概述
控制同一时间并发线程的数目。能够完成对于信号量的控制，可以控制某个资源可被同时访问的个数。提供了两个核心方法——acquire()方法和release()方法。acquire()方法表示获取一个许可，如果没有则等待，release()方法则是在操作完成后释放对应的许可。Semaphore维护了当前访问的个数，通过提供同步机制来控制同时访问的个数。Semaphore可以实现有限大小的链表。
使用场景
Semaphore常用于仅能提供有限访问的资源，比如：数据库连接数。
代码示例
每次获取并释放一个许可，示例代码如下：
代码语言：javascript复制package io.binghe.concurrency.example.aqs;
import lombok.extern.slf4j.Slf4j;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
@Slf4j
public class SemaphoreExample {
private static final int threadCount = 200;
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(); //获取一个许可
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);
}
}每次获取并释放多个许可，示例代码如下:
代码语言：javascript复制package io.binghe.concurrency.example.aqs;
import lombok.extern.slf4j.Slf4j;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
@Slf4j
public class SemaphoreExample {
private static final int threadCount = 200;
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); //获取多个许可
test(threadNum);
semaphore.release(3); //释放多个许可
} catch (InterruptedException e) {
e.printStackTrace();
}
});
}
log.info("finish");
exec.shutdown();
}
private static void test(int threadNum) throws InterruptedException {
log.info("{}", threadNum);
Thread.sleep(1000);
}
}假设有这样一个场景，并发太高了，即使使用Semaphore进行控制，处理起来也比较棘手。假设系统当前允许的最高并发数是3，超过3后就需要丢弃，使用Semaphore也能实现这样的场景，示例代码如下：
代码语言：javascript复制package io.binghe.concurrency.example.aqs;
import lombok.extern.slf4j.Slf4j;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
@Slf4j
public class SemaphoreExample {
private static final int threadCount = 200;
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 {
//尝试获取一个许可，也可以尝试获取多个许可，
//支持尝试获取许可超时设置，超时后不再等待后续线程的执行
//具体可以参见Semaphore的源码
if (semaphore.tryAcquire()) {
test(threadNum);
semaphore.release(); //释放一个许可
}
} catch (InterruptedException e) {
e.printStackTrace();
}
});
}
log.info("finish");
exec.shutdown();
}
private static void test(int threadNum) throws InterruptedException {
log.info("{}", threadNum);
Thread.sleep(1000);
}
}
