I am developing porable Java EE 7 application. For that, I am using the standard JAX-RS API. The application supports/deploys on Wildfly and Websphere. On Wildfly, the JAX-RS API is provided by RestEasy.
I would like to access a resource via a proxy-server and configure it in a way that i do not depend on RestEasy specific classes/interfaces as this would break the deployment on Websphere.
Under Websphere, i can easily provide the proxy-settins via properties on the clientBuilder:

ClientBuilder cb = ClientBuilder.newBuilder();
cb.property("com.ibm.ws.jaxrs.client.proxy.host", "hostname");
cb.property("com.ibm.ws.jaxrs.client.proxy.port", "8888";);
cb.property("com.ibm.ws.jaxrs.client.proxy.type", "HTTP");

Client c = cb.build();  

See here for the documentation.

On ReastEasy/Wildfly, the only option to set the proxy without compile-dependency on RestEasy, was to use refelection to access the defaultProxy-method that only exists on the ResteasyClientBuilder:

ClientBuilder cb = ClientBuilder.newBuilder();
cb = (ClientBuilder) (cb.getClass().getMethod("defaultProxy", String.class, int.class).invoke(cb,
				"proxy", 81));
Client c = cb.build();

It would be a nice enhancement to be able to set the proxy via properties like on Websphere.