http://stackoverflow.com/questions/9658488/how-to-implement-jsni-in-gwt/9658534#comment12266445_9658534

gistfile1.txt

public class MyEntryPoint implements EntryPoint {
  public void onModuleLoad() {
    doJsni();
  }

  private static void staticMethod(int num) {
    Window.alert("Static method called " + num);
  }

  private void instanceMethod(String str) {
    Window.alert("Instance method called " + str);
  }

  private native final void doJsni() /*-{
    var btn = document.createElement('button');
    btn.onClick = $entry(function() {
      // To call a static method...
      @com.this.package.client.MyEntryPoint::staticMethod(int)(3);

      // To call an instance method on this instance of MyEntryPoint...
      [email protected]::instanceMethod(Ljava/lang/String;)("string");
    });
  }-*/;
}