code: private void initServlet(Servlet servlet, ServletConfig config) 
      	throws ServletException
      {
          Principal user=null;
          try
          {
              // Handle run as
              if (_runAs!=null && _realm!=null)
                  user=_realm.pushRole(null,_runAs);
              servlet.init(config);
          }
          finally
          {
              // pop run-as role
              if (_runAs!=null && _realm!=null && user!=null)
                  _realm.popRole(user);
          }
      }
docs: /* ------------------------------------------------------------