private int counter = 0;
public MyPage() {
    final Label counterLabel = new Label("counter", counter);
    add(counterLabel);
    counterLabel.setOutputMarkupId(true);

    add(new AjaxLink("counterLabelLink")

    @Override
    public void onClick(AjaxRequestTarget target) {
        counter++;
        target.addComponent(counterLabel);
    }
});