如何使用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:

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<!-- 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:

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/**
* Created by chris 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:

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/**
* Created by chris 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:

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/**
* Created by chris 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:

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/**
* Created by chris 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:

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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 chris 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:

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@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)

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