Does Apache or some other CLIENT JAVA implementation support HTTP/2?

Jetty has support for HTTP2 starting from version 9.3. This includes the server and the client.

There is OkHttp: An HTTP & HTTP/2 client for Android and Java applications.

Jetty's provides two HTTP/2 Java client APIs. Both require Java 8 (or better) and the mandatory use of the ALPN, as explained here.

Low level APIs

These APIs are based on HTTP2Client, it's based on the HTTP/2 concepts of session and streams and uses listeners to be notified of the HTTP/2 frames that arrive from the server.

    // Setup and start the HTTP2Client.
    HTTP2Client client = new HTTP2Client();
    SslContextFactory sslContextFactory = new SslContextFactory();
    client.addBean(sslContextFactory);
    client.start();

    // Connect to the remote host to obtains a Session.
    FuturePromise<Session> sessionPromise = new FuturePromise<>();
    client.connect(sslContextFactory, new InetSocketAddress(host, port), new ServerSessionListener.Adapter(), sessionPromise);
    Session session = sessionPromise.get(5, TimeUnit.SECONDS);

    // Use the session to make requests.
    HttpFields requestFields = new HttpFields();
    requestFields.put("User-Agent", client.getClass().getName() + "/" + Jetty.VERSION);
    MetaData.Request metaData = new MetaData.Request("GET", new HttpURI("https://webtide.com/"), HttpVersion.HTTP_2, requestFields);
    HeadersFrame headersFrame = new HeadersFrame(metaData, null, true);
    session.newStream(headersFrame, new Promise.Adapter<>(), new Stream.Listener.Adapter()
    {
        @Override
        public void onHeaders(Stream stream, HeadersFrame frame)
        {
            // Response headers.
            System.err.println(frame);
        }

        @Override
        public void onData(Stream stream, DataFrame frame, Callback callback)
        {
            // Response content.
            System.err.println(frame);
            callback.succeeded();
        }
    });

High Level APIs

Jetty's HttpClient provides a way to use different transports, one of which is the HTTP/2 transport. Applications will use the higher level HTTP APIs, but underneath Jetty will use HTTP/2 to transport the HTTP semantic.

In this way, applications can use the high level APIs provided by HttpClient transparently, and factor out what transport to use in configuration or startup code.

    // Setup and start HttpClient with HTTP/2 transport.
    HTTP2Client http2Client = new HTTP2Client();
    SslContextFactory sslContextFactory = new SslContextFactory();
    HttpClient httpClient = new HttpClient(new HttpClientTransportOverHTTP2(http2Client), sslContextFactory);
    httpClient.start();

    // Make a request.
    ContentResponse response = httpClient.GET("https://webtide.com/");