@ServerEndpoint(
        value = Constants.JSON_ENDPOINT_PATH,
        configurator = GuiceServerEndpointConfigurator.class
)
public class JSONServerEndpoint {
   public JSONServerEndpoint(SomeDependency dep){...}
   ...
}

public class GuiceServerEndpointConfigurator extends ServerEndpointConfig.Configurator {
    @Override
    public <T> T getEndpointInstance(Class<T> endpointClass) throws InstantiationException {
        if(JSONServerEndpoint.class.equals(endpointClass){
            return new JSONServerEndpoint(new SomeDependency());
        }
        // PROD use case: instantiate the given class with Guice, so that stuff is injected!
        // return ServerBootstrap.injector().getInstance(endpointClass);
    }
}