code: public Resource<L> getResource(ID resourceId) {
          graphLockRead.lock();
          try {
              return resourceCache.get(resourceId);
          } finally {
              graphLockRead.unlock();
          }
      }
docs: Given a resource ID this will return the resource with that ID that is found in the internal
  hierarchical graph or <code>null</code> if there is no resource with that ID in the graph.
  
  @param resourceId the ID of the resource to retrieve
  @return the resource as found in the internal graph or <code>null</code>