基于redis的分布式锁二种应用场景

“分布式锁”是用来解决分布式应用中“并发冲突”的一种常用手段，实现方式一般有基于zookeeper及基于redis二种。具体到业务场景中，我们要考虑二种情况：

一、抢不到锁的请求，允许丢弃(即：忽略）

比如：一些不是很重要的场景，比如“监控数据持续上报”，某一篇文章的“已读/未读”标识位更新，对于同一个id，如果并发的请求同时到达，只要有一个请求处理成功，就算成功。

用活动图表示如下：

点击查看原图

二、并发请求，不论哪一条都必须要处理的场景(即：不允许丢数据)

比如：一个订单，客户正在前台修改地址，管理员在后台同时修改备注。地址和备注字段的修改，都必须正确更新，这二个请求同时到达的话，如果不借助db的事务，很容易造成行锁竞争，但用事务的话，db的性能显然比不上redis轻量。

解决思路：A,B二个请求，谁先抢到分布式锁（假设A先抢到锁），谁先处理，抢不到的那个（即：B），在一旁不停等待重试，重试期间一旦发现获取锁成功，即表示A已经处理完，把锁释放了。这时B就可以继续处理了。

但有二点要注意：

a、需要设置等待重试的最长时间，否则如果A处理过程中有bug，一直卡死，或者未能正确释放锁，B就一直会等待重试，但是又永远拿不到锁。

b、等待最长时间，必须小于锁的过期时间。否则，假设锁2秒过期自动释放，但是A还没处理完（即：A的处理时间大于2秒），这时锁会因为redis key过期“提前”误释放，B重试时拿到锁，造成A,B同时处理。（注：可能有同学会说，不设置锁的过期时间，不就完了么？理论上讲，确实可以这么做，但是如果业务代码有bug，导致处理完后没有unlock，或者根本忘记了unlock，分布式锁就会一直无法释放。所以综合考虑，给分布式锁加一个“保底”的过期时间，让其始终有机会自动释放，更为靠谱）

用活动图表示如下：

点击查看原图

写了一个简单的工具类：

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package com.cnblogs.yjmyzz.redisdistributionlock;
 
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.data.redis.core.StringRedisTemplate;
import org.springframework.util.StringUtils;
 
import java.util.UUID;
import java.util.concurrent.TimeUnit;
 
/**
 * 利用redis获取分布式锁
 *
 * @author 菩提树下的杨过
 * @blog http://yjmyzz.cnblogs.com/
 */
public class RedisLock {
 
    private StringRedisTemplate redisTemplate;
 
    private Logger logger = LoggerFactory.getLogger(this.getClass());
 
    /**
     * simple lock尝试获取锅的次数
     */
    private int retryCount = 3;
 
    /**
     * 每次尝试获取锁的重试间隔毫秒数
     */
    private int waitIntervalInMS = 100;
 
 
    public RedisLock(StringRedisTemplate redisTemplate) {
        this.redisTemplate = redisTemplate;
    }
 
    /**
     * 利用redis获取分布式锁(未获取锁的请求，允许丢弃!)
     *
     * @param redisKey       锁的key值
     * @param expireInSecond 锁的自动释放时间(秒)
     * @return
     * @throws DistributionLockException
     */
    public String simpleLock(final String redisKey, final int expireInSecond) throws DistributionLockException {
        String lockValue = UUID.randomUUID().toString();
        boolean flag = false;
        if (StringUtils.isEmpty(redisKey)) {
            throw new DistributionLockException("key is empty!");
        }
        if (expireInSecond <= 0) {
            throw new DistributionLockException("expireInSecond must be bigger than 0");
        }
        try {
            for (int i = 0; i < retryCount; i++) {
                boolean success = redisTemplate.opsForValue().setIfAbsent(redisKey, lockValue, expireInSecond, TimeUnit.SECONDS);
                if (success) {
                    flag = true;
                    break;
                }
                try {
                    TimeUnit.MILLISECONDS.sleep(waitIntervalInMS);
                } catch (Exception ignore) {
                    logger.warn("redis lock fail: " + ignore.getMessage());
 
                }
            }
            if (!flag) {
                throw new DistributionLockException(Thread.currentThread().getName() + " cannot acquire lock now ...");
            }
            return lockValue;
        } catch (DistributionLockException be) {
            throw be;
        } catch (Exception e) {
            logger.warn("get redis lock error, exception: " + e.getMessage());
            throw e;
        }
    }
 
