<form action="sys/login.action" method="post">
　　<input type="text" name="username">
　　<input type="submit" value="submit">
</form>

public class sysAction extends ActionSupport{
    private String username;

    public String login() throws Exception {
        System.out.println(username);
        return SUCCESS;
    }

    public String getUsername() {
        return username;
    }
    public void setUsername(String username) {
        this.username= username;
    }
}

<form action="sys/login.action" method="post">
　　<input type="text" name="username">
　　<input type="submit" value="submit">
</form>

public class sysAction extends ActionSupport implements ModelDriven<User>{
    private User user;

    public String login() throws Exception {
        System.out.println(getModel().getUsername());
        return SUCCESS;
    }

    public User getModel() {
        if (null == user) {
            return user = new User();
        }    
        return user;
    }
}

<form action="sys/login.action" method="post">
　　<input type="text" name="user.username">
　　<input type="text" name="teacher.level">
　　<input type="submit" value="submit">
</form>

public class sysAction extends ActionSupport{
　　 //using spring ioc or new manually　
    private User user=new User();
    private Teacher teacher=new Teacher();

    public String login() throws Exception {
        System.out.println(user.getUsername());
        System.out.println(teacher.getLevel());
        return SUCCESS;
    }

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