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：

  @AutowiredRedisLocker 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)