code: @Override
      public void checkin(K key, V resource) {
          super.checkin(key, resource);
          // NB: Blocking checkout calls for synchronous requests get the resource
          // checked in above before processQueueLoop() attempts checkout below.
          // There is therefore a risk that asynchronous requests will be starved.
          processQueueLoop(key);
      }
docs: Check the given resource back into the pool
  
  @param key The key for the resource
  @param resource The resource