http://stackoverflow.com/questions/9658488/how-to-implement-jsni-in-gwt/9658534#comment12266173_9658534

gistfile1.txt

public class MyEntryPoint implements EntryPoint {

  public void onModuleLoad() {
    jsniAlert();
    jsniAlertParam("Hello");
    jsniAlertParam("World");
  }

  private static final native void jsniAlert() /*-{
    $wnd.alert("Called JSNI alert");
  }-*/;

  private static final native void jsniAlertParam(String msg) /*-{
    $wnd.alert("Called JSNI alert with param: " + msg);
  }-*/;
}