Semaphore使用

Semaphore：信号量通常用于限制线程的数量访问一些(物理或逻辑)资源。个人理解：限流、控制访问量。
使用场景：竞争仅有的资源、一个车厢最多可容纳多少人数，超载则不能上车，或者每条通道一次仅能n个人进入，剩下的需要排队等等。。

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.concurrent.Semaphore;

/**
 * 信号量
 * Semaphore限流、控制访问量
 */
public class SemaphoreTest {

    public static final Logger logger = LoggerFactory.getLogger(SemaphoreTest.class);

    public static void main(String[] args) {

        /**
         * Creates a {@code Semaphore} with the given number of
         * permits and nonfair fairness setting.
         *
         * @param permits the initial number of permits available.
         *        This value may be negative, in which case releases
         *        must occur before any acquires will be granted.
         */

        // public Semaphore(int permits);

        /**
         * Creates a {@code Semaphore} with the given number of
         * permits and the given fairness setting.
         *
         * @param permits the initial number of permits available.
         *        This value may be negative, in which case releases
         *        must occur before any acquires will be granted.
         * @param fair {@code true} if this semaphore will guarantee
         *        first-in first-out granting of permits under contention,
         *        else {@code false}
         */

        // public Semaphore(int permits, boolean fair);


        //初始化2个信号量,也就是说同时最多只能有2个请求的访问
        Semaphore semaphore = new Semaphore(2);

        //模拟5次调用，循环5次，开启5个异步线程
        for (int i = 1; i <= 5; i++) {
            new Thread(() -> {
                try {
                    //获得访问机会
                    semaphore.acquire();
                    logger.info(Thread.currentThread().getName() + "获得访问机会!");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

                //假设这里有一个1秒钟的任务，阻塞一秒
                try {
                    Thread.sleep(1000L);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

                //释放许可，
                logger.info(Thread.currentThread().getName() + "释放许可，释放当前线程!");
                semaphore.release();
                logger.info(Thread.currentThread().getName() + "================================================");

            }).start();
        }
    }
}

Semaphore并不能保证程序的顺序和安全性。

输出日志:

11:59:54.157 [Thread-1] INFO com.test.tenant.SemaphoreTest - Thread-1获得访问机会!
11:59:54.157 [Thread-0] INFO com.test.tenant.SemaphoreTest - Thread-0获得访问机会!
11:59:55.165 [Thread-0] INFO com.test.tenant.SemaphoreTest - Thread-0释放许可，释放当前线程!
11:59:55.165 [Thread-1] INFO com.test.tenant.SemaphoreTest - Thread-1释放许可，释放当前线程!
11:59:55.165 [Thread-0] INFO com.test.tenant.SemaphoreTest - Thread-0================================================
11:59:55.165 [Thread-1] INFO com.test.tenant.SemaphoreTest - Thread-1================================================
11:59:55.165 [Thread-2] INFO com.test.tenant.SemaphoreTest - Thread-2获得访问机会!
11:59:55.165 [Thread-3] INFO com.test.tenant.SemaphoreTest - Thread-3获得访问机会!
11:59:56.167 [Thread-3] INFO com.test.tenant.SemaphoreTest - Thread-3释放许可，释放当前线程!
11:59:56.167 [Thread-2] INFO com.test.tenant.SemaphoreTest - Thread-2释放许可，释放当前线程!
11:59:56.168 [Thread-3] INFO com.test.tenant.SemaphoreTest - Thread-3================================================
11:59:56.168 [Thread-2] INFO com.test.tenant.SemaphoreTest - Thread-2================================================
11:59:56.168 [Thread-4] INFO com.test.tenant.SemaphoreTest - Thread-4获得访问机会!
11:59:57.172 [Thread-4] INFO com.test.tenant.SemaphoreTest - Thread-4释放许可，释放当前线程!
11:59:57.173 [Thread-4] INFO com.test.tenant.SemaphoreTest - Thread-4================================================