Semaphore

信号量，用来限制能同时访问共享资源的线程上限。

Semaphore应用

• 使用Semaphore限流，在访问高峰期时，让请求线程阻塞，高峰期过去再释放许可，当然它只适合限制单机线程数量，并且仅是限制线程数，而不是限制资源数（例如连接数，请对比Tomcat LimitLatch的实现)

import lombok.extern.slf4j.Slf4j;

import java.util.Random;
import java.util.concurrent.*;

@Slf4j
public class SemaphoreTest {
    public static void main(String[] args) {
        Semaphore semaphore = new Semaphore(3);

        for (int i = 0; i < 10; i++) {
            new Thread(() -> {
                try {
                    semaphore.acquire();
                    log.info("{} get lock", Thread.currentThread().getName());
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                }

                try {
                    int sleepTime = new Random().nextInt(5);
                    log.info("sleep time :{}", sleepTime);

                    TimeUnit.SECONDS.sleep(sleepTime);
                    log.info(Thread.currentThread().getName() + "start");
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                } finally {
                    semaphore.release();
                    log.info("{} unlock", Thread.currentThread().getName());
                }
            }).start();
        }
    }
}