Semaphore信号量介绍
官方的解释为
1)Semaphore是一个计数信号量
2)从概念上将,Semaphore包含一组许可证
3)如果有需要的话,每次调用acquire()方法都会阻塞,直到获取一个可用的许可证
4)每次调用release()方法都会释放持有许可证的线程,并且归还Semaphore一个可用的许可证
5)实际上并没有真实的许可证对象供线程使用, Semaphore只是对可用的数量进行管理维护
Semaphore内部主要通过AQS(AbstractQueuedSynchronizer)实现线程的管理 .
一、获取单个许可
信号量设置为3，每次获取1个许可，那么就可以连续获得3个许可。
如下面的信号量Demo
@Slf4j
public class SemaphoreExample1 {
private static int threadCount = 20;
public static void main(String[] args) throws InterruptedException {
ExecutorService exec = Executors.newCachedThreadPool();
final Semaphore semaphore = new Semaphore(3);
for(int i = 0; i < threadCount; i++){
final int threadNum = i;
exec.execute(()->{
try {
semaphore.acquire();//获得一个许可
test(threadNum);
semaphore.release();//释放一个许可
} catch (InterruptedException e) {
e.printStackTrace();
}
});
}
exec.shutdown();
}
private static void test(int threadNum) throws InterruptedException {
log.info("{}", threadNum);
Thread.sleep(1000);
}
}
打印结果如下。信号量设置为3。 差不多每隔1秒钟打印3条数据。
二、获得多个许可
线程数为20，信号量为3，每次获得3个许可。相当于单线程。
@Slf4j
public class SemaphoreExample2 {
private static int threadCount = 20;
public static void main(String[] args) throws InterruptedException {
ExecutorService exec = Executors.newCachedThreadPool();
final Semaphore semaphore = new Semaphore(3);
for(int i = 0; i < threadCount; i++){
final int threadNum = i;
exec.execute(()->{
try {
semaphore.acquire(3);//获得3个许可， 信号量为3，相当于单线程执行
test(threadNum);
semaphore.release(3);//释放3个许可
} catch (InterruptedException e) {
e.printStackTrace();
}
});
}
exec.shutdown();
}
private static void test(int threadNum) throws InterruptedException {
log.info("{}", threadNum);
Thread.sleep(1000);
}
}
打印结果： 差不多每隔1秒打印一次结果
21:58:55.766 [pool-1-thread-1] INFO com.bingfa.concurrency.example.aqs.SemaphoreExample2 - 0 21:58:56.771 [pool-1-thread-2] INFO com.bingfa.concurrency.example.aqs.SemaphoreExample2 - 1 21:58:57.771 [pool-1-thread-3] INFO com.bingfa.concurrency.example.aqs.SemaphoreExample2 - 2 21:58:58.772 [pool-1-thread-4] INFO com.bingfa.concurrency.example.aqs.SemaphoreExample2 - 3 21:58:59.773 [pool-1-thread-5] INFO com.bingfa.concurrency.example.aqs.SemaphoreExample2 - 4 21:59:00.774 [pool-1-thread-6] INFO com.bingfa.concurrency.example.aqs.SemaphoreExample2 - 5 21:59:01.774 [pool-1-thread-8] INFO com.bingfa.concurrency.example.aqs.SemaphoreExample2 - 7 21:59:02.775 [pool-1-thread-7] INFO com.bingfa.concurrency.example.aqs.SemaphoreExample2 - 6 21:59:03.776 [pool-1-thread-9] INFO com.bingfa.concurrency.example.aqs.SemaphoreExample2 - 8 21:59:04.776 [pool-1-thread-10] INFO com.bingfa.concurrency.example.aqs.SemaphoreExample2 - 9 21:59:05.777 [pool-1-thread-11] INFO com.bingfa.concurrency.example.aqs.SemaphoreExample2 - 10 21:59:06.778 [pool-1-thread-12] INFO com.bingfa.concurrency.example.aqs.SemaphoreExample2 - 11 21:59:07.779 [pool-1-thread-13] INFO com.bingfa.concurrency.example.aqs.SemaphoreExample2 - 12 21:59:08.780 [pool-1-thread-14] INFO com.bingfa.concurrency.example.aqs.SemaphoreExample2 - 13 21:59:09.780 [pool-1-thread-16] INFO com.bingfa.concurrency.example.aqs.SemaphoreExample2 - 15 21:59:10.781 [pool-1-thread-15] INFO com.bingfa.concurrency.example.aqs.SemaphoreExample2 - 14 21:59:11.782 [pool-1-thread-17] INFO com.bingfa.concurrency.example.aqs.SemaphoreExample2 - 16 21:59:12.783 [pool-1-thread-18] INFO com.bingfa.concurrency.example.aqs.SemaphoreExample2 - 17 21:59:13.784 [pool-1-thread-19] INFO com.bingfa.concurrency.example.aqs.SemaphoreExample2 - 18 21:59:14.784 [pool-1-thread-20] INFO com.bingfa.concurrency.example.aqs.SemaphoreExample2 - 19
三、尝试等待获取一个许可
@Slf4j
public class SemaphoreExample3 {
private static int threadCount = 20;
public static void main(String[] args) throws InterruptedException {
ExecutorService exec = Executors.newCachedThreadPool();
final Semaphore semaphore = new Semaphore(3);
for(int i = 0; i < threadCount; i++){
final int threadNum = i;
exec.execute(()->{
try {
if(semaphore.tryAcquire(3, TimeUnit.SECONDS)) {//尝试获得一个许可
test(threadNum);
semaphore.release();//释放一个许可
}
} catch (InterruptedException e) {
e.printStackTrace();
}
});
}
exec.shutdown();
}
private static void test(int threadNum) throws InterruptedException {
log.info("{}", threadNum);
Thread.sleep(1000);
}
}
打印结果： 只执行了前面9个线程
