Atmosphere 4.0 is a major release with significant changes from 2.x:

All javax.* packages are now jakarta.*:

// 2.x
import javax.inject.Inject;
import javax.servlet.http.HttpServletRequest;

// 4.0
import jakarta.inject.Inject;
import jakarta.servlet.http.HttpServletRequest;

The core annotation model is the same. Your @ManagedService classes work with minimal changes:

// Same in both 2.x and 4.0
@ManagedService(path = "/chat")
public class Chat {
    @Ready
    public void onReady() { }

    @Disconnect
    public void onDisconnect() { }

    @Message(encoders = {JacksonEncoder.class}, decoders = {JacksonDecoder.class})
    public Message onMessage(Message message) { return message; }
}

The following have been removed or replaced in 4.0:

var socket = atmosphere;
var request = {
    url: '/chat',
    transport: 'websocket',
    fallbackTransport: 'long-polling'
};
request.onMessage = function(response) {
    var message = response.responseBody;
};
var subSocket = socket.subscribe(request);
subSocket.push(JSON.stringify(data));

import { Atmosphere } from 'atmosphere.js';

const atm = new Atmosphere();
const sub = await atm.subscribe(
    {
        url: '/chat',
        transport: 'websocket',
        fallbackTransport: 'long-polling',
    },
    {
        message: (response) => {
            const data = JSON.parse(response.responseBody);
        },
    }
);
sub.push(JSON.stringify(data));

The global atmosphere object is still available for <script> tag usage (no build step):

const sub = await atmosphere.atmosphere.subscribe(request, handlers);

1. Update JDK to 21+
2. Replace javax.* imports with jakarta.*
3. Update atmosphere-runtime dependency to 4.0.0
4. Replace Jersey/Meteor code with @ManagedService (if not already using it)
5. Replace atmosphere-javascript with atmosphere.js 5.0
6. Optionally adopt Rooms for group messaging patterns
7. Optionally adopt Spring Boot starter or Quarkus extension

Update the web-app namespace for Jakarta EE:

<!-- 2.x -->
<web-app xmlns="http://java.sun.com/xml/ns/javaee" version="3.0">

<!-- 4.0 -->
<web-app xmlns="https://jakarta.ee/xml/ns/jakartaee" version="6.0">

The AtmosphereServlet class name is unchanged: org.atmosphere.cpr.AtmosphereServlet.