayhello,ThreadName:Thread-0~~end
end sayHello:2019-03-21
SemaphoreTest.sayhello,ThreadName:Thread-1~~end
SemaphoreTest.sayhello,ThreadName:Thread-5~~start
SemaphoreTest.sayhello,ThreadName:Thread-3~~start
begin sayHello:2019-03-21
Thread-4 say:3
Thread-5 say:1
begin sayHello:2019-03-21
Thread-5 say:2
Thread-4 say:4
Thread-4 say:5
end sayHello:2019-03-21
SemaphoreTest.sayhello,ThreadName:Thread-4~~end
Thread-5 say:3
Thread-5 say:4
Thread-5 say:5
Thread-3 say:1
end sayHello:2019-03-21
SemaphoreTest.sayhello,ThreadName:Thread-6~~start
SemaphoreTest.sayhello,ThreadName:Thread-7~~start
SemaphoreTest.sayhello,ThreadName:Thread-5~~end
Thread-3 say:2
Thread-3 say:3
Thread-3 say:4
Thread-3 say:5
end sayHello:2019-03-21
SemaphoreTest.sayhello,ThreadName:Thread-3~~end
begin sayHello:2019-03-21
Thread-7 say:1
begin sayHello:2019-03-21
Thread-7 say:2
SemaphoreTest.sayhello,ThreadName:Thread-8~~start
Thread-7 say:3
begin sayHello:2019-03-21
Thread-6 say:1
Thread-8 say:1
Thread-8 say:2
Thread-8 say:3
Thread-8 say:4
Thread-6 say:2
Thread-6 say:3
Thread-6 say:4
Thread-7 say:4
Thread-7 say:5
end sayHello:2019-03-21
SemaphoreTest.sayhello,ThreadName:Thread-7~~end
Thread-6 say:5
Thread-8 say:5
SemaphoreTest.sayhello,ThreadName:Thread-9~~start
end sayHello:2019-03-21
end sayHello:2019-03-21
SemaphoreTest.sayhello,ThreadName:Thread-10~~start
SemaphoreTest.sayhello,ThreadName:Thread-8~~end
begin sayHello:2019-03-21
Thread-10 say:1
Thread-10 say:2
Thread-10 say:3
begin sayHello:2019-03-21
Thread-10 say:4
SemaphoreTest.sayhello,ThreadName:Thread-11~~start
SemaphoreTest.sayhello,ThreadName:Thread-6~~end
begin sayHello:2019-03-21
Thread-11 say:1
Thread-11 say:2
Thread-10 say:5
Thread-9 say:1
end sayHello:2019-03-21
Thread-11 say:3
SemaphoreTest.sayhello,ThreadName:Thread-10~~end
Thread-9 say:2
Thread-9 say:3
Thread-9 say:4
Thread-11 say:4
Thread-9 say:5
Thread-11 say:5
end sayHello:2019-03-21
SemaphoreTest.sayhello,ThreadName:Thread-9~~end
end sayHello:2019-03-21
SemaphoreTest.sayhello,ThreadName:Thread-11~~end
2、多进路-单处理-多出路实验
本实验目标是允许多个线程同时处理任务，但是执行的书序确是同步的，也就是阻塞的，所以也称作单处理。
2.1 修改SemaphoreTest.java
import java.time.LocalDate;
import java.util.concurrent.Semaphore;
import java.util.concurrent.locks.ReentrantLock;

