code: private void initializeLayer() {
  		// Set the URL to the service and the file extension:
  		String[] urls = new String[] { 
  				"http://a.tile.openstreetmap.org/{z}/{x}/{y}.png",
  				"http://b.tile.openstreetmap.org/{z}/{x}/{y}.png",
  				"http://c.tile.openstreetmap.org/{z}/{x}/{y}.png" };
  		// Create a new layer with the configurations and add it to the maps:
  		OsmLayer osmLayer = TileBasedLayerClient.getInstance().createOsmLayer("osmCountries", 19, urls);
  		mapPresenter.getLayersModel().addLayer(osmLayer);
  	}
docs: Initialize the OSM layer by creating a new {@link org.geomajas.gwt2.client.map.layer.tile.TileBasedLayer}
  and a {@link org.geomajas.gwt2.client.map.layer.tile.TileConfiguration}.
  <p/>
  Values such as {@code {z}, {x}, {y}} are optional and will be used to substitute the tile level, X-ordinate and
  Y-ordinate.
  <p/>
  The tile based layer service can be given different URLs in which case Round-robin will be performed to
  determine the next URL to load tiles from.