public abstract class CustomServlet extends HttpServlet {

    private static AtomicBoolean loaded = new AtomicBoolean();

    @Override
    public void init() throws ServletException {
        if (!loaded.getAndSet(true)) {
            InputStream input = getServletContext().getResourceAsStream("/WEB-INF/custom-config.xml");
            // ...
        }
    }

    // ...
}

@WebListener
public class Config implements ServletContextListener {

    @Override
    public void contextInitialized(ServletContextEvent event) {
        InputStream input = event.getServletContext().getResourceAsStream("/WEB-INF/custom-config.xml");
        // ...
    }

    // ...
}

public abstract class BaseServlet extends HttpServlet{

public Base(){
loadXml();
}
private void loadXml(){
...
}
}

public class Servlet1 extends BaseServlet {
...
}