public class SemaphoreTest {
    private Semaphore semaphore = new Semaphore(3);
    private ReentrantLock lock = new ReentrantLock();
    void sayhello() {
        try {
            semaphore.acquire();
            System.out.println("SemaphoreTest.sayhello,ThreadName:" + Thread.currentThread().getName() + "~~start");
            lock.lock();
            System.out.println("begin sayHello:" + LocalDate.now());
            for (int i = 0; i < 5; i++) {
                System.out.println(Thread.currentThread().getName() + " say:" + (i + 1));
            }
            System.out.println("end sayHello:" + LocalDate.now());
            lock.unlock();
            semaphore.release();
            System.out.println("SemaphoreTest.sayhello,ThreadName:" + Thread.currentThread().getName() + "~~end");
        } catch (Exception e) {
        }
    }
}
结果：
SemaphoreTest.sayhello,ThreadName:Thread-0~~start
SemaphoreTest.sayhello,ThreadName:Thread-2~~start
SemaphoreTest.sayhello,ThreadName:Thread-1~~start
begin sayHello:2019-03-21
Thread-0 say:1
Thread-0 say:2
Thread-0 say:3
Thread-0 say:4
Thread-0 say:5
end sayHello:2019-03-21
SemaphoreTest.sayhello,ThreadName:Thread-0~~end
SemaphoreTest.sayhello,ThreadName:Thread-3~~start
begin sayHello:2019-03-21
Thread-1 say:1
Thread-1 say:2
Thread-1 say:3
Thread-1 say:4
Thread-1 say:5
end sayHello:2019-03-21
SemaphoreTest.sayhello,ThreadName:Thread-1~~end
begin sayHello:2019-03-21
SemaphoreTest.sayhello,ThreadName:Thread-4~~start
Thread-2 say:1
Thread-2 say:2
Thread-2 say:3
Thread-2 say:4
Thread-2 say:5
end sayHello:2019-03-21
SemaphoreTest.sayhello,ThreadName:Thread-2~~end
begin sayHello:2019-03-21
Thread-3 say:1
Thread-3 say:2
Thread-3 say:3
Thread-3 say:4
Thread-3 say:5
end sayHello:2019-03-21
SemaphoreTest.sayhello,ThreadName:Thread-3~~end
begin sayHello:2019-03-21
SemaphoreTest.sayhello,ThreadName:Thread-6~~start
SemaphoreTest.sayhello,ThreadName:Thread-5~~start
Thread-4 say:1
Thread-4 say:2
Thread-4 say:3
Thread-4 say:4
Thread-4 say:5
end sayHello:2019-03-21
SemaphoreTest.sayhello,ThreadName:Thread-4~~end
begin sayHello:2019-03-21
Thread-6 say:1
Thread-6 say:2
Thread-6 say:3
Thread-6 say:4
SemaphoreTest.sayhello,ThreadName:Thread-7~~start
Thread-6 say:5
end sayHello:2019-03-21
SemaphoreTest.sayhello,ThreadName:Thread-6~~end
begin sayHello:2019-03-21
Thread-5 say:1
Thread-5 say:2
Thread-5 say:3
Thread-5 say:4
SemaphoreTest.sayhello,ThreadName:Thread-8~~start
Thread-5 say:5
end sayHello:2019-03-21
SemaphoreTest.sayhello,ThreadName:Thread-5~~end
begin sayHello:2019-03-21
Thread-7 say:1
SemaphoreTest.sayhello,ThreadName:Thread-9~~start
Thread-7 say:2
Thread-7 say:3
Thread-7 say:4
Thread-7 say:5
end sayHello:2019-03-21
SemaphoreTest.sayhello,ThreadName:Thread-7~~end
begin sayHello:2019-03-21
Thread-8 say:1
Thread-8 say:2
Thread-8 say:3
Thread-8 say:4
Thread-8 say:5
end sayHello:2019-03-21
SemaphoreTest.sayhello,ThreadName:Thread-8~~end
SemaphoreTest.sayhello,ThreadName:Thread-10~~start
begin sayHello:2019-03-21
Thread-10 say:1
Thread-10 say:2
Thread-10 say:3
Thread-10 say:4
Thread-10 say:5
end sayHello:2019-03-21
SemaphoreTest.sayhello,ThreadName:Thread-10~~end
begin sayHello:2019-03-21
Thread-9 say:1
Thread-9 say:2
Thread-9 say:3
Thread-9 say:4
Thread-9 say:5
end sayHello:2019-03-21
SemaphoreTest.sayhello,ThreadName:Thread-9~~end
SemaphoreTest.sayhello,ThreadName:Thread-11~~start
begin sayHello:2019-03-21
Thread-11 say:1
Thread-11 say:2
Thread-11 say:3
Thread-11 say:4
Thread-11 say:5
end sayHello:2019-03-21

五、总结

Semaphore主要用于控制当前活动线程数目，就如同停车场系统一般，而Semaphore则相当于看守的人，用于控制总共允许停车的停车位的个数，而对于每辆车来说就如同一个线程，线程需要通过acquire()方法获取许可，而release()释放许可。如果许可数达到最大活动数，那么调用acquire()之后，便进入等待队列，等待已获得许可的线程释放许可，从而使得多线程能够合理的运行。

Semaphore（一）学习

一、定义

二、构造方法

三、常用方法

四、Model

五、总结

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