MessageBoardEndpoint.java

public class MessageBoardEndpoint extends Endpoint {
  
    private final MessageQueue messageQueue = MessageQueue.instance();
  
    @Override
    public void onOpen(Session session, EndpointConfig endpointConfig) {
        session.addMessageHandler(new MessageHandler.Whole<String>() {
            @Override
            public void onMessage(String message) {
                try {
                    if (message.equals("SEND")) {
                        while (!messageQueue.isEmpty()) {
                            session.getBasicRemote().sendObject(messageQueue.pop());
                        }
                    }
                } catch (Exception e) {
                    // ...
                }
            }
        });
    }
}