The Java WebSocket specification also defines a programmatic Java interface for connections. Without annotation, a POJO has to extend the abstract class javax.websocket.Endpoint and write the implementation methods: onOpen(), onClose() and onError().

Here is an implementation of the echo WebSocket from earlier:

public class EchoProgramServerEndpoint extends Endpoint {
  public void onOpen( final Session session, 
            EndpointConfig config) {

    session.addMessageHandler(
      new MessageHandler.Whole<String>() {
      @Override
      public void onMessage(String message) {
        System.out.printf(
          "Received message=%s
", message);
        try {
          session.getRemoteBasic()
            .sendText("ECHO "+message);
        }
        catch (Exception e) { }
      }
    });

  }

  public void onClose(Session session, 
            CloseReason closeReason) {
    /* ... */
  }

  public void onError(Session session, 
            Throwable throwable) {
    /* ... */
  }
}

In the onOpen() method inside the EchoProgramServerEndpoint, we register a MessageHandler in order to receive messages from remote peers. A MessageHandler has two sub interfaces: Whole or Partial. The Whole interface is designed for applications that want to consume entire messages as they arrive. The Partial interface is designed for applications that consume partial messages.

The anonymous message handler in EchoProgramServerEndpoint accepts a String text message from the remote peer. It then sends the echo message back to the peer with the captured Session instance from the onOpen method parameter.