code: protected Retriever getRetriever()
      {
          if (retriever == null)
          {
              synchronized (Lock.CREATE_RETRIEVER)
              {
                  if (retriever == null)
                  {
                      retriever = createRetriever();
                  }
              }
          }
  
          return retriever;
      }
docs: Get the {@link Retriever} instance used internally to fetch values using an
  {@link Evaluator}.
  
  @see #createRetriever()
  @see Evaluator
  
  @return the retriever instance