概述
信号量，用来限制能同时访问共享资源的线程上限。
public static void main(String[] args) {
// 1. 创建 semaphore 对象
Semaphore semaphore = new Semaphore(3);
// 2. 10个线程同时运行
for (int i = 0; i < 10; i++) {
new Thread(() -> {
// 3. 获取许可
try {
semaphore.acquire();
} catch (InterruptedException e) {
e.printStackTrace();
}
try {
log.debug("running...");
sleep(1);
log.debug("end...");
} finally {
// 4. 释放许可
semaphore.release();
}
}).start();
}
}
输出：
07:35:15.485 c.TestSemaphore [Thread-2] - running...
07:35:15.485 c.TestSemaphore [Thread-1] - running...
07:35:15.485 c.TestSemaphore [Thread-0] - running...
07:35:16.490 c.TestSemaphore [Thread-2] - end...
07:35:16.490 c.TestSemaphore [Thread-0] - end...
07:35:16.490 c.TestSemaphore [Thread-1] - end...
07:35:16.490 c.TestSemaphore [Thread-3] - running...
07:35:16.490 c.TestSemaphore [Thread-5] - running...
07:35:16.490 c.TestSemaphore [Thread-4] - running...
07:35:17.490 c.TestSemaphore [Thread-5] - end...
07:35:17.490 c.TestSemaphore [Thread-4] - end...
07:35:17.490 c.TestSemaphore [Thread-3] - end...
07:35:17.490 c.TestSemaphore [Thread-6] - running...
07:35:17.490 c.TestSemaphore [Thread-7] - running...
07:35:17.490 c.TestSemaphore [Thread-9] - running...
07:35:18.491 c.TestSemaphore [Thread-6] - end...
07:35:18.491 c.TestSemaphore [Thread-7] - end...
07:35:18.491 c.TestSemaphore [Thread-9] - end...
07:35:18.491 c.TestSemaphore [Thread-8] - running...
07:35:19.492 c.TestSemaphore [Thread-8] - end...
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）
- 用 Semaphore 实现简单连接池，线程数和数据库连接数是相等的
class Pool {
// 1. 连接池大小
private final int poolSize;
// 2. 连接对象数组
private Connection[] connections;
// 3. 连接状态数组 0 表示空闲， 1 表示繁忙
private AtomicIntegerArray states;
private Semaphore semaphore;
// 4. 构造方法初始化
public Pool(int poolSize) {
this.poolSize = poolSize;
// 让许可数与资源数一致
this.semaphore = new Semaphore(poolSize);
this.connections = new Connection[poolSize];
this.states = new AtomicIntegerArray(new int[poolSize]);
for (int i = 0; i < poolSize; i++) {
connections[i] = new MockConnection("连接" + (i+1));
}
}
// 5. 借连接
public Connection borrow() {// t1, t2, t3
// 获取许可
try {
semaphore.acquire(); // 没有许可的线程，在此等待
} catch (InterruptedException e) {
e.printStackTrace();
}
for (int i = 0; i < poolSize; i++) {
// 获取空闲连接
if(states.get(i) == 0) {
if (states.compareAndSet(i, 0, 1)) {
log.debug("borrow {}", connections[i]);
return connections[i];
}
}
}
// 不会执行到这里
return null;
}
// 6. 归还连接
public void free(Connection conn) {
for (int i = 0; i < poolSize; i++) {
if (connections[i] == conn) {
states.set(i, 0);
log.debug("free {}", conn);
semaphore.release();
break;
}
}
}
}
原理
AQS使用共享模式，state代表资源数，state大于0才能获得锁，否则进入等待队列。
原创文章，作者：彭晨涛，如若转载，请注明出处：https://www.codetool.top/article/juc%e5%8c%85%e4%b8%8b%e7%9a%84%e4%bf%a1%e5%8f%b7%e9%87%8fsemaphore/
