...

public class JsonWebPage extends WebPage {

	public JsonWebPage(PageParameters pageParameters) {

		getRequestCycle().scheduleRequestHandlerAfterCurrent(new IRequestHandler() {

			@Override
			public void detach(RequestCycle requestCycle) {
				// Nothing to do here.
			}

			@Override
			public void respond(RequestCycle requestCycle) {
				// Add JSON-encoded string to the response.
				Response response = requestCycle.getResponse();
                                response.setContentType("application/json");
                                response.write("{\"jsonKey\":\"jsonValue\"}");
			}
		});
	}
}
...