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

        final IOReactorConfig ioReactorConfig = IOReactorConfig.custom()
                .setSoTimeout(Timeout.ofSeconds(5))
                .build();

        final PoolingAsyncClientConnectionManager connectionManager = PoolingAsyncClientConnectionManagerBuilder.create()
                .setDefaultTlsConfig(TlsConfig.custom()
                        .setVersionPolicy(HttpVersionPolicy.FORCE_HTTP_2)
                        .build())
                .setMessageMultiplexing(true)
                .build();

        final CloseableHttpAsyncClient client = HttpAsyncClients.custom()
                .setConnectionManager(connectionManager)
                .setIOReactorConfig(ioReactorConfig)
                .build();

        client.start();

        final HttpHost target = new HttpHost("https", "nghttp2.org");

        final SimpleHttpRequest warmup = SimpleRequestBuilder.get()
                .setHttpHost(target)
                .setPath("/httpbin")
                .build();

        // Make sure there is an open HTTP/2 connection in the pool
        System.out.println("Executing warm-up request " + warmup);
        final Future<SimpleHttpResponse> future = client.execute(
                SimpleRequestProducer.create(warmup),
                SimpleResponseConsumer.create(),
                new FutureCallback<SimpleHttpResponse>() {

                    @Override
                    public void completed(final SimpleHttpResponse response) {
                        System.out.println(warmup + "->" + new StatusLine(response));
                        System.out.println(response.getBody());
                    }

                    @Override
                    public void failed(final Exception ex) {
                        System.out.println(warmup + "->" + ex);
                    }

                    @Override
                    public void cancelled() {
                        System.out.println(warmup + " cancelled");
                    }

                });
        future.get();

        Thread.sleep(1000);

        System.out.println("Connection pool stats: " + connectionManager.getTotalStats());

        // Execute multiple requests over the HTTP/2 connection from the pool
        final String[] requestUris = new String[]{"/httpbin", "/httpbin/ip", "/httpbin/user-agent", "/httpbin/headers"};
        final CountDownLatch countDownLatch = new CountDownLatch(requestUris.length);

        for (final String requestUri : requestUris) {
            final SimpleHttpRequest request = SimpleRequestBuilder.get()
                    .setHttpHost(target)
                    .setPath(requestUri)
                    .build();

            System.out.println("Executing request " + request);
            client.execute(
                    SimpleRequestProducer.create(request),
                    SimpleResponseConsumer.create(),
                    new FutureCallback<SimpleHttpResponse>() {

                        @Override
                        public void completed(final SimpleHttpResponse response) {
                            countDownLatch.countDown();
                            System.out.println(request + "->" + new StatusLine(response));
                            System.out.println(response.getBody());
                        }

                        @Override
                        public void failed(final Exception ex) {
                            countDownLatch.countDown();
                            System.out.println(request + "->" + ex);
                        }

                        @Override
                        public void cancelled() {
                            countDownLatch.countDown();
                            System.out.println(request + " cancelled");
                        }

                    });
        }

        countDownLatch.await();

        // There still should be a single connection in the pool
        System.out.println("Connection pool stats: " + connectionManager.getTotalStats());

        System.out.println("Shutting down");
        client.close(CloseMode.GRACEFUL);
    }
Example of asynchronous HTTP/1.1 request execution with message exchange multiplexing
over HTTP/2 connections.
httpclient5/src/test/java/org/apache/hc/client5/http/examples/AsyncClientH2Multiplexing.java