/**
 * 分段鎖，系統(tǒng)提供一定數(shù)量的原始鎖，根據(jù)傳入對象的哈希值獲取對應(yīng)的鎖并加鎖
 * 注意：要鎖的對象的哈希值如果發(fā)生改變，有可能導(dǎo)致鎖無法成功釋放!!!
 */
public class SegmentLock<T> {
  private Integer segments = 16;//默認(rèn)分段數(shù)量
  private final HashMap<Integer, ReentrantLock> lockMap = new HashMap<>();

  public SegmentLock() {
    init(null, false);
  }

  public SegmentLock(Integer counts, boolean fair) {
    init(counts, fair);
  }

  private void init(Integer counts, boolean fair) {
    if (counts != null) {
      segments = counts;
    }
    for (int i = 0; i < segments; i++) {
      lockMap.put(i, new ReentrantLock(fair));
    }
  }

  public void lock(T key) {
    ReentrantLock lock = lockMap.get((key.hashCode()>>>1) % segments);
    lock.lock();
  }

  public void unlock(T key) {
    ReentrantLock lock = lockMap.get((key.hashCode()>>>1) % segments);
    lock.unlock();
  }
}

public class HashLock<T> {
  private boolean isFair = false;
  private final SegmentLock<T> segmentLock = new SegmentLock<>();//分段鎖
  private final ConcurrentHashMap<T, LockInfo> lockMap = new ConcurrentHashMap<>();

  public HashLock() {
  }

  public HashLock(boolean fair) {
    isFair = fair;
  }

  public void lock(T key) {
    LockInfo lockInfo;
    segmentLock.lock(key);
    try {
      lockInfo = lockMap.get(key);
      if (lockInfo == null) {
        lockInfo = new LockInfo(isFair);
        lockMap.put(key, lockInfo);
      } else {
        lockInfo.count.incrementAndGet();
      }
    } finally {
      segmentLock.unlock(key);
    }
    lockInfo.lock.lock();
  }

  public void unlock(T key) {
    LockInfo lockInfo = lockMap.get(key);
    if (lockInfo.count.get() == 1) {
      segmentLock.lock(key);
      try {
        if (lockInfo.count.get() == 1) {
          lockMap.remove(key);
        }
      } finally {
        segmentLock.unlock(key);
      }
    }
    lockInfo.count.decrementAndGet();
    lockInfo.unlock();
  }

  private static class LockInfo {
    public ReentrantLock lock;
    public AtomicInteger count = new AtomicInteger(1);

    private LockInfo(boolean fair) {
      this.lock = new ReentrantLock(fair);
    }

    public void lock() {
      this.lock.lock();
    }

    public void unlock() {
      this.lock.unlock();
    }
  }
}

/**
 * 弱引用鎖，為每個獨立的哈希值提供獨立的鎖功能
 */
public class WeakHashLock<T> {
  private ConcurrentHashMap<T, WeakLockRef<T, ReentrantLock>> lockMap = new ConcurrentHashMap<>();
  private ReferenceQueue<ReentrantLock> queue = new ReferenceQueue<>();

  public ReentrantLock get(T key) {
    if (lockMap.size() > 1000) {
      clearEmptyRef();
    }
    WeakReference<ReentrantLock> lockRef = lockMap.get(key);
    ReentrantLock lock = (lockRef == null ? null : lockRef.get());
    while (lock == null) {
      lockMap.putIfAbsent(key, new WeakLockRef<>(new ReentrantLock(), queue, key));
      lockRef = lockMap.get(key);
      lock = (lockRef == null ? null : lockRef.get());
      if (lock != null) {
        return lock;
      }
      clearEmptyRef();
    }
    return lock;
  }

  @SuppressWarnings("unchecked")
  private void clearEmptyRef() {
    Reference<? extends ReentrantLock> ref;
    while ((ref = queue.poll()) != null) {
      WeakLockRef<T, ? extends ReentrantLock> weakLockRef = (WeakLockRef<T, ? extends ReentrantLock>) ref;
      lockMap.remove(weakLockRef.key);
    }
  }

  private static final class WeakLockRef<T, K> extends WeakReference<K> {
    final T key;

    private WeakLockRef(K referent, ReferenceQueue<? super K> q, T key) {
      super(referent, q);
      this.key = key;
    }
  }
}