How can I send a Ping frame over WebSocket? My Playframework version is 1.2.7

I just found a Ping frame of some source code in \ framework \ src \ play \ server \ PlayHandler.java. And it will handle the ping frame with the following codes. But I can't find what time the ping frame will be sent? Or any other api to send the ping framework?

private void websocketFrameReceived(final ChannelHandlerContext ctx, WebSocketFrame webSocketFrame) {
        Http.Inbound inbound = channels.get(ctx);
        // Check for closing frame
        if (webSocketFrame instanceof CloseWebSocketFrame) {
            this.handshaker.close(ctx.getChannel(), (CloseWebSocketFrame) webSocketFrame);
        } else if (webSocketFrame instanceof PingWebSocketFrame) {
            ctx.getChannel().write(new PongWebSocketFrame(webSocketFrame.getBinaryData()));
        } else if (webSocketFrame instanceof BinaryWebSocketFrame) {
            inbound._received(new Http.WebSocketFrame(webSocketFrame.getBinaryData().array()));
        } else if (webSocketFrame instanceof TextWebSocketFrame) {
            inbound._received(new Http.WebSocketFrame(((TextWebSocketFrame)webSocketFrame).getText()));
        }
    }