Only one single annotation is accepted for @OnOpen, @OnError, @OnClose.

Other websockets implementations accept multiple @ for different methods with different parameters. The single annotation which is not accepted is @OnMessage - and this way it respects JSR-356

Each websocket endpoint may only have one message handling method for each of the native websocket message formats: text, binary and pong

https://docs.oracle.com/javaee/7/api/javax/websocket/OnMessage.html