Semaphore：信號(hào)量通常用于限制線程的數(shù)量訪問一些(物理或邏輯)資源祟牲。個(gè)人理解：限流枷邪、控制訪問量榛搔。
使用場(chǎng)景：競(jìng)爭(zhēng)僅有的資源、一個(gè)車廂最多可容納多少人數(shù)东揣，超載則不能上車践惑，或者每條通道一次僅能n個(gè)人進(jìn)入，剩下的需要排隊(duì)等等嘶卧。尔觉。

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

import java.util.concurrent.Semaphore;

/**
 * 信號(hào)量
 * Semaphore限流、控制訪問量
 */
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個(gè)信號(hào)量,也就是說同時(shí)最多只能有2個(gè)請(qǐng)求的訪問
        Semaphore semaphore = new Semaphore(2);

        //模擬5次調(diào)用芥吟，循環(huán)5次侦铜，開啟5個(gè)異步線程
        for (int i = 1; i <= 5; i++) {
            new Thread(() -> {
                try {
                    //獲得訪問機(jī)會(huì)
                    semaphore.acquire();
                    logger.info(Thread.currentThread().getName() + "獲得訪問機(jī)會(huì)!");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

                //假設(shè)這里有一個(gè)1秒鐘的任務(wù)专甩，阻塞一秒
                try {
                    Thread.sleep(1000L);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

                //釋放許可，
                logger.info(Thread.currentThread().getName() + "釋放許可钉稍，釋放當(dāng)前線程!");
                semaphore.release();
                logger.info(Thread.currentThread().getName() + "================================================");

            }).start();
        }
    }
}

Semaphore并不能保證程序的順序和安全性涤躲。

輸出日志:

11:59:54.157 [Thread-1] INFO com.test.tenant.SemaphoreTest - Thread-1獲得訪問機(jī)會(huì)!
11:59:54.157 [Thread-0] INFO com.test.tenant.SemaphoreTest - Thread-0獲得訪問機(jī)會(huì)!
11:59:55.165 [Thread-0] INFO com.test.tenant.SemaphoreTest - Thread-0釋放許可，釋放當(dāng)前線程!
11:59:55.165 [Thread-1] INFO com.test.tenant.SemaphoreTest - Thread-1釋放許可贡未，釋放當(dāng)前線程!
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獲得訪問機(jī)會(huì)!
11:59:55.165 [Thread-3] INFO com.test.tenant.SemaphoreTest - Thread-3獲得訪問機(jī)會(huì)!
11:59:56.167 [Thread-3] INFO com.test.tenant.SemaphoreTest - Thread-3釋放許可种樱，釋放當(dāng)前線程!
11:59:56.167 [Thread-2] INFO com.test.tenant.SemaphoreTest - Thread-2釋放許可，釋放當(dāng)前線程!
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獲得訪問機(jī)會(huì)!
11:59:57.172 [Thread-4] INFO com.test.tenant.SemaphoreTest - Thread-4釋放許可羞秤，釋放當(dāng)前線程!
11:59:57.173 [Thread-4] INFO com.test.tenant.SemaphoreTest - Thread-4================================================