Question

在动态网络应用程序中，我遇到了一些问题，所以我发布了我的代码：

这是我的控制器（servlet）：

        String select = request.getParameter("select");  // getting proper value
        String search = request.getParameter("search");  // getting proper value
        request.setAttribute("select", select);
        request.setAttribute("search", search);
        System.out.println("Select : "+select+" Search : "+search);

            int page = 1;
            int recordsPerPage = 20;
            if(request.getParameter("page") != null)
                page = Integer.parseInt(request.getParameter("page"));
            SearchDAO searchDAO=new SearchDAO();
            List<User> list=searchDAO.searchAllUsers((page-1)*recordsPerPage,recordsPerPage,select,search);
            int noOfRecords = searchDAO.getNoOfRecords();
                    System.out.println("4> NoOfRecords : "+noOfRecords);
            int noOfPages = (int) Math.ceil(noOfRecords * 1.0 / recordsPerPage);
                    System.out.println("5> NoOfPages : "+noOfPages);
            request.setAttribute("searchList", list);
            System.out.println("6> List : "+list);
            request.setAttribute("noOfPages", noOfPages); // Getting null value, 0
            request.setAttribute("currentPage", page); // Getting null value, 0
            RequestDispatcher view = request.getRequestDispatcher("/jsp/Securex_Anti_Theft_SearchUserList.jsp");
            view.forward(request, response);

这是我的DAO（简单的java类）：

public class SearchDAO {
private int noOfRecords;
Connection connection;
Statement stmt;

    public List<User> searchAllUsers(int offset ,int noOfRecords,String select,String search){
    private static Connection getConnection() throws SQLException,ClassNotFoundException{
    Connection con = ConnectionFactory.getInstance().getConnection();
    return con;  //ConnectionFactory is class for making the connection to DB.
}
     String query="select SQL_CALC_FOUND_ROWS * from info where '"+select+
"' like '%"+search+"%' order by serialNo asc limit 
" + offset + " , " + noOfRecords;

    List<User> list1 = new ArrayList<User>();
    User user1=null;

    try {
        connection = getConnection();
        stmt = connection.createStatement();
        ResultSet rs=stmt.executeQuery(query);
        System.out.println("1> :"+rs);
        while(rs.next()){
        user1=new User();
        user1.setSerial(rs.getInt(1));
        System.out.println("I'm inside a loop");
        user1.setName(rs.getString(2));
        user1.setEmail(rs.getString(3));
        list1.add(user1);
        }

        rs.close();
        rs = stmt.executeQuery("SELECT FOUND_ROWS()");
        System.out.println("2> :" +rs);
        if(rs.next())
            this.noOfRecords = rs.getInt(1); 
           System.out.println("3> :" +this.noOfRecords);

    } catch (SQLException e) {
        e.printStackTrace();
    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    }finally
    {
        try {
            if(stmt != null)
                stmt.close();
            if(connection != null)
                connection.close();
            } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    return list1;

}

public int getNoOfRecords() {
    return noOfRecords;
}
}

输出是：

1> :com.mysql.jdbc.JDBC4ResultSet@1af8502
2> :com.mysql.jdbc.JDBC4ResultSet@455aa8
3> :0
4> NoOfRecords : 0
5> NoOfPages : 0
6> List : []

我为select all用户提供相同的servlet和类，并且正常工作，bbut从此处获取null值.Even ResultSet返回值即com.mysql.jdbc.JDBC4ResultSet @ 88d319，但无法从DAO获取值的servlet。

从//属性获取空值，对我而言，一切都应该没问题，但仍然会得到空值，随时指出我的错误，如果我做了什么

Answer 1

很可能你的DAO返回空列表。所有其他代码看起来很好。所以，首先我会检查这个电话

searchDAO.searchAllUsers((page-1)*recordsPerPage,recordsPerPage,select,search);

page变量的值是多少？其他参数？请在以下后面记录查询：

 String query="select SQL_CALC_FOUND_ROWS * from info where '"+select+
"' like '%"+search+"%' order by serialNo asc limit 
" + offset + " , " + noOfRecords;

并针对DB单独运行，它应该给你一个答案。

从属性获取Null值

1 个答案: