code: public static OMMapManager getInstance() {
      if (instanceRef.get() == null) {
        synchronized (instanceRef) {
          if (instanceRef.compareAndSet(null, createInstance())) {
            instanceRef.get().init();
          }
        }
      }
      return instanceRef.get();
    }
docs: This method returns instance of mmap manager.
  
  @return mmap manager instance. If it is not exist create new one.