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To detect connection events on your WebSocket server using Spring 4, you can follow these steps:

1. Implement the WebSocketHandler interface, which includes methods to handle connection events. You can extend the BinaryWebSocketHandler or TextWebSocketHandler class to implement this interface and handle binary or text WebSocket messages.

2. Override the afterConnectionEstablished method to handle the connection open event. This method is called when a client establishes a WebSocket connection with the server.

3. Override the handleBinaryMessage or handleTextMessage method to handle incoming WebSocket messages.

4. Override the afterConnectionClosed method to handle the connection close event. This method is called when a client closes the WebSocket connection with the server.

5. Register your WebSocket handler with Spring's WebSocketHandlerRegistry using the registerWebSocketHandler method.

6. Enable WebSocket support in your Spring configuration using the @EnableWebSocket annotation.

Here's an example implementation:

@Component
public class MyWebSocketHandler extends TextWebSocketHandler {

  @Override
  public void afterConnectionEstablished(WebSocketSession session) throws Exception {
    // Handle connection open event
  }

  @Override
  protected void handleTextMessage(WebSocketSession session, TextMessage message) throws Exception {
    // Handle incoming text message
  }

  @Override
  public void afterConnectionClosed(WebSocketSession session, CloseStatus status) throws Exception {
    // Handle connection close event
  }
}

@Configuration
@EnableWebSocket
public class WebSocketConfig implements WebSocketConfigurer {

  @Autowired
  private MyWebSocketHandler myWebSocketHandler;

  @Override
  public void registerWebSocketHandlers(WebSocketHandlerRegistry registry) {
    registry.addHandler(myWebSocketHandler, "/my-websocket");
  }
}

In this example, we have implemented a MyWebSocketHandler class that extends TextWebSocketHandler and overrides the methods to handle connection open, incoming text messages, and connection close events. We have also registered this handler with Spring's WebSocketHandlerRegistry using the registerWebSocketHandler method in the WebSocketConfig class, which is enabled for WebSocket support using the @EnableWebSocket annotation.