A Jetty-based HTTP proxy. As easy as it can be.

Main.java

import org.eclipse.jetty.server.Server;
import org.eclipse.jetty.servlet.ServletHandler;
import org.eclipse.jetty.servlets.ProxyServlet;

import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import java.io.IOException;

public class Main {

    public static class MyProxyServlet extends ProxyServlet {
        @Override
        public void init(ServletConfig config) throws ServletException {
            super.init(config);
            System.out.println(">> init done !");
        }

        @Override
        public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
            System.out.println(">>> got a request !");
            super.service(req, res);
        }
    }

    public static void main(String... args) throws Exception {
        Server server = new Server(8080);

        ServletHandler servletHandler = new ServletHandler();
        servletHandler.addServletWithMapping(MyProxyServlet.class, "/*");

        server.setHandler(servletHandler);
        server.start();
        server.join();
    }
}