How do we implement a daemon (a program running all the time, like a scheduled program with a fixed-rate) inside Weblogic?

The best approach is to create an empty servlet, let's call it StartupServlet, that has standard web.xml option set to non-zero integer value. Place your code in the implementation of the servlet's "init" method. It's guaranteed that this method will be called on the server startup, that it will be called once, and that by that time all the server's services, including EJB, will be up.

Example:
import javax.servlet.*;
public class StartupServlet extends HttpServlet {

public void init(ServletConfig servletConfig) throws ServletException {
super.init(servletConfig);
ServiceManager serviceManager = ServiceManager.getInstance();
serviceManager.startServices();
}


public void destroy() {
ServiceManager serviceManager = ServiceManager.getInstance();
serviceManager.stopServices();
super.destroy();
}
}


web.xml