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:


        <dependency>
            <groupId>org.springframework.bootgroupId> <artifactId>spring-boot-starter-data-redisartifactId> dependency>  <dependency> <groupId>org.redissongroupId> <artifactId>redissonartifactId> <version>3.3.2version> dependency> 
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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 invokeAfterLockAquire() throws Exception;
}
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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  * @return 处理完具体的业务逻辑要返回的数据 * @throws UnableToAquireLockException * @throws Exception */  T lock(String resourceName, AquiredLockWorker worker) throws UnableToAquireLockException, Exception;  T lock(String resourceName, AquiredLockWorker worker, int lockTime) throws UnableToAquireLockException, Exception; }
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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); } } 
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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; } } 
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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 lock(String resourceName, AquiredLockWorker worker) throws InterruptedException, UnableToAquireLockException, Exception { return lock(resourceName, worker, 100); } @Override public  T lock(String resourceName, AquiredLockWorker 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(); } } 
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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() { @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(); } }  
   
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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)

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