J.U.C工具包之Semaphore

技术2020年8月4日

什么是Semaphore：

Semaphore俗称信号量，可以控制某个资源可被同时访问的线程个数。

通过acquire获取一个许可，如果没有就等待，一旦利用这个资源执行完业务逻辑之后，线程就会调用release方法释放一个许可出来。

例子：

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

public class SemaphoreDemo {
    public static void main(String[] args) {
        ExecutorService executorService = Executors.newCachedThreadPool();
        // 最多5个线程同时访问
        Semaphore semaphore = new Semaphore(5);
        for (int i = 0; i < 10; i++) {
            int num = i;
            Runnable task = new Runnable() {
                @Override
                public void run() {
                    try {
                        semaphore.acquire();
                        System.out.println("Thread:" + Thread.currentThread().getName() + "num is:" + num
                            + " starttime:" + System.currentTimeMillis());
                        Thread.sleep(5000);
                        semaphore.release();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            };
            executorService.execute(task);
        }
        executorService.shutdown();
    }

}

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.concurrent.Semaphore;

public class SemaphoreDemo {

public static void main(String[] args) {

ExecutorService executorService = Executors.newCachedThreadPool();

// 最多5个线程同时访问

Semaphore semaphore = new Semaphore(5);

for (int i = 0; i < 10; i++) {

int num = i;

Runnable task = new Runnable() {

@Override

public void run() {

try {

semaphore.acquire();

System.out.println("Thread:" + Thread.currentThread().getName() + "num is:" + num

+ " starttime:" + System.currentTimeMillis());

Thread.sleep(5000);

semaphore.release();

} catch (InterruptedException e) {

e.printStackTrace();

}

};

executorService.execute(task);

}

executorService.shutdown();

}

执行结果：

Thread:pool-1-thread-2num is:1 starttime:1596530701355
Thread:pool-1-thread-1num is:0 starttime:1596530701355
Thread:pool-1-thread-3num is:2 starttime:1596530701355
Thread:pool-1-thread-5num is:4 starttime:1596530701355
Thread:pool-1-thread-4num is:3 starttime:1596530701355
/**这里开始因为已经有5个线程同时访问了，所以要等待那5个执行完成释放后，后续的才能继续获取资源执行*/
Thread:pool-1-thread-6num is:5 starttime:1596530706357
Thread:pool-1-thread-10num is:9 starttime:1596530706357
Thread:pool-1-thread-9num is:8 starttime:1596530706357
Thread:pool-1-thread-7num is:6 starttime:1596530706357
Thread:pool-1-thread-8num is:7 starttime:1596530706357

Thread:pool-1-thread-2num is:1 starttime:1596530701355

Thread:pool-1-thread-1num is:0 starttime:1596530701355

Thread:pool-1-thread-3num is:2 starttime:1596530701355

Thread:pool-1-thread-5num is:4 starttime:1596530701355

Thread:pool-1-thread-4num is:3 starttime:1596530701355

/**这里开始因为已经有5个线程同时访问了，所以要等待那5个执行完成释放后，后续的才能继续获取资源执行*/

Thread:pool-1-thread-6num is:5 starttime:1596530706357

Thread:pool-1-thread-10num is:9 starttime:1596530706357

Thread:pool-1-thread-9num is:8 starttime:1596530706357

Thread:pool-1-thread-7num is:6 starttime:1596530706357

Thread:pool-1-thread-8num is:7 starttime:1596530706357

J.U.C工具包之Semaphore

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