Method A:
ServletActionContext.getResponse() (works internally using a ThreadLocal)

Method B:
Have the action implements ResponseAware and the response will be set through setServletResponse(HttpServletResponse). The action needs to have 'servlet-config' interceptor added to it.

@see webwork-default.xml
@see com.opensymphony.webwork.interceptor.ServletResponseAware
@see com.opensymphony.webwork.interceptor.ServletConfigInterceptor