The class org.jboss.ws.core.CommonClient contains the following member variable: -

// Output parameters
protected EndpointInvocation epInv;

Within CommonClient this variable is only accessed from within the invoke method and each time the invoke method is called the EndpointInvocation is recreated - however the member variable is in place as org.jboss.ws.core.jaxrpc.client.CallImpl which extends CommonClient access it for subsequent calls.