请问：1：用smack+ejabberd做即时通讯，用户注册时如何判断用户已经注册过？
2：修改用户密码时：可不可以做到不登陆就可以由管理员修改用户的密码？
3：网上某位大神写的查询用户的代码，在我的电脑上运行，报503异常，怎么解决
public static List<UserBean> searchUsers(XMPPConnection connection,String serverDomain,String userName) throws XMPPException  
    {  
        List<UserBean> results = new ArrayList<UserBean>();  
        System.out.println("查询开始..............."+connection.getHost()+connection.getServiceName());  
         
        UserSearchManager usm = new UserSearchManager(connection);  
         
         
        Form searchForm = usm.getSearchForm(serverDomain);  
        Form answerForm = searchForm.createAnswerForm();  
        answerForm.setAnswer("Username", true);  
        answerForm.setAnswer("search", userName);  
        ReportedData data = usm.getSearchResults(answerForm, serverDomain);  
           
         Iterator<Row> it = data.getRows();  
         Row row = null;  
         UserBean user = null;  
         while(it.hasNext())  
         {  
             user = new UserBean();  
             row = it.next();  
             user.setUserName(row.getValues("Username").next().toString());  
             user.setName(row.getValues("Name").next().toString());  
             user.setEmail(row.getValues("Email").next().toString());  
             System.out.println(row.getValues("Username").next());  
             System.out.println(row.getValues("Name").next());  
             System.out.println(row.getValues("Email").next());  
             results.add(user);  
             //若存在，则有返回,UserName一定非空，其他两个若是有设，一定非空  
         }  
           
         return results;  
    }