Semaphore

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;

public class Phonebooth {
	public static final int N = 5;

	public static void main(String[] args) {
		Semaphore semaphore = new Semaphore(N);
		ExecutorService service = Executors.newCachedThreadPool();
		for (int i = 0; i < 2 * N; i++) {
			service.execute(new Person(semaphore));
		}
		service.shutdown();
	}
}

import java.util.Random;
import java.util.concurrent.Semaphore;
import java.util.concurrent.TimeUnit;

public class Person implements Runnable {
	private Semaphore semaphore;

	public Person(Semaphore semaphore) {
		this.semaphore = semaphore;
	}

	public void run() {
		String name = Thread.currentThread().getName();
		try {
			System.out.println(name + " 等待进入电话亭...");
			semaphore.acquire();
			System.out.println(name + " 进入电话亭通话中...");
			TimeUnit.SECONDS.sleep(new Random().nextInt(10));
			System.out.println(name + " 出电话亭...");
			semaphore.release();
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}
}

Console
pool-1-thread-1 等待进入电话亭...
pool-1-thread-1 进入电话亭通话中...
pool-1-thread-3 等待进入电话亭...
pool-1-thread-3 进入电话亭通话中...
pool-1-thread-3 出电话亭...
pool-1-thread-5 等待进入电话亭...
pool-1-thread-2 等待进入电话亭...
pool-1-thread-2 进入电话亭通话中...
pool-1-thread-5 进入电话亭通话中...
pool-1-thread-4 等待进入电话亭...
pool-1-thread-4 进入电话亭通话中...
pool-1-thread-5 出电话亭...
pool-1-thread-6 等待进入电话亭...
pool-1-thread-6 进入电话亭通话中...
pool-1-thread-8 等待进入电话亭...
pool-1-thread-8 进入电话亭通话中...
pool-1-thread-10 等待进入电话亭...
pool-1-thread-7 等待进入电话亭...
pool-1-thread-9 等待进入电话亭...
pool-1-thread-1 出电话亭...
pool-1-thread-10 进入电话亭通话中...
pool-1-thread-2 出电话亭...
pool-1-thread-7 进入电话亭通话中...
pool-1-thread-6 出电话亭...
pool-1-thread-9 进入电话亭通话中...
pool-1-thread-7 出电话亭...
pool-1-thread-4 出电话亭...
pool-1-thread-10 出电话亭...
pool-1-thread-9 出电话亭...
pool-1-thread-8 出电话亭...