ReproducerTcpClose.java

package ch.oli.vertx;

import io.vertx.core.Vertx;
import io.vertx.core.http.HttpClient;
import io.vertx.core.http.HttpClientOptions;
import io.vertx.core.http.HttpClientRequest;
import io.vertx.core.http.HttpServer;

public class ReproducerTcpClose {

    private static final Vertx VERTX = Vertx.vertx();

    public static void main(String[] args) throws Exception {

        startBackendServer(); // listen on Port 7020

        HttpClientOptions clOpts = new HttpClientOptions().setMaxPoolSize(1).setKeepAlive(true);
        final HttpClient httpClient = VERTX.createHttpClient(clOpts);

        for (int i = 1; ; i++) {
            String uri = "id-" + i;
            HttpClientRequest req = httpClient.get(7020, "localhost", uri);
            req.handler(resp -> {
                int status = resp.statusCode();
                if (status != 200) {
                    System.out.println("Request " + uri + ": Non-200-Status " + status);
                }
                resp.handler(data -> {
//                    System.out.println("Request #" + uri + ": received " + data.length() + " bytes");
                });
                resp.endHandler(end -> {
                    System.out.println("Request " + uri + ": finished (response complete)");
                });
            });
            req.connectionHandler(conn -> {
                System.out.println("Connection established for " + uri);
            });
            req.end();
            System.out.println("\n\nRequest " + uri + ": starting");

            Thread.sleep(5000);
        }

    }

    private static void startBackendServer() {
        final HttpServer httpServer = VERTX.createHttpServer();
        httpServer.requestHandler(req -> {
            System.out.println("-------------------------- Backend-Server received request " + req.uri());
            VERTX.setTimer(500, id -> {
                req.response().end("my data");
                System.out.println("-------------------------- Backend-Server finished response " + req.uri());
            });
        });
        httpServer.listen(7020);
    }
}

With vertx-3.5.0 we see

Request id-1: starting
Connection established for id-1
-------------------------- Backend-Server received request id-1
-------------------------- Backend-Server finished response id-1
Request id-1: finished (response complete)

Request id-2: starting
-------------------------- Backend-Server received request id-2
-------------------------- Backend-Server finished response id-2
Request id-2: finished (response complete)

Request id-3: starting
-------------------------- Backend-Server received request id-3
-------------------------- Backend-Server finished response id-3
Request id-3: finished (response complete)

With vertx-3.5.1 we see:

Request id-1: starting
Connection established for id-1
-------------------------- Backend-Server received request id-1
-------------------------- Backend-Server finished response id-1
Request id-1: finished (response complete)

Request id-2: starting
Connection established for id-2
-------------------------- Backend-Server received request id-2
-------------------------- Backend-Server finished response id-2
Request id-2: finished (response complete)

Request id-3: starting
Connection established for id-3
-------------------------- Backend-Server received request id-3
-------------------------- Backend-Server finished response id-3
Request id-3: finished (response complete)

Note the three Connection established for ... - a new TCP-connection is established for every single http cycle

Note: vertx-core master branch (3.6.0-SNAPSHOT) again behaves like vertx-3.5.0 - so it really makes use of "keep-alive" connections between two http-requests