先说一下redis实现分布式锁的指令吧
> set lock:sixj true ex 5 nx
......do something......
> del lock:sixj
加锁的指令是setnx与expire组合的原子指令,key=lock:sixj,value=true,过期时间=5秒
redis分布式锁存在的问题:
如果加锁和释放锁之间的逻辑执行时间超出了锁的超时时间,临界区的逻辑还没有执行完,另一个线程就会重新持有这把锁,导致临界区的代码不能严格控制串行执行。
言归正传,说redis可重入锁
所谓可重入锁就是一个锁支持同一个线程的多次加锁
使用线程的ThreadLocal变量存储当前线程持有锁的计数
public class RedisWithReentrantLock {
private ThreadLocal<Map<String,Integer>> lockers = new ThreadLocal<>();
private Jedis jedis;
public RedisWithReentrantLock(Jedis jedis) {
this.jedis = jedis;
}
private boolean _lock(String key){
return jedis.set(key,"","nx","ex",5L) != null;
}
private void _unlock(String key){
jedis.del(key);
}
private Map<String,Integer> currentLockers(){
Map<String, Integer> refs = lockers.get();
if(refs != null){
return refs;
}
lockers.set(new HashMap<>());
return lockers.get();
}
public boolean lock(String key){
Map<String, Integer> refs = currentLockers();
Integer refCnt = refs.get(key);
if(refCnt != null){
refs.put(key,refCnt+1);
return true;
}
boolean ok = this._lock(key);
if(! ok){
return false;
}
refs.put(key,1);
return true;
}
public boolean unlock(String key){
Map<String, Integer> refs = currentLockers();
Integer refCnt = refs.get(key);
if(refCnt == null){
return false;
}
refCnt -= 1;
if(refCnt > 0){
refs.put(key,refCnt);
}else {
refs.remove(key);
this._unlock(key);
}
return true;
}
}
测试:
RedisWithReentrantLock redis = new RedisWithReentrantLock(jedis);
System.out.println(redis.lock("sixj"));
System.out.println(redis.lock("sixj"));
System.out.println(redis.unlock("sixj"));
System.out.println(redis.unlock("sixj"));
运行结果:
true
true
true
true