Server.java

// A simple web server using Java's built-in HttpServer

// Examples from https://dzone.com/articles/simple-http-server-in-java were useful references

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

import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.PrintWriter;

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

interface URLHandler {
    String handleRequest(URI url);
}

class ServerHttpHandler implements HttpHandler {
    URLHandler handler;
    ServerHttpHandler(URLHandler handler) {
      this.handler = handler;
    }
    public void handle(final HttpExchange exchange) throws IOException {
        // form return body after being handled by program
        try {
            URI uri = exchange.getRequestURI();

            try(FileWriter fw = new FileWriter("session.log", true);
              BufferedWriter bw = new BufferedWriter(fw);
              PrintWriter out = new PrintWriter(bw)) {
              out.println(uri.toString());
            } catch (IOException e) {
              //exception handling left as an exercise for the reader
            }

            String ret = handler.handleRequest(uri);
            // form the return string and write it on the browser
            exchange.sendResponseHeaders(200, ret.getBytes().length);
            OutputStream os = exchange.getResponseBody();
            os.write(ret.getBytes());
            os.close();
        } catch(Exception e) {
            String response = e.toString();
            exchange.sendResponseHeaders(500, response.getBytes().length);
            OutputStream os = exchange.getResponseBody();
            os.write(response.getBytes());
            os.close();
        }
    }
}

public class Server {
    public static void start(int port, URLHandler handler) throws IOException {
        HttpServer server = HttpServer.create(new InetSocketAddress(port), 0);

        //create request entrypoint
        server.createContext("/", new ServerHttpHandler(handler));

        //start the server
        server.start();
        System.out.println("Server Started! Visit http://localhost:" + port + " to visit.");
    }
}