--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/goodjava/webserver/ServerSentEvents.java	Thu Nov 03 19:23:53 2022 -0600
@@ -0,0 +1,96 @@
+package goodjava.webserver;
+
+import java.io.Writer;
+import java.io.OutputStreamWriter;
+import java.io.BufferedWriter;
+import java.io.IOException;
+import java.net.Socket;
+import java.util.List;
+import java.util.ArrayList;
+import java.util.Map;
+import java.util.HashMap;
+import java.util.Collections;
+import java.util.Iterator;
+
+
+public class ServerSentEvents {
+
+	private static class Con {
+		final Socket socket;
+		final Writer writer;
+
+		Con(Socket socket) throws IOException {
+			this.socket = socket;
+			this.writer = new BufferedWriter( new OutputStreamWriter( socket.getOutputStream(), "UTF-8" ) );
+		}
+	}
+
+	private static class EventHandler {
+		private final List<Con> cons = new ArrayList<Con>();
+
+		synchronized void add(Con con) {
+			cons.add(con);
+		}
+
+		synchronized void write( String url, String content ) {
+			Iterator<Con> iter = cons.iterator();
+			while( iter.hasNext() ) {
+				Con con = iter.next();
+				Writer writer = con.writer;
+				try {
+					writer.write(content);
+					writer.write("\n\n");
+					writer.flush();
+				} catch(IOException e) {
+					iter.remove();
+				}
+			}
+			if( cons.isEmpty() )
+				map.remove(url);
+		}
+	}
+
+	private static final Map<String,EventHandler> map
+		= Collections.synchronizedMap(new HashMap<String,EventHandler>());
+
+	static void add(Socket socket,Request request) throws IOException {
+		Con con = new Con(socket);
+
+		Writer writer = con.writer;
+		writer.write("HTTP/1.1 200 OK\r\n");
+		writer.write("Access-Control-Allow-Origin: *\r\n");
+		writer.write("Cache-Control: no-cache\r\n");
+		writer.write("Content-Type: text/event-stream\r\n");
+		writer.write("\r\n");
+		writer.flush();
+
+		String url = request.url();
+		EventHandler handler;
+		synchronized(map) {
+			handler = map.get(url);
+			if( handler==null ) {
+				handler = new EventHandler();
+				map.put(url,handler);
+			}
+		}
+		handler.add(con);
+	}
+
+	public static void write( String url, String content ) {
+		EventHandler handler = map.get(url);
+		if( handler != null )
+			handler.write(url,content);
+	}
+
+	public static String toData(String message) {
+		if( message.endsWith("\n") )
+			message = message.substring( 0, message.length() - 1 );
+		return "data: " + message.replace( "\n", "\ndata: " ) + "\n";
+	}
+
+	public static void writeMessage( String url, String message ) {
+		write( url, toData(message) );
+	}
+
+	private ServerSentEvents() {}  // never
+}