i am new to GWT GWT-EXT and i mimic it's demo the problem is where i should put xml file

final TreePanel treePanel = new TreePanel() {
        {
            setAnimate(true);
            setEnableDD(true);
            setContainerScroll(true);
            setRootVisible(true);
        }
    };

 final XMLTreeLoader loader = new XMLTreeLoader() {
        {
            setDataUrl("countries-cb.xml");
            setMethod("get");
            setRootTag("countries");
            setFolderIdMapping("@id");
            setLeafIdMapping("@id");
            setFolderTitleMapping("@title");
            setFolderTag("team");
            setLeafTitleMapping("@title");
            setLeafTag("country");
            setQtipMapping("@qtip");
            setDisabledMapping("@disabled");
            setCheckedMapping("@checked");
            setIconMapping("@icon");
            setAttributeMappings(new String[]{"@rank"});
        }
    };
    AsyncTreeNode root = new AsyncTreeNode("Countries", loader);

    treePanel.setRootNode(root);
    treePanel.render();

    root.expand();
    treePanel.expandAll();