wro4j-core/src/main/java/ro/isdc/wro/cache/support/AbstractSynchronizedCacheStrategyDecorator.java
private ReadWriteLock getLockForKey(final K key) {
    final ReadWriteLock lock = locks.putIfAbsent(key, new ReentrantReadWriteLock());
    return lock == null ? locks.get(key) : lock;
  }
private ReadWriteLock getLockForKey(final K key) {
    final ReadWriteLock lock = locks.putIfAbsent(key, new ReentrantReadWriteLock());
    return lock == null ? locks.get(key) : lock;
  }
Ensure that the returned lock will never be null.