A bug in HttpURLConnection ?

HttpURLConnectionBug.java

import java.io.IOException;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.concurrent.atomic.AtomicInteger;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.eclipse.jetty.server.Request;
import org.eclipse.jetty.server.Server;
import org.eclipse.jetty.server.handler.AbstractHandler;
import org.eclipse.jetty.util.ConcurrentHashSet;

public class HttpURLConnectionBug {

    public static void main(String[] args) throws Exception {
        Server server = new Server(8080);
        server.setHandler(new UniqueCheckHanlder());
        server.start();

        final AtomicInteger i = new AtomicInteger();
        Runnable r = new Runnable() {
            @Override
            public void run() {
                for (int n = 0; n < 1000; n++) {
                    String id = Long.toString(i.incrementAndGet());
                    int code;
                    String message;
                    try {
                        HttpURLConnection connection = (HttpURLConnection) new URL("http://localhost:8080/?id=" + id)
                                .openConnection();
                        connection.connect();
                        code = connection.getResponseCode();
                        message = connection.getResponseMessage();
                    } catch (Exception e) {
                        throw new RuntimeException(e);
                    }
                    if (code != 200) {
                        throw new RuntimeException("Unexpected code: " + code + " " + message);
                    }
                }
            }
        };
        for (int n = 0; n < 8; n++) {
            new Thread(r).start();
        }
    }

    private static class UniqueCheckHanlder extends AbstractHandler {

        private ConcurrentHashSet<String> ids = new ConcurrentHashSet<>();

        public void handle(String target, Request baseRequest, HttpServletRequest request, HttpServletResponse response)
                throws IOException, ServletException {
            String id = request.getParameter("id");
            baseRequest.setHandled(true);
            if (ids.add(id)) {
                response.setContentType("text/html;charset=utf-8");
                response.setStatus(HttpServletResponse.SC_OK);
                response.getWriter().println("OK");
            } else {
                response.setContentType("text/html;charset=utf-8");
                response.setStatus(HttpServletResponse.SC_BAD_REQUEST);
                response.getWriter().println("DUPLICATE");
            }
        }
    }

}