信号量是对锁的扩展，无论是synchronized还是ReetrantLock，一次都只允许一个线程访问一个资源，而信号量却可以指定多个线程，同时访问一个资源。
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import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
public class implements Runnable {
final Semaphore semp = new Semaphore(5);
public static void main(String[] args) throws Exception {
ExecutorService exec = Executors.newFixedThreadPool(20);
final Shit shit = new Shit();
for (int i = 0; i < 20; i++) {
exec.submit(shit);
}
}
public void run() {
try {
semp.acquire();
Thread.sleep(2000);
System.out.println(Thread.currentThread().getId() + ":done");
semp.release();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
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上面这个例子中，将会输出以5个线程为一组的提示文本。
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