再谈使用java实现分布式锁-redis篇

水深无声 2022-05-28 11:49 145阅读 0赞

当前网上可以找到许多基于redis使用java实现的分布式锁的代码，其主要实现方式主要有以下几种：

1.      SETNX、GETSET、GET、DEL

加锁时，使用SETNX设置锁名和锁的到期时间，若设置成功则获取锁；否则再检查锁是否已过期，是则使用GETSET设置新的到期时间，设置成功则获取到锁，获取到锁后记一下状态；解锁时，若锁已过期则直接解锁，否则根据状态判断是否由自己执有锁，是则解锁。

2.      SETNX、EXPIRE、GET、DEL

加锁时，使用SETNX设置锁名和锁的执有者，使用EXPIRE设置锁的过期时间；解锁时，先查询锁是否还由锁执有者执有，是则直接DEL解锁。

这两种实现方式在实际代码中都会各种各样的问题，上代码来分析：

import org.apache.logging.log4j.LogManager;
    import org.apache.logging.log4j.Logger;
    import redis.clients.jedis.Jedis;
    
    public class WrongARedisDLock implements DLock {
        private static final Logger LOG = LogManager.getLogger(WrongARedisDLock.class);
        private final String lockName;
        private final int lockDuration;
        private final String lockSubject;
        private final Jedis jedis;
    
        public WrongARedisDLock(String lockName, int lockDuration, String lockSubject, Jedis jedis) {
            this.lockName = lockName;
            this.lockDuration = lockDuration;
            this.lockSubject = lockSubject;
            this.jedis = jedis;
        }
        public void lock() {
            tryLock(Long.MAX_VALUE);
        }
        public boolean tryLock() {
            return tryLock(0);
        }
        public boolean tryLock(long waitTimeout) {
            long startTime = System.currentTimeMillis();
            while (true) {
                try {
                    // 设置锁名和锁申请主体的唯一标识
                    if (jedis.setnx(lockName, lockSubject) != null) {
                        // 若此时程序故障退出，锁将无法释放
                        jedis.expire(lockName, lockDuration);
                        return true;
                    }
                    String lockOwner = jedis.get(lockName);
                    if (lockSubject.equals(lockOwner)) {
                        return true;
                    }
                    if (lockOwner == null) {
                        continue;
                    }
                    if (System.currentTimeMillis() - startTime < waitTimeout) {
                        try {
                            Thread.sleep(100);
                        } catch (InterruptedException ie) {
                        }
                        continue;
                    }
                    return false;
                } catch (Exception e) {
                    LOG.error("tryLock error", e);
                    if (System.currentTimeMillis() - startTime < waitTimeout) {
                        try {
                            Thread.sleep(100);
                        } catch (InterruptedException ie) {
                        }
                        continue;
                    }
                    return false;
                }
            }
        }
    
        public void unlock() {
            try {
                String lockOwner = jedis.get(lockName);
                if (lockSubject.equals(lockOwner)) {
                    // 若此时锁过期了并且被其它申请者成功获取，则将误删锁
                    jedis.del(lockName);
                }
            } catch (Exception e) {
                LOG.error("unlock error", e);
            }
        }
    }

import org.apache.logging.log4j.LogManager;
    import org.apache.logging.log4j.Logger;
    import redis.clients.jedis.Jedis;
    
    public class WrongBRedisDLock implements DLock {
        private static final Logger LOG = LogManager.getLogger(WrongBRedisDLock.class);
        private final String lockName;
        private final long lockDuration;
        private final String lockSubject;
        private final Jedis jedis;
        private boolean isLocked = false;
    
        public WrongBRedisDLock(String lockName, long lockDuration, String lockSubject, Jedis jedis) {
            this.lockName = lockName;
            this.lockDuration = lockDuration;
            this.lockSubject = lockSubject;
            this.jedis = jedis;
        }
        public void lock() {
            tryLock(Long.MAX_VALUE);
        }
        public boolean tryLock() {
            return tryLock(0);
        }
        public boolean tryLock(long waitTimeout) {
            long loopStartTime = System.currentTimeMillis();
            while (true) {
                try {
                    long startTime = System.currentTimeMillis();
                    String expireTimeStr = String.valueOf(startTime + lockDuration + 1);
                    // 设置锁名和锁的到期时间，此处就存在多个不同进程或服务器当前时间可能会不一致的问题
                    if (jedis.setnx(lockName, expireTimeStr) != null) {
                        isLocked = true;
                        return true;
                    }
                    // 获取锁的过期时间
                    String oldExpireTimeStr = jedis.get(lockName);
                    if (oldExpireTimeStr == null) {
                        continue;
                    }
                    long oldExpireTime = Long.parseLong(oldExpireTimeStr);
                    if (startTime > oldExpireTime) {
                        // 在并发量较大时，可能会有多个锁申请主体同时进入到这里，并且都会修改锁的过期时间，
                        // 这样会造成锁的实际过期时间比锁执有者设置的时间靠后
                        oldExpireTimeStr = jedis.getSet(lockName, expireTimeStr);
                        if (oldExpireTimeStr != null) {
                            // 以下代码可以保证第一个申请成功的主体获取到锁
                            oldExpireTime = Long.parseLong(oldExpireTimeStr);
                            if (startTime > oldExpireTime) {
                                isLocked = true;
                                return true;
                            }
                        }
                    }
                    if (System.currentTimeMillis() - loopStartTime < waitTimeout) {
                        try {
                            Thread.sleep(100);
                        } catch (InterruptedException ie) {
                        }
                        continue;
                    }
                    return false;
                } catch (Exception e) {
                    LOG.error("tryLock error", e);
                    if (System.currentTimeMillis() - loopStartTime < waitTimeout) {
                        try {
                            Thread.sleep(100);
                        } catch (InterruptedException ie) {
                        }
                        continue;
                    }
                    return false;
                }
            }
        }
        public void unlock() {
            try {
                // 获取锁的到期时间
                String oldExpireTimeStr = jedis.get(lockName);
                if (oldExpireTimeStr != null) {
                    long oldExpireTime = Long.parseLong(oldExpireTimeStr);
                    if (oldExpireTime < System.currentTimeMillis()) {
                        // 若此时锁过期了并且被其它申请者成功获取，则将误删锁
                        jedis.del(lockName);
                    } else {
                        if (isLocked) {
                            // 若此时锁过期了并且被其它申请者成功获取，则将误删锁
                            jedis.del(lockName);
                        }
                    }
                }
                isLocked = false;
            } catch (Exception e) {
                LOG.error("unlock error", e);
            }
        }
    }

正确的实现方式如下：

import org.apache.logging.log4j.LogManager;
    import org.apache.logging.log4j.Logger;
    import redis.clients.jedis.Jedis;
    import redis.clients.jedis.Transaction;
    
    public class RedisDLock implements DLock {
        private static final Logger LOG = LogManager.getLogger(RedisDLock.class);
        private final String lockName;
        private final long lockDuration;
        private final String lockSubject;
        private final Jedis jedis;
    
        public RedisDLock(String lockName, long lockDuration, String lockSubject, Jedis jedis) {
            this.lockName = lockName;
            this.lockDuration = lockDuration;
            this.lockSubject = lockSubject;
            this.jedis = jedis;
        }
        public void lock() {
            tryLock(Long.MAX_VALUE);
        }
        public boolean tryLock() {
            return tryLock(0);
        }
        public boolean tryLock(long waitTimeout) {
            long startTime = System.currentTimeMillis();
            while (true) {
                try {
                    // 尝试加锁。等同于 SETNX + EXPIRE，通过设置一个属性值完成加锁过程
                    if (jedis.set(lockName, //  锁名
                            lockSubject,  //  锁申请人
                            "NX",           //  锁名不存在时插入
                            "PX",           //  锁的有效时长的时间单位为millisecond
                            lockDuration    //  锁的有效时长
                    ) != null) {
                        // 加锁成功
                        return true;
                    }
                    // 若加锁不成功，获取锁的拥有者。加锁不成功有可能是申请者已执有此锁，也可能是其他人执有此锁
                    String lockOwner = jedis.get(lockName);
                    // 若锁执有者与锁申请者相同，则返回申请者已占用此锁
                    if (lockSubject.equals(lockOwner)) {
                        return true;
                    }
                    // 若锁未被占用，则再次申请锁
                    if (lockOwner == null) {
                        continue;
                    }
                    if (System.currentTimeMillis() - startTime < waitTimeout) {
                        try {
                            Thread.sleep(100);
                        } catch (InterruptedException ie) {
                        }
                        continue;
                    }
                    // 返回锁已被其他申请主体占用
                    return false;
                } catch (Exception e) {
                    LOG.error("tryLock error", e);
                    if (System.currentTimeMillis() - startTime < waitTimeout) {
                        try {
                            Thread.sleep(100);
                        } catch (InterruptedException ie) {
                        }
                        continue;
                    }
                    return false;
                }
            }
        }
        // 使用transaction解锁
        public void unlock1() {
            try {
                jedis.watch(lockName); //  添加对锁的监控，若锁的相关属性发生变化，则整个事务将不执行
                String lockOwner = jedis.get(lockName);
                if (lockSubject.equals(lockOwner)) {
                    // 若锁未超时，则del操作会删除锁记录
                    // 若锁已超时，当前锁无执有者，则del操作不会造成影响
                    // 若锁已超时，当前锁被其他人执有，由于watch监控到锁的属性有变化，则事务中的del操作不会真正执行
                    Transaction tx = jedis.multi();
                    tx.del(lockName);
                    tx.exec();
                } else {
                    jedis.unwatch(); // 若不再执有锁，则放开监控
                }
            } catch (Exception e) {
                LOG.error("unlock error", e);
            }
        }
        // 使用lua script解锁
        public void unlock() {
            try {
                String script =
                "if redis.call(\"get\",KEYS[1]) == ARGV[1] then return redis.call(\"del\",KEYS[1]) else return 0 end";
                jedis.eval(script,Collections.singletonList(lockName),Collections.singletonList(lockSubject));
            } catch (Exception e) {
                LOG.error("unlock error", e);
            }
        }
    }