code: static synchronized DocFileFactory getFactory(Configuration configuration) {
          DocFileFactory f = factories.get(configuration);
          if (f == null) {
              JavaFileManager fm = configuration.getFileManager();
              if (fm instanceof StandardJavaFileManager) {
                  f = new StandardDocFileFactory(configuration);
              } else {
                  throw new IllegalStateException();
              }
              factories.put(configuration, f);
          }
          return f;
      }
docs: Get the appropriate factory, based on the file manager given in the
  configuration.