Question

我是java servlet中的初学者...我正在进行curd操作... ...数据不显示请告诉我哪里错了....

DAO.java

public void select(){

        try{
            Class.forName(jdbcDriver);
            Connection conn = DriverManager .getConnection(connectionString,username,password);
        PreparedStatement create = conn.prepareStatement("select * from student.users");

            ResultSet rs;
            rs = create.executeQuery();
            while(rs.next()){
                int id =rs.getInt("id");
                String name = rs.getString("fname");

            }
            }
            catch(Exception e){
                e.printStackTrace();
            }

    }

servlet.java

DAO user = new DAO();   /// i create object of DAO
user.select();  //calling select method

out.write("<td>");
                    out.write("<p>"+user.id+"</p>");    ///tried many from different way
            out.write("</td>");
            out.write("<td>"); 
                    out.write("<p>"+user.name+"</p>");    ///
            out.write("</td>");

我从不同的方式尝试了许多但没有运气....你能告诉我我错在哪里

Answer 1

我看到一些问题。首先，我认为你的select方法应该返回一个用户列表。所以你可以按照下面的方式实现它

public List<User> select(){
    List<User> result = new ArrayList<User>();
    try{
        Class.forName(jdbcDriver);
        Connection conn = DriverManager .getConnection(connectionString,username,password);
    PreparedStatement create = conn.prepareStatement("select * from student.users");

        ResultSet rs;
        rs = create.executeQuery();
        while(rs.next()){
            int id =rs.getInt("id");
            String name = rs.getString("fname");
            User user = new User(); // Creating a user object to fill with user data (I imagine that you have a user class in your model)
            user.setId(id);
            user.setName(name);
            //Add the retrived user to the list
            result.add(user);

        }
        //Returning the list of users.
        return result;
    }catch(Exception e){
        e.printStackTrace();
        return null;
    }

}

因此，在您的servlet中，您可以调用此方法来返回此列表并展示它。

 DAO daoUser = new DAO();   /// i create object of DAO
 List<User> users = daoUser.select();  //calling select to get list of users
 out.write("<table>");
 for(User user : users){ //Running through the list to show all users retrived
    out.write("<td>");
    out.write("<p>"+user.id+"</p>");    
    out.write("</td>");
    out.write("<td>"); 
    out.write("<p>"+user.name+"</p>");    ///
    out.write("</td>");
 }
 out.write("</table>");

就是这样。我不确定它是否会起作用的逻辑可能会忘记一些细节，但这是可行的方法。希望有所帮助。

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