分布式锁之三：Redlock实现分布式锁

之前写过一篇文章《如何在springcloud分布式系统中实现分布式锁？》，由于自己仅仅是阅读了相关的书籍，和查阅了相关的资料，就认为那样的是可行的。那篇文章实现的大概思路是用setNx命令和setEx配合使用。 setNx是一个耗时操作，因为它需要查询这个键是否存在，就算redis的百万的qps，在高并发的场景下，这种操作也是有问题的。关于redis实现分布式锁，redis官方推荐使用redlock。

一、redlock简介

在不同进程需要互斥地访问共享资源时，分布式锁是一种非常有用的技术手段。实现高效的分布式锁有三个属性需要考虑：

安全属性：互斥，不管什么时候，只有一个客户端持有锁
效率属性A:不会死锁
效率属性B：容错，只要大多数redis节点能够正常工作，客户端端都能获取和释放锁。

Redlock是redis官方提出的实现分布式锁管理器的算法。这个算法会比一般的普通方法更加安全可靠。关于这个算法的讨论可以看下官方文档。

二、怎么用java使用 redlock

在pom文件引入redis和redisson依赖：

<!-- redis-->

        <dependency>

            <groupId>org.springframework.boot</groupId>

            <artifactId>spring-boot-starter-data-redis</artifactId>

        </dependency>

        <!-- redisson-->

        <dependency>

            <groupId>org.redisson</groupId>

            <artifactId>redisson</artifactId>

            <version>3.3.2</version>

        </dependency>

AquiredLockWorker接口类，，主要是用于获取锁后需要处理的逻辑：

/**

 * Created by fangzhipeng on 2017/4/5.

 * 获取锁后需要处理的逻辑

 */

public interface AquiredLockWorker<T> {

     T invokeAfterLockAquire() throws Exception;

}

DistributedLocker 获取锁管理类：



/**

 * Created by fangzhipeng on 2017/4/5.

 * 获取锁管理类

 */

public interface DistributedLocker {

     /**

      * 获取锁

      * @param resourceName  锁的名称

      * @param worker 获取锁后的处理类

      * @param <T>

      * @return 处理完具体的业务逻辑要返回的数据

      * @throws UnableToAquireLockException

      * @throws Exception

      */

     <T> T lock(String resourceName, AquiredLockWorker<T> worker) throws UnableToAquireLockException, Exception;

     <T> T lock(String resourceName, AquiredLockWorker<T> worker, int lockTime) throws UnableToAquireLockException, Exception;

}

UnableToAquireLockException ，不能获取锁的异常类：

/**

 * Created by fangzhipeng on 2017/4/5.

 * 异常类

 */

public class UnableToAquireLockException extends RuntimeException {

    public UnableToAquireLockException() {

    }

    public UnableToAquireLockException(String message) {

        super(message);

    }

    public UnableToAquireLockException(String message, Throwable cause) {

        super(message, cause);

    }

}

RedissonConnector 连接类：

/**

 * Created by fangzhipeng on 2017/4/5.

 * 获取RedissonClient连接类

 */

@Component

public class RedissonConnector {

    RedissonClient redisson;

    @PostConstruct

    public void init(){

        redisson = Redisson.create();

    }

    public RedissonClient getClient(){

        return redisson;

    }

}

RedisLocker 类，实现了DistributedLocker：

import org.redisson.api.RLock;

import org.redisson.api.RedissonClient;

import org.springframework.beans.factory.annotation.Autowired;

import org.springframework.stereotype.Component;

import java.util.concurrent.TimeUnit;

/**

 * Created by fangzhipeng on 2017/4/5.

 */

@Component

public class RedisLocker  implements DistributedLocker{

    private final static String LOCKER_PREFIX = "lock:";

    @Autowired

    RedissonConnector redissonConnector;

    @Override

    public <T> T lock(String resourceName, AquiredLockWorker<T> worker) throws InterruptedException, UnableToAquireLockException, Exception {

        return lock(resourceName, worker, 100);

    }

    @Override

    public <T> T lock(String resourceName, AquiredLockWorker<T> worker, int lockTime) throws UnableToAquireLockException, Exception {

        RedissonClient redisson= redissonConnector.getClient();

        RLock lock = redisson.getLock(LOCKER_PREFIX + resourceName);

      // Wait for 100 seconds seconds and automatically unlock it after lockTime seconds

        boolean success = lock.tryLock(100, lockTime, TimeUnit.SECONDS);

        if (success) {

            try {

                return worker.invokeAfterLockAquire();

            } finally {

                lock.unlock();

            }

        }

        throw new UnableToAquireLockException();

    }

}

测试类：

  @Autowired

    RedisLocker distributedLocker;

    @RequestMapping(value = "/redlock")

    public String testRedlock() throws Exception{

        CountDownLatch startSignal = new CountDownLatch(1);

        CountDownLatch doneSignal = new CountDownLatch(5);

        for (int i = 0; i < 5; ++i) { // create and start threads

            new Thread(new Worker(startSignal, doneSignal)).start();

        }

        startSignal.countDown(); // let all threads proceed

        doneSignal.await();

        System.out.println("All processors done. Shutdown connection");

        return "redlock";

    }

     class Worker implements Runnable {

        private final CountDownLatch startSignal;

        private final CountDownLatch doneSignal;

        Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {

            this.startSignal = startSignal;

            this.doneSignal = doneSignal;

        }

        public void run() {

            try {

                startSignal.await();

                distributedLocker.lock("test",new AquiredLockWorker<Object>() {

                    @Override

                    public Object invokeAfterLockAquire() {

                        doTask();

                        return null;

                    }

                });

            }catch (Exception e){

            }

        }

        void doTask() {

            System.out.println(Thread.currentThread().getName() + " start");

            Random random = new Random();

            int _int = random.nextInt(200);

            System.out.println(Thread.currentThread().getName() + " sleep " + _int + "millis");

            try {

                Thread.sleep(_int);

            } catch (InterruptedException e) {

                e.printStackTrace();

            }

            System.out.println(Thread.currentThread().getName() + " end");

            doneSignal.countDown();

        }

    }

运行测试类：

Thread-48 start
Thread-48 sleep 99millis
Thread-48 end
Thread-49 start
Thread-49 sleep 118millis
Thread-49 end
Thread-52 start
Thread-52 sleep 141millis
Thread-52 end
Thread-50 start
Thread-50 sleep 28millis
Thread-50 end
Thread-51 start
Thread-51 sleep 145millis
Thread-51 end

从运行结果上看，在异步任务的情况下，确实是获取锁之后才能运行线程。不管怎么样，这是redis官方推荐的一种方案，可靠性比较高。有什么问题欢迎留言。

三、参考资料

https://github.com/redisson/redisson

《Redis官方文档》用Redis构建分布式锁

A Look at the Java Distributed In-Memory Data Model (Powered by Redis)