code: @Override
      public synchronized Lock obtainReadLock() {
          File writeLock = getWriteLock();
  
          if (writeLock != null && writeLock.exists()) {
              return null;
          }
  
          return createLock(false);
      }
docs: {@inheritDoc}
  <p>
  A {@link Lock read lock} cannot be obtained if the {@link Lock write
  lock} has been obtained.
  </p>