java基于jedisLock―redis分布式锁实现示例代码

long N=0L;
//N从redis获取值
if(N<5){
N++；
//N写回redis
}

redis> EXISTS job        # job 不存在
(integer) 0

redis> SETNX job "programmer"  # job 设置成功
(integer) 1

redis> SETNX job "code-farmer"  # 尝试覆盖 job ，失败
(integer) 0

redis> GET job          # 没有被覆盖
"programmer"

SETNX lock.foo <current Unix time + lock timeout + 1> 

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.dao.DataAccessException;
import org.springframework.data.redis.connection.RedisConnection;
import org.springframework.data.redis.core.RedisCallback;
import org.springframework.data.redis.core.RedisTemplate;
import org.springframework.data.redis.serializer.StringRedisSerializer;

/**
 * Redis distributed lock implementation.
 *
 * @author zhengcanrui
 */
public class RedisLock {

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

  private RedisTemplate redisTemplate;

  private static final int DEFAULT_ACQUIRY_RESOLUTION_MILLIS = 100;

  /**
   * Lock key path.
   */
  private String lockKey;

  /**
   * 锁超时时间，防止线程在入锁以后，无限的执行等待
   */
  private int expireMsecs = 60 * 1000;

  /**
   * 锁等待时间，防止线程饥饿
   */
  private int timeoutMsecs = 10 * 1000;

  private volatile boolean locked = false;

  /**
   * Detailed constructor with default acquire timeout 10000 msecs and lock expiration of 60000 msecs.
   *
   * @param lockKey lock key (ex. account:1,...)
   */
  public RedisLock(RedisTemplate redisTemplate,String lockKey) {
    this.redisTemplate = redisTemplate;
    this.lockKey = lockKey + "_lock";
  }

  /**
   * Detailed constructor with default lock expiration of 60000 msecs.
   *
   */
  public RedisLock(RedisTemplate redisTemplate,String lockKey,int timeoutMsecs) {
    this(redisTemplate,lockKey);
    this.timeoutMsecs = timeoutMsecs;
  }

  /**
   * Detailed constructor.
   *
   */
  public RedisLock(RedisTemplate redisTemplate,int timeoutMsecs,int expireMsecs) {
    this(redisTemplate,lockKey,timeoutMsecs);
    this.expireMsecs = expireMsecs;
  }

  /**
   * @return lock key
   */
  public String getLockKey() {
    return lockKey;
  }

  private String get(final String key) {
    Object obj = null;
    try {
      obj = redisTemplate.execute(new RedisCallback<Object>() {
        @Override
        public Object doInRedis(RedisConnection connection) throws DataAccessException {
          StringRedisSerializer serializer = new StringRedisSerializer();
          byte[] data = connection.get(serializer.serialize(key));
          connection.close();
          if (data == null) {
            return null;
          }
          return serializer.deserialize(data);
        }
      });
    } catch (Exception e) {
      logger.error("get redis error,key : {}",key);
    }
    return obj != null ? obj.toString() : null;
  }

  private boolean setNX(final String key,final String value) {
    Object obj = null;
    try {
      obj = redisTemplate.execute(new RedisCallback<Object>() {
        @Override
        public Object doInRedis(RedisConnection connection) throws DataAccessException {
          StringRedisSerializer serializer = new StringRedisSerializer();
          Boolean success = connection.setNX(serializer.serialize(key),serializer.serialize(value));
          connection.close();
          return success;
        }
      });
    } catch (Exception e) {
      logger.error("setNX redis error,key);
    }
    return obj != null ? (Boolean) obj : false;
  }

  private String getSet(final String key,final String value) {
    Object obj = null;
    try {
      obj = redisTemplate.execute(new RedisCallback<Object>() {
        @Override
        public Object doInRedis(RedisConnection connection) throws DataAccessException {
          StringRedisSerializer serializer = new StringRedisSerializer();
          byte[] ret = connection.getSet(serializer.serialize(key),serializer.serialize(value));
          connection.close();
          return serializer.deserialize(ret);
        }
      });
    } catch (Exception e) {
      logger.error("setNX redis error,key);
    }
    return obj != null ? (String) obj : null;
  }

  /**
   * 获得 lock.
   * 实现思路: 主要是使用了redis 的setnx命令,缓存了锁.
   * reids缓存的key是锁的key,所有的共享,value是锁的到期时间(注意:这里把过期时间放在value了,没有时间上设置其超时时间)
   * 执行过程:
   * 1.通过setnx尝试设置某个key的值,成功(当前没有这个锁)则返回,成功获得锁
   * 2.锁已经存在则获取锁的到期时间,和当前时间比较,超时的话,则设置新的值
   *
   * @return true if lock is acquired,false acquire timeouted
   * @throws InterruptedException in case of thread interruption
   */
  public synchronized boolean lock() throws InterruptedException {
    int timeout = timeoutMsecs;
    while (timeout >= 0) {
      long expires = System.currentTimeMillis() + expireMsecs + 1;
      String expiresStr = String.valueOf(expires); //锁到期时间
      if (this.setNX(lockKey,expiresStr)) {
        // lock acquired
        locked = true;
        return true;
      }

      String currentValueStr = this.get(lockKey); //redis里的时间
      if (currentValueStr != null && Long.parseLong(currentValueStr) < System.currentTimeMillis()) {
        //判断是否为空，不为空的情况下，如果被其他线程设置了值，则第二个条件判断是过不去的
        // lock is expired

        String oldValueStr = this.getSet(lockKey,expiresStr);
        //获取上一个锁到期时间，并设置现在的锁到期时间，
        //只有一个线程才能获取上一个线上的设置时间，因为jedis.getSet是同步的
        if (oldValueStr != null && oldValueStr.equals(currentValueStr)) {
          //防止误删（覆盖，因为key是相同的）了他人的锁――这里达不到效果，这里值会被覆盖，但是因为什么相差了很少的时间，所以可以接受

          //[分布式的情况下]:如过这个时候，多个线程恰好都到了这里，但是只有一个线程的设置值和当前值相同，他才有权利获取锁
          // lock acquired
          locked = true;
          return true;
        }
      }
      timeout -= DEFAULT_ACQUIRY_RESOLUTION_MILLIS;

      /*
        延迟100 毫秒,这里使用随机时间可能会好一点,可以防止饥饿进程的出现,即,当同时到达多个进程,只会有一个进程获得锁,其他的都用同样的频率进行尝试,后面有来了一些进行,也以同样的频率申请锁,这将可能导致前面来的锁得不到满足.
        使用随机的等待时间可以一定程度上保证公平性
       */
      Thread.sleep(DEFAULT_ACQUIRY_RESOLUTION_MILLIS);

    }
    return false;
  }


  /**
   * Acqurired lock release.
   */
  public synchronized void unlock() {
    if (locked) {
      redisTemplate.delete(lockKey);
      locked = false;
    }
  }

}

 RedisLock lock = new RedisLock(redisTemplate,key,10000,20000);
 try {
      if(lock.lock()) {
          //需要加锁的代码
        }
      }
    } catch (InterruptedException e) {
      e.printStackTrace();
    }finally {
      //为了让分布式锁的算法更稳键些，持有锁的客户端在解锁之前应该再检查一次自己的锁是否已经超时，再去做DEL操作，因为可能客户端因为某个耗时的操作而挂起，
      //操作完的时候锁因为超时已经被别人获得，这时就不必解锁了。 ――――这里没有做
      lock.unlock();
    }

lock(key,expireSec){
isSuccess = setnx key
if (isSuccess)
expire key expireSec
}