Issue #11765 - Introduce EventSourceHandler for Server-Sent Events in jetty-core

[afarber]

Closes #11765

This PR adds EventSourceHandler to jetty-core/jetty-server for SSE support without Servlet API dependencies.

• Abstract handler with newEventSource(Request) factory method
• EventSource interface with onOpen(Emitter) and onClose() callbacks
• Emitter with event(), data(), comment(), close() methods
• Configurable heartbeat period for connection keep-alive
• Documentation in programming guide

[afarber]

Successfully tested on my Macbook M1 macOS 26.2 by running the curl -N -H "Accept: text/event-stream" http://localhost:8080/ command against the EventSourceDemo.java listed below:

package org.eclipse.jetty.server.handler;

import java.io.IOException;
import java.time.Duration;
import java.time.Instant;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;

import org.eclipse.jetty.server.Request;
import org.eclipse.jetty.server.Server;
import org.eclipse.jetty.server.ServerConnector;

public class EventSourceDemo
{
    public static void main(String[] args) throws Exception
    {
        Server server = new Server();
        ServerConnector connector = new ServerConnector(server);
        connector.setPort(8080);
        server.addConnector(connector);

        EventSourceHandler handler = new EventSourceHandler()
        {
            {
                setHeartBeatPeriod(Duration.ofSeconds(15));
            }

            @Override
            protected EventSource newEventSource(Request request)
            {
                return new EventSource()
                {
                    private final ScheduledExecutorService executor = Executors.newSingleThreadScheduledExecutor();
                    private final AtomicInteger counter = new AtomicInteger();

                    @Override
                    public void onOpen(Emitter emitter) throws IOException
                    {
                        emitter.comment("Connected to Jetty SSE demo");
                        emitter.data("Welcome! You will receive events every 2 seconds.");

                        executor.scheduleAtFixedRate(() ->
                        {
                            try
                            {
                                int n = counter.incrementAndGet();
                                emitter.event("tick", "Event #" + n + " at " + Instant.now());
                            }
                            catch (IOException e)
                            {
                                executor.shutdown();
                            }
                        }, 2, 2, TimeUnit.SECONDS);
                    }

                    @Override
                    public void onClose()
                    {
                        executor.shutdown();
                        System.out.println("Client disconnected");
                    }
                };
            }
        };

        server.setHandler(handler);
        server.start();
        System.out.println("SSE Demo server started on http://localhost:8080/");
        System.out.println("Test with: curl -N -H 'Accept: text/event-stream' http://localhost:8080/");
        server.join();
    }
}