Ein Handbuch zur Java-API für WebSocket

@ServerEndpoint(value = "/chat/{username}")
public class ChatEndpoint {

    @OnOpen
    public void onOpen(Session session) throws IOException {
        // Get session and WebSocket connection
    }

    @OnMessage
    public void onMessage(Session session, Message message) throws IOException {
        // Handle new messages
    }

    @OnClose
    public void onClose(Session session) throws IOException {
        // WebSocket connection closes
    }

    @OnError
    public void onError(Session session, Throwable throwable) {
        // Do error handling here
    }
}

@ServerEndpoint(value="/chat/{username}")
public class ChatEndpoint {

    private Session session;
    private static Set chatEndpoints
      = new CopyOnWriteArraySet<>();
    private static HashMap users = new HashMap<>();

    @OnOpen
    public void onOpen(
      Session session,
      @PathParam("username") String username) throws IOException {

        this.session = session;
        chatEndpoints.add(this);
        users.put(session.getId(), username);

        Message message = new Message();
        message.setFrom(username);
        message.setContent("Connected!");
        broadcast(message);
    }

    @OnMessage
    public void onMessage(Session session, Message message)
      throws IOException {

        message.setFrom(users.get(session.getId()));
        broadcast(message);
    }

    @OnClose
    public void onClose(Session session) throws IOException {

        chatEndpoints.remove(this);
        Message message = new Message();
        message.setFrom(users.get(session.getId()));
        message.setContent("Disconnected!");
        broadcast(message);
    }

    @OnError
    public void onError(Session session, Throwable throwable) {
        // Do error handling here
    }

    private static void broadcast(Message message)
      throws IOException, EncodeException {

        chatEndpoints.forEach(endpoint -> {
            synchronized (endpoint) {
                try {
                    endpoint.session.getBasicRemote().
                      sendObject(message);
                } catch (IOException | EncodeException e) {
                    e.printStackTrace();
                }
            }
        });
    }
}

public class Message {
    private String from;
    private String to;
    private String content;

    //standard constructors, getters, setters
}

public class MessageEncoder implements Encoder.Text {

    private static Gson gson = new Gson();

    @Override
    public String encode(Message message) throws EncodeException {
        return gson.toJson(message);
    }

    @Override
    public void init(EndpointConfig endpointConfig) {
        // Custom initialization logic
    }

    @Override
    public void destroy() {
        // Close resources
    }
}

public class MessageDecoder implements Decoder.Text {

    private static Gson gson = new Gson();

    @Override
    public Message decode(String s) throws DecodeException {
        return gson.fromJson(s, Message.class);
    }

    @Override
    public boolean willDecode(String s) {
        return (s != null);
    }

    @Override
    public void init(EndpointConfig endpointConfig) {
        // Custom initialization logic
    }

    @Override
    public void destroy() {
        // Close resources
    }
}

@ServerEndpoint(
  value="/chat/{username}",
  decoders = MessageDecoder.class,
  encoders = MessageEncoder.class )