浅析Redis分布式锁

近期工作遇到需要业务场景如下,需要每天定时推送给另一系统一批数据,但是由于系统是集群部署的,会造成统一情况下任务争用的情况,所以需要增加分布式锁来保证一定时间范围内有一个Job来完成定时任务. 前期考虑的方案有采用ZooKeeper分布式任务,Quartz分布式任务调度,但是由于Zookeeper需要增加额外组件,Quartz需要增加表,并且项目中现在已经有Redis这一组件存在,所以考虑采用Redis分布式锁的情况来完成分布式任务抢占这一功能

记录一下走过的弯路.

第一版本:

@Override
	public <T> Long set(String key,T value, Long cacheSeconds) {
		if (value instanceof HashMap) {
			BoundHashOperations valueOperations = redisTemplate.boundHashOps(key);
			valueOperations.putAll((Map) value);
			valueOperations.expire(cacheSeconds, TimeUnit.SECONDS);
		}
		else{
		//使用map存储
		BoundHashOperations valueOperations = redisTemplate.boundHashOps(key);
		valueOperations.put(key, value);
		//秒
		valueOperations.expire(cacheSeconds, TimeUnit.SECONDS);
		}
		return null;
	}

	@Override
	public void del(String key) {
		redisTemplate.delete(key);
	}

采用set 和 del 完成锁的占用与释放,后经测试得知,set不是线程安全,在并发情况下常常会导致数据不一致.

第二版本:

/**
   * 分布式锁
   * @param range 锁的长度 允许有多少个请求抢占资源
   * @param key
   * @return
   */
  public boolean getLock(int range, String key) {
    ValueOperations<String, Integer> valueOper1 = template.opsForValue();
    return valueOper1.increment(key, 1) <= range;
  }

  /**
   * 初始化锁, 设置等于0
   * @param key
   * @param expireSeconds
   * @return
   */
  public void initLock(String key, Long expireSeconds) {
    ValueOperations<String, Integer> operations = template.opsForValue();
    template.setKeySerializer(new GenericJackson2JsonRedisSerializer());
    template.setValueSerializer(new GenericJackson2JsonRedisSerializer());
    operations.set(key, 0, expireSeconds * 1000);
  }

  /**
   * 释放锁
   * @param key
   */
  public void releaseLock(String key) {
    ValueOperations<String, Integer> operations = template.opsForValue();
    template.setKeySerializer(new GenericJackson2JsonRedisSerializer());
    template.setValueSerializer(new GenericJackson2JsonRedisSerializer());
    template.delete(key);
  }

采用redis的 increament操作完成锁的抢占.但是释放锁时,是每个线程都可以删除redis中的key值. 并且initLock会降上一次的操作给覆盖掉，所以也废弃掉此方法

最终版本:

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.data.redis.connection.RedisConnectionFactory;
import org.springframework.data.redis.connection.jedis.JedisConnection;
import org.springframework.stereotype.Service;
import org.springframework.util.ReflectionUtils;
import redis.clients.jedis.Jedis;
import java.lang.reflect.Field;
import java.util.Collections;
@Service
public class RedisLock {
  private static final String LOCK_SUCCESS = "OK";
  private static final String SET_IF_NOT_EXIST = "NX";
  private static final String SET_WITH_EXPIRE_TIME = "PX";
  private static final Long RELEASE_SUCCESS = 1L;
  @Autowired
  private RedisConnectionFactory connectionFactory;
  /**
   * 尝试获取分布式锁
   * @param lockKey 锁
   * @param requestId 请求标识
   * @param expireTime 超期时间
   * @return 是否获取成功
   */
  public boolean lock(String lockKey, String requestId, int expireTime) {
    Field jedisField = ReflectionUtils.findField(JedisConnection.class, "jedis");
    ReflectionUtils.makeAccessible(jedisField);
    Jedis jedis = (Jedis) ReflectionUtils.getField(jedisField, connectionFactory.getConnection());

    String result = jedis.set(lockKey, requestId, SET_IF_NOT_EXIST, SET_WITH_EXPIRE_TIME, expireTime);

    if (LOCK_SUCCESS.equals(result)) {
      return true;
    }
    return false;

  }
  /**
   * 释放分布式锁
   * @param lockKey 锁
   * @param requestId 请求标识
   * @return 是否释放成功
   */
  public boolean releaseLock(String lockKey, String requestId) {

    String script = "if redis.call('get', KEYS[1]) == ARGV[1] then return redis.call('del', KEYS[1]) else return 0 end";
    Object result = getJedis().eval(script, Collections.singletonList(lockKey), Collections.singletonList(requestId));

    if (RELEASE_SUCCESS.equals(result)) {
      return true;
    }
    return false;
  }
  public Jedis getJedis() {
    Field jedisField = ReflectionUtils.findField(JedisConnection.class, "jedis");
    ReflectionUtils.makeAccessible(jedisField);
    Jedis jedis = (Jedis) ReflectionUtils.getField(jedisField, connectionFactory.getConnection());
    return jedis;
  }
}