code: private void buildDependencyTree()
          throws DependencyTreeBuilderException
      {
          if ( rootNode == null )
          {
              rootNode =
                  dependencyTreeBuilder.buildDependencyTree( project, localRepository, artifactFactory,
                                                             artifactMetadataSource, null, artifactCollector );
          }
      }
docs: copied from dependency:tree mojo (v2.4)