SSH用户注册老掉牙报attempt to create saveOrUpdate event with null entity这个

SSH用户注册老报attempt to create saveOrUpdate event with null entity这个
代码都在这儿求给为大侠给瞅瞅
数据库里有一个user表，三个字段id,username,password
_________________________________
User.java:
public class User implements java.io.Serializable
{
private int id;
private String username;
private String password;
public int getId()
{
return id;
}
public void setId(int id)
{
this.id = id;
}
public String getUsername()
{
return username;
}
public void setUsername(String username)
{
this.username = username;
}
public String getPassword()
{
return password;
}
public void setPassword(String password)
{
this.password = password;
}
public User(String username, String password)
{
super();
this.username = username;
this.password = password;
}

}
___________________________________________________
UserDao.java

public interface UserDao
{
public abstract boolean addUser(User user);
}
---------------------
UserDaoImpl.java
public class UserDaoImpl implements UserDao
{
private SessionFactory sessionFactory;


public void setSessionFactory(SessionFactory sessionFactory)
{
this.sessionFactory = sessionFactory;
}

@Override
public boolean addUser(User user)
{
try {
HibernateTemplate hibernateTemplate =new HibernateTemplate(sessionFactory);
hibernateTemplate.saveOrUpdate(user);
return true;
} catch (DataAccessException e) {
e.printStackTrace();
return false;
}

}

}
--------------------------
RegisterAction.java:

{
private User user;
private UserDaoImpl userDaoImpl;

public User getUser()
{
return user;
}

public void setUser(User user)
{
this.user = user;
}

public void setUserDaoImpl(UserDaoImpl userDaoImpl)
{
this.userDaoImpl = userDaoImpl;
}

@Override
public String execute() throws Exception
{
if(userDaoImpl.addUser(user))
return SUCCESS;
else
return INPUT;
}

}
---------------------
applicationContext.xml: