浅谈redis实现的分布式锁

/**
 * Created by BingZhong on 2017/7/29.
 *
 * 基于Redis实现的分布式锁
 */
public final class RedisLockHelper {

    private static Logger logger = LoggerFactory.getLogger(RedisLockHelper.class);

    /**
     * redis操作帮助类,可以是其他封装了redis操作的类
     */
    private RedisHelper redisHelper;

    public static final long DEFAULT_TIMEOUT = 30 * 1000;

    public static final long DEFAULT_SLEEP_TIME = 100;

    private RedisLockHelper(RedisHelper redisHelper) {
        this.redisHelper = redisHelper;
    }

    public static RedisLockHelper getInstance(RedisHelper redisHelper) {
        return new RedisLockHelper(redisHelper);
    }

    /**
     * 创建锁
     *
     * @param mutex     互斥量
     * @param timeout   锁的超时时间
     * @param sleepTime 线程自旋尝试获取锁时的休眠时间
     * @param timeUnit  时间单位
     */
    public RedisLock newLock(String mutex, long timeout, long sleepTime, TimeUnit timeUnit) {
        logger.info("创建分布式锁,互斥量为{}", mutex);
        return new RedisLock(mutex, timeout, sleepTime, timeUnit);
    }

    public RedisLock newLock(String mutex, long timeout, TimeUnit timeUnit) {
        return newLock(mutex, timeout, DEFAULT_SLEEP_TIME, timeUnit);
    }

    public RedisLock newLock(String mutex) {
        return newLock(mutex, DEFAULT_TIMEOUT, TimeUnit.MILLISECONDS);
    }

    public class RedisLock {
        /**
         * 用于创建redis健值对的键，相当于互斥量
         */
        private final String mutex;

        /**
         * 锁过期的绝对时间
         */
        private volatile long lockExpiresTime = 0;

        /**
         * 锁的超时时间
         */
        private final long timeout;

        /**
         * 每次循环获取锁的休眠时间
         */
        private final long sleepTime;

        /**
         * 锁的线程持有者
         */
        private volatile Thread lockHolder = null;

        private final ReentrantLock threadLock = new ReentrantLock();

        public RedisLock(String mutex, long timeout, long sleepTime, TimeUnit timeUnit) {
            this.mutex = mutex;
            this.timeout = timeUnit.toMillis(timeout);
            this.sleepTime = timeUnit.toMillis(sleepTime);
        }

        /**
         * 加锁,将会一直尝试获取锁,直到超时
         */
        public boolean lock(long acquireTimeout, TimeUnit timeUnit) throws InterruptedException {
            acquireTimeout = timeUnit.toMillis(acquireTimeout);
            long acquireTime = acquireTimeout + System.currentTimeMillis();
            threadLock.tryLock(acquireTimeout, timeUnit);
            try {
                while (true) {
                    boolean hasLock = tryLock();
                    if (hasLock) {
                        //获取锁成功
                        return true;
                    } else if (acquireTime < System.currentTimeMillis()) {
                        break;
                    }
                    Thread.sleep(sleepTime);
                }
            } finally {
                if (threadLock.isHeldByCurrentThread()) {
                    threadLock.unlock();
                }
            }

            return false;
        }

        /**
         * 尝试获取锁,无论是否获取到锁都将直接返回而不会阻塞
         * 不支持重入锁
         */
        public boolean tryLock() {
            if (lockHolder == Thread.currentThread()) {
                throw new IllegalMonitorStateException("不支持重入锁");
            }
            long currentTime = System.currentTimeMillis();
            String expires = String.valueOf(timeout + currentTime);
            //尝试设置互斥量
            if (redisHelper.setNx(mutex, expires) > 0) {
                setLockStatus(expires);
                return true;
            } else {
                String currentLockTime = redisHelper.get(mutex);
                //检查锁是否超时
                if (Objects.nonNull(currentLockTime) && Long.parseLong(currentLockTime) < currentTime) {
                    //获取旧的锁时间并设置互斥量
                    String oldLockTime = redisHelper.getSet(mutex, expires);
                    //判断获取到的旧值是否一致,不一致证明已经有另外的进程(线程)成功获取到了锁
                    if (Objects.nonNull(oldLockTime) && Objects.equals(oldLockTime, currentLockTime)) {
                        setLockStatus(expires);
                        return true;
                    }
                }

                return false;
            }
        }

        /**
         * 该锁是否被锁住
         */
        public boolean isLock() {
            String currentLockTime = redisHelper.get(mutex);
            //存在互斥量且锁还为过时即锁住
            return Objects.nonNull(currentLockTime) && Long.parseLong(currentLockTime) > System.currentTimeMillis();
        }

        public String getMutex() {
            return mutex;
        }

        /**
         * 解锁
         */
        public boolean unlock() {
            //只有锁的持有线程才能解锁
            if (lockHolder == Thread.currentThread()) {
                //判断锁是否超时，没有超时才将互斥量删除
                if (lockExpiresTime > System.currentTimeMillis()) {
                    redisHelper.del(mutex);
                    logger.info("删除互斥量[{}]", mutex);
                }
                lockHolder = null;
                logger.info("释放[{}]锁成功", mutex);

                return true;
            } else {
                throw new IllegalMonitorStateException("没有获取到锁的线程无法执行解锁操作");
            }
        }

        private void setLockStatus(String expires) {
            lockExpiresTime = Long.parseLong(expires);
            lockHolder = Thread.currentThread();
            logger.info("获取[{}]锁成功", mutex);
        }
    }
}