如题 这是错误是什么意思呢？？？？？？
  

package QQsever.Model;
import *;
import *;
import java.util.*;
import *;
public class QqseverModel {
   

    public QqseverModel(){
        try{
            System.out.println("现在在9998端口监听");            
            ServerSocket ss=new ServerSocket(9998);            
            Socket s=ss.accept();        
            ObjectInputStream ois=new ObjectInputStream(s.getInputStream());
             u=(User)ois.readObject();
            Message ms=new Message();
            if(u.getMima().equals("123456")){
                //返回一个成功登入的信息包            
                ms.setMesType("1");
            }else{
                ms.setMesType("2");
            }
            ObjectOutputStream oos=new ObjectOutputStream(s.getOutputStream());
            oos.writeObject(ms);
            s.close();
           
        }catch (Exception e) {
            e.printStackTrace();
        }finally{
            
        }
        
    }
}
出错在    u=(User)ois.readObject();
 这句话

现在在9998端口监听
java.lang.ClassNotFoundException: com.User
    at (Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at (Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClassInternal(Unknown Source)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Unknown Source)
    at (Unknown Source)
    at (Unknown Source)
    at (Unknown Source)
    at (Unknown Source)
    at (Unknown Source)
    at (Unknown Source)
    at QQsever.Model.QqseverModel.<init>(QqseverModel.java:15)
    at QQsever.jiemian.QQsverzjm.main(QQsverzjm.java:10)