Example analysis of the implementation principle of redis distributed lock

First of all, in order to ensure that distributed locks are available, we must at least ensure that the lock implementation meets the following four conditions at the same time:

1. Mutual exclusivity. At any time, only one client can hold the lock.

2. No deadlock will occur. Even if a client crashes while holding the lock without actively unlocking it, it is guaranteed that other clients can subsequently lock it.

3. Fault-tolerant. As long as most Redis nodes are running normally, the client can lock and unlock.

4. To untie the bell, you must also tie the bell. The locking and unlocking must be done by the same client. The client itself cannot unlock the lock added by others.

The following is the code implementation. First, we need to introduce the Jedis open source component through Maven, and add the following code to the pom.xml file:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-data-redis</artifactId>
</dependency>
<dependency>
    <groupId>redis.clients</groupId>
    <artifactId>jedis</artifactId>
    <version>3.1.0</version>
</dependency>

Distributed lock implementation code, DistributedLock.java

import redis.clients.jedis.Jedis;
import redis.clients.jedis.JedisPool;
import redis.clients.jedis.JedisPoolConfig;
import redis.clients.jedis.Transaction;
import redis.clients.jedis.exceptions.JedisException;
import java.util.List;
import java.util.UUID;
/**
 * @author swadian
 * @date 2022/3/4
 * @Version 1.0
 * @describetion Redis分布式锁原理
 */
public class DistributedLock {
    //redis连接池
    private static JedisPool jedisPool;
    static {
        JedisPoolConfig config = new JedisPoolConfig();
        // 设置最大连接数
        config.setMaxTotal(200);
        // 设置最大空闲数
        config.setMaxIdle(8);
        // 设置最大等待时间
        config.setMaxWaitMillis(1000 * 100);
        // 在borrow一个jedis实例时，是否需要验证，若为true，则所有jedis实例均是可用的
        config.setTestOnBorrow(true);
        jedisPool = new JedisPool(config, "192.168.3.27", 6379, 3000);
    }
    /**
     * 加锁
     * @param lockName       锁的key
     * @param acquireTimeout 获取锁的超时时间
     * @param timeout        锁的超时时间
     * @return 锁标识
     * Redis Setnx（SET if Not eXists） 命令在指定的 key 不存在时，为 key 设置指定的值。
     * 设置成功，返回 1 。 设置失败，返回 0 。
     */
    public String lockWithTimeout(String lockName, long acquireTimeout, long timeout) {
        Jedis jedis = null;
        String retIdentifier = null;
        try {
            // 获取连接
            jedis = jedisPool.getResource();
            // value值->随机生成一个String
            String identifier = UUID.randomUUID().toString();
            // key值->即锁名
            String lockKey = "lock:" + lockName;
            // 超时时间->上锁后超过此时间则自动释放锁 毫秒转成->秒
            int lockExpire = (int) (timeout / 1000);
            // 获取锁的超时时间->超过这个时间则放弃获取锁
            long end = System.currentTimeMillis() + acquireTimeout;
            while (System.currentTimeMillis() < end) { //在获取锁时间内
                if (jedis.setnx(lockKey, identifier) == 1) {//关键:设置锁
                    jedis.expire(lockKey, lockExpire);
                    // 返回value值，用于释放锁时间确认
                    retIdentifier = identifier;
                    return retIdentifier;
                }
                // ttl以秒为单位返回 key 的剩余过期时间，返回-1代表key没有设置超时时间，为key设置一个超时时间
                if (jedis.ttl(lockKey) == -1) {
                    jedis.expire(lockKey, lockExpire);
                }
                try {
                    Thread.sleep(10);
                } catch (InterruptedException e) {
                    Thread.currentThread().interrupt();
                }
            }
        } catch (JedisException e) {
            e.printStackTrace();
        } finally {
            if (jedis != null) {
                jedis.close();
            }
        }
        return retIdentifier;
    }
    /**
     * 释放锁
     * @param lockName   锁的key
     * @param identifier 释放锁的标识
     * @return
     */
    public boolean releaseLock(String lockName, String identifier) {
        Jedis jedis = null;
        String lockKey = "lock:" + lockName;
        boolean retFlag = false;
        try {
            jedis = jedisPool.getResource();
            while (true) {
                // 监视lock，准备开始redis事务
                jedis.watch(lockKey);
                // 通过前面返回的value值判断是不是该锁，若是该锁，则删除，释放锁
                if (identifier.equals(jedis.get(lockKey))) {
                    Transaction transaction = jedis.multi();//开启redis事务
                    transaction.del(lockKey);
                    List<Object> results = transaction.exec();//提交redis事务
                    if (results == null) {//提交失败
                        continue;//继续循环
                    }
                    retFlag = true;//提交成功
                }
                jedis.unwatch();//解除监控
                break;
            }
        } catch (JedisException e) {
            e.printStackTrace();
        } finally {
            if (jedis != null) {
                jedis.close();
            }
        }
        return retFlag;
    }
}

In order to verify it, we create the SkillService.java business class

import lombok.extern.slf4j.Slf4j;
@Slf4j
public class SkillService {
    final DistributedLock lock = new DistributedLock();
    public static final String LOCK_KEY = "lock_resource";
    int n = 500;
    /**
     * 线程业务方法
     */
    public void seckill() {
        // 返回锁的value值，供释放锁时候进行判断
        String identifier = lock.lockWithTimeout(LOCK_KEY, 5000, 1000);
        log.info("线程:"+Thread.currentThread().getName() + "获得了锁");
        log.info("剩余数量:{}",--n);
        lock.releaseLock(LOCK_KEY, identifier);
    }
}

If the @Slf4j log cannot be found, inpom.xmlAdd the following code to the file:

<!--@Slf4j日志依赖组件-->
  <dependency>
      <groupId>org.projectlombok</groupId>
      <artifactId>lombok</artifactId>
</dependency>

Edit a test class TestLock.java

/**
 * @author swadian
 * @date 2022/3/4
 * @Version 1.0
 */
public class TestLock {
    public static void main(String[] args) {
        SkillService service = new SkillService();
        for (int i = 10; i < 60; i++) { //开50个线程
            SkillThread skillThread = new SkillThread(service, "skillThread->" + i);
            skillThread.start();
        }
    }
}
class SkillThread extends Thread {
    private SkillService skillService;
    public SkillThread(SkillService skillService, String skillThreadName) {
        super(skillThreadName);
        this.skillService = skillService;
    }
    @Override
    public void run() {
        skillService.seckill();
    }
}

The test results show that the remaining numbers after locking are all sequential and serial, 499,498,497...

We modify the SkillService.java business class and comment out the locking logic

@Slf4j
public class SkillService {
    final DistributedLock lock = new DistributedLock();
    public static final String LOCK_KEY = "lock_resource";
    int n = 500;
    /**
     * 线程业务方法
     */
    public void seckill() {
        // 返回锁的value值，供释放锁时候进行判断
        //String identifier = lock.lockWithTimeout(LOCK_KEY, 5000, 1000);
        log.info("线程:"+Thread.currentThread().getName() + "获得了锁");
        log.info("剩余数量:{}",--n);
        //lock.releaseLock(LOCK_KEY, identifier);
    }
}

Re-execute the test and after commenting out the locking logic, the remaining quantities are all messed up In sequence, 472,454,452...

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