ReentrantLock 可重入鎖

        可重入性是指線程在持有鎖的情況下再次請求加鎖，如果一個鎖支持同一個線程的多次加鎖，那麼這個鎖就是可重入的。比如 Java 語言裏有個 ReentrantLock 就是可重入鎖。如果Redis 分佈式鎖要支持可重入，需要對客戶端的 set 方法進行包裝，使用線程的 Threadlocal 變量存儲當前持有鎖的計數。

代碼如下：

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 (null != refs){
            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 (null != refCnt){
            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 (null ==  refCnt){
            return false;
        }
        refCnt -= 1;
        if (refCnt > 0){
            refs.put(key, refCnt);
        }else {
            refs.remove(key);
            this._lock(key);
        }
        return true;
    }

    /**
    public static void main(String[] args) {
        Jedis jedis = new Jedis("localhost");
        RedisWithReentrantLock redisWithReentrantLock = new RedisWithReentrantLock(jedis);
        System.out.println(redisWithReentrantLock.lock("TestLock"));
        System.out.println(redisWithReentrantLock.lock("TestLock"));
        System.out.println(redisWithReentrantLock.unlock("TestLock"));
        System.out.println(redisWithReentrantLock.unlock("TestLock"));
    }
    **/
     
}