    /**
     * 利用redis获取分布式锁(未获取锁的请求，将在timeoutSecond时间范围内，一直等待重试)
     *
     * @param redisKey       锁的key值
     * @param expireInSecond 锁的自动释放时间(秒)
     * @param timeoutSecond  未获取到锁的请求，尝试重试的最久等待时间(秒)
     * @return
     * @throws DistributionLockException
     */
    public String lock(final String redisKey, final int expireInSecond, final int timeoutSecond) throws DistributionLockException {
        String lockValue = UUID.randomUUID().toString();
        boolean flag = false;
        if (StringUtils.isEmpty(redisKey)) {
            throw new DistributionLockException("key is empty!");
        }
        if (expireInSecond <= 0) {
            throw new DistributionLockException("expireInSecond must be greater than 0");
        }
        if (timeoutSecond <= 0) {
            throw new DistributionLockException("timeoutSecond must be greater than 0");
        }
        if (timeoutSecond >= expireInSecond) {
            throw new DistributionLockException("timeoutSecond must be less than expireInSecond");
        }
        try {
            long timeoutAt = System.currentTimeMillis() + timeoutSecond * 1000;
            while (true) {
                boolean success = redisTemplate.opsForValue().setIfAbsent(redisKey, lockValue, expireInSecond, TimeUnit.SECONDS);
                if (success) {
                    flag = true;
                    break;
                }
                if (System.currentTimeMillis() >= timeoutAt) {
                    break;
                }
                try {
                    TimeUnit.MILLISECONDS.sleep(waitIntervalInMS);
                } catch (Exception ignore) {
                    logger.warn("redis lock fail: " + ignore.getMessage());
                }
            }
            if (!flag) {
                throw new DistributionLockException(Thread.currentThread().getName() + " cannot acquire lock now ...");
            }
            return lockValue;
        } catch (DistributionLockException be) {
            throw be;
        } catch (Exception e) {
            logger.warn("get redis lock error, exception: " + e.getMessage());
            throw e;
        }
    }
 
 
    /**
     * 锁释放
     *
     * @param redisKey
     * @param lockValue
     */
    public void unlock(final String redisKey, final String lockValue) {
        if (StringUtils.isEmpty(redisKey)) {
            return;
        }
        if (StringUtils.isEmpty(lockValue)) {
            return;
        }
        try {
            String currLockVal = redisTemplate.opsForValue().get(redisKey);
            if (currLockVal != null && currLockVal.equals(lockValue)) {
                boolean result = redisTemplate.delete(redisKey);
                if (!result) {
                    logger.warn(Thread.currentThread().getName() + " unlock redis lock fail");
                } else {
                    logger.info(Thread.currentThread().getName() + " unlock redis lock:" + redisKey + " successfully!");
                }
            }
        } catch (Exception je) {
            logger.warn(Thread.currentThread().getName() + " unlock redis lock error:" + je.getMessage());
        }
    }
}

然后写个spring-boot来测试一下：

package com.cnblogs.yjmyzz.redisdistributionlock;
 
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.data.redis.core.StringRedisTemplate;
 
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
 
@SpringBootApplication
public class RedisDistributionLockApplication {
 
    private static Logger logger = LoggerFactory.getLogger(RedisDistributionLockApplication.class);
 
    public static void main(String[] args) throws InterruptedException {
        ConfigurableApplicationContext applicationContext = SpringApplication.run(RedisDistributionLockApplication.class, args);
 
        //初始化
        StringRedisTemplate redisTemplate = applicationContext.getBean(StringRedisTemplate.class);
        RedisLock redisLock = new RedisLock(redisTemplate);
        String lockKey = "lock:test";
 
 
        CountDownLatch start = new CountDownLatch(1);
        CountDownLatch threadsLatch = new CountDownLatch(2);
 
        final int lockExpireSecond = 5;
        final int timeoutSecond = 3;
 
        Runnable lockRunnable = () -> {
            String lockValue = "";
            try {
                //等待发令枪响，防止线程抢跑
                start.await();
 
                //允许丢数据的简单锁示例
                lockValue = redisLock.simpleLock(lockKey, lockExpireSecond);
 
 
                //不允许丢数据的分布式锁示例
                //lockValue = redisLock.lock(lockKey, lockExpireSecond, timeoutSecond);
 
                //停一会儿，故意让后面的线程抢不到锁
                TimeUnit.SECONDS.sleep(2);
                logger.info(String.format("%s get lock successfully, value:%s", Thread.currentThread().getName(), lockValue));
 
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                redisLock.unlock(lockKey, lockValue);
                //执行完后，计数减1
                threadsLatch.countDown();
            }
 
        };
 
        Thread t1 = new Thread(lockRunnable, "T1");
        Thread t2 = new Thread(lockRunnable, "T2");
 
        t1.start();
        t2.start();
 
        //预备：开始！
        start.countDown();
 
        //等待所有线程跑完
        threadsLatch.await();
 
        logger.info("======>done!!!");
 
    }
 
}