public class Login extends ActionSupport implements SessionAware,ServletResponseAware

 {

     private HttpServletResponse response = null;

 　　　　....

     public void setServletResponse(HttpServletResponse response) {

         // TODO Auto-generated method stub

         this.response = response;

     }

     public String execute()

     {

         Person person = new Person();

         PersonDAO personDAO = new PersonDAO();

         if(personDAO.findById(id)==null)

         {

             System.out.println("用户不存在");

             try {

                 response.setContentType("text/html; charset=UTF-8");

                 response.setCharacterEncoding("UTF-8");        //防止乱码；

                 PrintWriter out = response.getWriter();

                 out.print("<Script>alert('用户不存在!')</Script>");

                 out.flush();

                 out.close();

             }

             catch (IOException e) {

                 // TODO Auto-generated catch block

                 e.printStackTrace();

             }

         }

     ....

     }

 }