Question

我正在编写一个Webapp，其中有数据必须从数据库中提取并显示在文本框中。一切都很好。但我需要在No Data Available这些文本框中显示if(!rs.next())的值，目前我正在尝试以下代码。

package org.DAO;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.Statement;
import java.util.ArrayList;
import java.util.List;

import org.bean.UserBean;
import org.code.general.DBConnection;
import org.code.general.GetTheUserDetails;

public class GetDataDAO {
    DBConnection dbConnection = new DBConnection();
    GetTheUserDetails getTheUserDetails = new GetTheUserDetails();

    public List<UserBean> list(String systemUserName) throws Exception {

        List<UserBean> userBeans = new ArrayList<UserBean>();
        try {

            Connection conn = dbConnection.getConn();
            Statement stmt = dbConnection.getStmt();
            ResultSet rs = dbConnection.getRs();
            String excelPath = dbConnection.getExcelPath();
            String queryString = null;
            dbConnection.createClassForNameForExcel();
            conn = DriverManager.getConnection(
                    "jdbc:odbc:Driver={Microsoft Excel Driver (*.xls)};DBQ=" + excelPath + "; READONLY=FALSE;");
            System.out.println("Connecting to database…");
            System.out.println("Oracle JDBC Driver Registered!");
            if (conn != null) {
                System.out.println("You made it, take control your database now!");
            } else {
                System.out.println("Failed to make connection!");
            }
            stmt = conn.createStatement();
            queryString = "Select * from [" + dbConnection.getSheetPath()
                    + "$] where STATE IS NULL and [Case Owner] = '"
                    + getTheUserDetails.getUserNameDetails(systemUserName) + "'";
            System.out.println("Query is " + queryString);
            rs = stmt.executeQuery(queryString);
            ResultSetMetaData rsmd = rs.getMetaData();
            System.out.println("bno of cols are " + rsmd.getColumnCount());
            while (rs.next()) {
                if (rs.next()) {
                    UserBean userBean = new UserBean();
                    userBean.setCaseNumber(rs.getString(1));
                    userBean.setCaseOwner(rs.getString(2));
                    userBean.setStatus(rs.getString(4));
                    userBean.setIssue(rs.getString(5));
                    userBean.setReason(rs.getString(6));
                    userBean.setDateOpened(rs.getString(7));
                    userBean.setAge(rs.getInt(8));
                    userBeans.add(userBean);
                } else {
                    UserBean userBean = new UserBean();
                    userBean.setCaseNumber("None");
                    userBean.setCaseOwner("None");
                    userBean.setStatus("None");
                    userBean.setIssue("None");
                    userBean.setReason("None");
                    userBean.setDateOpened("None");
                    userBean.setAge(Integer.parseInt("None"));
                    userBeans.add(userBean);
                }
            }

            rs.close();
            conn.commit();
            conn.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
        return userBeans;
    }
}

根据我的理解，我写过尝试以下算法，

if(rs.next)
   then create the bean and add the values from database,
 else 
   create the bean and add the values as None

请让我知道我哪里出错了，我该如何解决这个问题。

由于

Answer 1

if(rs.next)
   then create the bean and add the values from database,
 else 
     set the bean as null

在jsp中检查bean是否为空，然后在文本框下为No Data Available。理想情况下，您应该在servlet或控制器中执行此操作，因为jsp应该尽可能轻便

Answer 2

每次迭代代码都跳过一行：

while (rs.next()) {
    if (rs.next())

如果结果集为0或1行，则只需使用rs.next()：

if (rs.next())
{
    // Use 'rs'.
}
else
{
    // "None".
}

此外：

对null的{{1}}检查是多余的，因为DriverManager.getConnection()如果失败则会引发异常。
使用try-with-resources确保始终释放conn，Connection和Statement。

Answer 3

public boolean next（）

将光标移动到下一行。如果结果集中没有更多行，则此方法返回false。

因此，当您拨打rs.next()两次，一次在while，一次在if时，您每次都会错过一行。

尝试删除if条件

Answer 4

如果行为空，则必须具有这样的逻辑：

        rs = stmt.executeQuery(queryString);
        ResultSetMetaData rsmd = rs.getMetaData();

        System.out.println("bno of cols are " + rsmd.getColumnCount());

        while (rs.next()) {
                UserBean userBean = new UserBean();
                userBean.setCaseNumber(rs.getString(1));
                userBean.setCaseOwner(rs.getString(2));
                userBean.setStatus(rs.getString(4));
                userBean.setIssue(rs.getString(5));
                userBean.setReason(rs.getString(6));
                userBean.setDateOpened(rs.getString(7));
                userBean.setAge(rs.getInt(8));
                userBeans.add(userBean);
        }
        if(userBeans.size() == 0) {
                UserBean userBean = new UserBean();
                userBean.setCaseNumber("None");
                userBean.setCaseOwner("None");
                userBean.setStatus("None");
                userBean.setIssue("None");
                userBean.setReason("None");
                userBean.setDateOpened("None");
                userBean.setAge(Integer.parseInt("None"));
                userBeans.add(userBean);
        }

        rs.close();
        conn.commit();
        conn.close();

但是如果行不为空，只包含空值，则可以使用如下逻辑：

        rs = stmt.executeQuery(queryString);
        ResultSetMetaData rsmd = rs.getMetaData();
        System.out.println("bno of cols are " + rsmd.getColumnCount());

        while (rs.next()) {
                UserBean userBean = new UserBean();
                String value = rs.getString(1) == null ? "None" : rs.getString(1);
                // you do this for all the columns, from 1 to 8
                userBean.setCaseNumber(rs.getString(1));
                userBean.setCaseOwner(rs.getString(2));
                userBean.setStatus(rs.getString(4));
                userBean.setIssue(rs.getString(5));
                userBean.setReason(rs.getString(6));
                userBean.setDateOpened(rs.getString(7));
                userBean.setAge(rs.getInt(8));
                userBeans.add(userBean);
        }

        rs.close();
        conn.commit();
        conn.close();

填充空值为

4 个答案: