code: public Resource getMissingResource(final ResourceList resources) {
          try {
              return _withLock(new Callable<Resource>() {
                  @Override
                  public Resource call() {
                      return resources.getConflict(inUse);
                  }
              });
          } catch (Exception e) {
              throw new IllegalStateException("Inner callable does not throw exception");
          }
      }
docs: Of the resource in the given resource list, return the one that's
  currently in use.
  
  <p>
  If more than one such resource exists, one is chosen and returned.
  This method is used for reporting what's causing the blockage.