One clear advantage of using the Helidon connector, as opposed to the default one provided by Jersey, is the ability to issue HTTP/2 requests. There are three ways to enable HTTP/2:

1. Via content negotiation from HTTP/1.1, where the initial request is HTTP/1.1 (text) and the first response is HTTP/2 (binary), assuming the negotiation is successful.
2. Similar to (1) except that a TLS extension called ALPN is used to convey the upgrade negotiation. Naturally, this only works with secure connections, so TLS is a requirement here.
3. Using prior knowledge, where the client simply sends an HTTP/2 request knowing a priori that the server is capable of handling it. This option always requires TLS.

Without TLS, HTTP/2 negotiation is accomplished by setting a single property. In the example below, the property is set on the correspoding WebTarget, which indicates that it applies to all requests created from it.

ClientConfig clientConfig = new ClientConfig();
clientConfig.connectorProvider(HelidonConnectorProvider.create());
Client client = ClientBuilder.newClient(clientConfig);

WebTarget webTarget = client.target(uri)
        .property(HelidonProperties.PROTOCOL_ID, Http2Client.PROTOCOL_ID);      // HTTP2 upgrade
try (Response response = webTarget.request().get()) {
    // ...
}

The request invocation in the example above will include an HTTP/2 protocol upgrade request which may be granted by the server if HTTP/2 support is enabled.

ALPN is a TLS extension that can be used for HTTP/2 negotiation. The Helidon connector accepts a Tls instance to enable protocol security and also to negotiate an HTTP/2 upgrade as shown below.

Tls tls = Tls.builder()
        .trustAll(true)
        .addApplicationProtocol(Http2Client.PROTOCOL_ID)        // HTTP/2 upgrade
        .endpointIdentificationAlgorithm(Tls.ENDPOINT_IDENTIFICATION_NONE)
        .build();

ClientConfig clientConfig = new ClientConfig();
clientConfig.connectorProvider(HelidonConnectorProvider.create());
clientConfig.property(HelidonProperties.TLS, tls);
Client client = ClientBuilder.newClient(clientConfig);

WebTarget webTarget = client.target(uri);
try (Response response = webTarget.request().get()) {
    // ...
}

The call to addApplicationProtocol() indicates the desire to negotiate a protocol upgrade. Naturally, ALPN only works on secure connections, so TLS is always configured at the same time.

The last example shows how to enable HTTP/2 when prior knowledge of the server’s capabilities is known ahead of time. In order to force HTTP/2 for the initial request, we must provide an Http2ClientProtocolConfig instance that is properly configured for that purpose. Passing protocol configurations is a general mechanism supported by the connector; in this example, we take advantage of this mechanism to pre-configure the desired HTTP/2 support as shown next.

Tls tls = Tls.builder()
        .trustAll(true)
        .endpointIdentificationAlgorithm(Tls.ENDPOINT_IDENTIFICATION_NONE)
        .build();

ClientConfig clientConfig = new ClientConfig();
clientConfig.connectorProvider(HelidonConnectorProvider.create());
clientConfig.property(HelidonProperties.TLS, tls);
clientConfig.property(HelidonProperties.PROTOCOL_CONFIGS,
                      List.of(Http2ClientProtocolConfig.builder()
                                      .priorKnowledge(true)    // HTTP/2 knowlege
                                      .build()));
Client client = ClientBuilder.newClient(clientConfig);

WebTarget webTarget = client.target(uri);
try (Response response = webTarget.request().get()) {
    // ...
}

The property HelidonProperties.PROTOCOL_CONFIGS accepts a list of protocol configurations that are passed directly to the underlying WebClient layer.