public class User {
    private String userid ;
    private String name ;
    private String password ;

    public void setUserid(String userid){
        this.userid = userid ;
    }
    public void setName(String name){
        this.name = name ;
    }
    public void setPassword(String password){
        this.password = password ;
    }
    public String getUserid(){
        return this.userid ;
    }
    public String getName(){
        return this.name ;
    }
    public String getPassword(){
        return this.password ;
    }
}

public class UserDAOImpl implements IUserDAO {
    private Connection conn = null ;
    private PreparedStatement pstmt = null ;
    public UserDAOImpl(Connection conn){
        this.conn = conn ;
    }
    public boolean findLogin(User user) throws Exception{
        boolean flag = false ;
        String sql = "SELECT name FROM user WHERE userid=? AND password=?" ;
        this.pstmt = this.conn.prepareStatement(sql) ;
        this.pstmt.setString(1,user.getUserid()) ;
        this.pstmt.setString(2,user.getPassword()) ;
        ResultSet rs = this.pstmt.executeQuery() ;
        if(rs.next()){
            user.setName(rs.getString(1)) ;    // 取出一个用户的真实姓名
            flag = true ;
        }
        this.pstmt.close() ;
        return flag ;
    }
} 

public class UserDAOProxy implements IUserDAO {
    private DatabaseConnection dbc = null ;
    private IUserDAO dao = null ;
    public UserDAOProxy(){
        try{
            this.dbc = new DatabaseConnection() ;
        }catch(Exception e){
            e.printStackTrace() ;
        }
        this.dao = new UserDAOImpl(dbc.getConnection()) ;
    }
    public boolean findLogin(User user) throws Exception{
        boolean flag = false ;
        try{
            flag = this.dao.findLogin(user) ;    // 调用真实主体，完成操作
        }catch(Exception e){
            throw e ;
        }finally{
            this.dbc.close() ;
        }
        return flag ;
    }
} 

public class DAOFactory {
    public static IUserDAO getIUserDAOInstance(){
        return new UserDAOProxy() ;
    }
}

public class LoginServlet extends HttpServlet {
    public void doGet(HttpServletRequest req,HttpServletResponse resp) throws ServletException,IOException{
        String path = "login.jsp" ;
        String userid = req.getParameter("userid") ;
        String userpass = req.getParameter("userpass") ;
        List<String> info = new ArrayList<String>() ;    // 收集错误
        if(userid==null || "".equals(userid)){
            info.add("用户id不能为空！") ;
        }
        if(userpass==null || "".equals(userpass)){
            info.add("密码不能为空！") ;
        }
        if(info.size()==0){    // 里面没有记录任何的错误
            User user = new User() ;
            user.setUserid(userid) ;
            user.setPassword(userpass) ;
            try{
                if(DAOFactory.getIUserDAOInstance().findLogin(user)){
                    info.add("用户登陆成功，欢迎" + user.getName() + "光临！") ;
                } else {
                    info.add("用户登陆失败，错误的用户名和密码！") ;
                }
            }catch(Exception e){
                e.printStackTrace() ;
            }
        }
        req.setAttribute("info",info) ;
        req.getRequestDispatcher(path).forward(req,resp) ;
    }
    public void doPost(HttpServletRequest req,HttpServletResponse resp) throws ServletException,IOException{
        this.doGet(req,resp) ;
    }