App.java

import java.io.IOException;

public class App {
    public static void main(String[] args) throws IOException {
       Server server = new Server();
       server.register("/", new HandleRoot());
       server.register("/abc", new HandleAbc());
       server.start();
    }
}

HandleAbc.java

public class HandleAbc extends Handler {
    private int num = 0;
    public String handle() {
        return "" + (num++);
    }
}

Handler.java

import com.sun.net.httpserver.HttpExchange;

import java.io.IOException;

public class Handler {
    public Handler() {
    }

    public String handle() {
        return "";
    }
}

HandleRoot.java

import com.sun.net.httpserver.HttpExchange;

import java.io.IOException;
import java.io.OutputStream;

public class HandleRoot extends Handler {
    public String handle() {
        return "hi there";
    }
}

Server.java

import com.sun.net.httpserver.HttpContext;
import com.sun.net.httpserver.HttpExchange;
import com.sun.net.httpserver.HttpServer;

import java.io.IOException;
import java.io.OutputStream;
import java.net.InetSocketAddress;
import java.net.URI;
import java.util.HashMap;

public class Server {
    private HttpServer http;
    private HttpContext ctx;
    private HashMap<String, Handler> handlers = new HashMap<>();

    public Server() throws IOException {
        http = HttpServer.create(new InetSocketAddress(8500),  0);
        ctx = http.createContext("/");
        ctx.setHandler(this::handleRequest);
    }

    public void register(String path, Handler handler) {
        handlers.put(path, handler);
    }

    public void start() {
        http.start();
    }

    private void handleRequest(HttpExchange conn) throws IOException {
        URI pathURI = conn.getRequestURI();
        String path = pathURI.toString();
        System.out.println(path);

        Handler handler = handlers.get(path);

        String response = handler.handle();
        conn.sendResponseHeaders(200,  response.getBytes().length);
        OutputStream os = conn.getResponseBody();
        os.write(response.getBytes());
        os.close();
    }

}