在 JDBC 中查看 ResultSet 时不检索记录
Vin*_*ino 2 java mysql jdbc resultset data-retrieval
现在我正在学习 Java 中的 ResultSet 类型。在这里，我已经编码以不同方式查看记录。起初我显示了 emp4 表中的全部记录，然后我开始以不同的方式查看这些记录（最后，第一，下一个） 这正是我正在寻找的，但它不会显示所有的记录在 emp4 表中。请参阅第一个程序，它不起作用，但如果我记录了第 41 行（请参阅第二个程序中的此内容），它就可以正常工作。有什么问题 ？我的代码有问题吗？？？
代码示例 1
package demojdbc;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.Statement;
import java.sql.ResultSet;
import java.sql.SQLException;
public class MysqlCon{
private static final String DB_DRIVER = "com.mysql.jdbc.Driver";
private static final String DB_CONNECTION = "jdbc:mysql://localhost:3306/vinoth";
private static final String DB_USER = "root";
private static final String DB_PASSWORD = "vino";
public static void main(String args[])throws SQLException{
//Creating statement and connection
Connection dbConnection = null;
Statement stmt = null;
try{
//Creating class driver
Class.forName(DB_DRIVER);
//Creating Database Connection
dbConnection = DriverManager.getConnection(DB_CONNECTION,DB_USER,DB_PASSWORD);
//Creating statement
stmt = dbConnection.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE,ResultSet.CONCUR_READ_ONLY);
//Creating query
String sql = "SELECT id,gmail,yahoo from emp4";
//Creating ResultSet
ResultSet rs = stmt.executeQuery(sql);
//Displaying database
System.out.println("Displaying records before doing some operations");
System.out.println(rs.getInt(1)+" "+rs.getString(2)+" "+rs.getString(3));
System.out.println("Displaying records for last row");
rs.last();
int id = rs.getInt("id");
String gmail = rs.getString("gmail");
String yahoo = rs.getString("yahoo");
//Displaying records in last row
System.out.println("ID : "+id);
System.out.println("GMAIL : "+gmail);
System.out.println("YAHOO : "+yahoo);
System.out.println();
rs.first();
System.out.println("Displaying records for first row");
id = rs.getInt("id");
gmail = rs.getString("gmail");
yahoo = rs.getString("yahoo");
//Displaying records in last row
System.out.println("ID : "+id);
System.out.println("GMAIL : "+gmail);
System.out.println("YAHOO : "+yahoo);
System.out.println();
rs.next();
System.out.println("Displaying records for next row");
id = rs.getInt("id");
gmail = rs.getString("gmail");
yahoo = rs.getString("yahoo");
//Displaying records in last row
System.out.println("ID : "+id);
System.out.println("GMAIL : "+gmail);
System.out.println("YAHOO : "+yahoo);
}catch(SQLException e){
e.printStackTrace();
}catch(ClassNotFoundException e){
System.out.println("Plese check the driver class path "+e.getMessage());
}finally{
if(stmt != null){
stmt.close();
}
if(dbConnection != null){
dbConnection.close();
}
}
}
}
在这里，代码将正常工作......
代码示例 2
package demojdbc;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.Statement;
import java.sql.ResultSet;
import java.sql.SQLException;
public class MysqlCon{
private static final String DB_DRIVER = "com.mysql.jdbc.Driver";
private static final String DB_CONNECTION = "jdbc:mysql://localhost:3306/vinoth";
private static final String DB_USER = "root";
private static final String DB_PASSWORD = "vino";
public static void main(String args[])throws SQLException{
//Creating statement and connection
Connection dbConnection = null;
Statement stmt = null;
try{
//Creating class driver
Class.forName(DB_DRIVER);
//Creating Database Connection
dbConnection = DriverManager.getConnection(DB_CONNECTION,DB_USER,DB_PASSWORD);
//Creating statement
stmt = dbConnection.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE,ResultSet.CONCUR_READ_ONLY);
//Creating query
String sql = "SELECT id,gmail,yahoo from emp4";
//Creating ResultSet
ResultSet rs = stmt.executeQuery(sql);
//Displaying database
System.out.println("Displaying records before doing some operations");
//System.out.println(rs.getInt(1)+" "+rs.getString(2)+" "+rs.getString(3));
System.out.println("Displaying records for last row");
rs.last();
int id = rs.getInt("id");
String gmail = rs.getString("gmail");
String yahoo = rs.getString("yahoo");
//Displaying records in last row
System.out.println("ID : "+id);
System.out.println("GMAIL : "+gmail);
System.out.println("YAHOO : "+yahoo);
System.out.println();
rs.first();
System.out.println("Displaying records for first row");
id = rs.getInt("id");
gmail = rs.getString("gmail");
yahoo = rs.getString("yahoo");
//Displaying records in last row
System.out.println("ID : "+id);
System.out.println("GMAIL : "+gmail);
System.out.println("YAHOO : "+yahoo);
System.out.println();
rs.next();
System.out.println("Displaying records for next row");
id = rs.getInt("id");
gmail = rs.getString("gmail");
yahoo = rs.getString("yahoo");
//Displaying records in last row
System.out.println("ID : "+id);
System.out.println("GMAIL : "+gmail);
System.out.println("YAHOO : "+yahoo);
}catch(SQLException e){
e.printStackTrace();
}catch(ClassNotFoundException e){
System.out.println("Plese check the driver class path "+e.getMessage());
}finally{
if(stmt != null){
stmt.close();
}
if(dbConnection != null){
dbConnection.close();
}
}
}
}
输出
显示最后一行 ID 的记录：5 GMAIL : naveen YAHOO : naveenrockz
显示第一行 ID 的记录：1 GMAIL : vinothvino YAHOO : vinothasd
显示下一行 ID 的记录：2 GMAIL：ajithvirje YAHOO：ajith234
请让我明白。为什么我的代码没有在 CODE SAMPLE 1 程序中获取任何记录？
下图表示 emp4 表中的以下记录
原因是您没有ResultSet通过next()在使用getInt()/访问数据之前调用来推进 的游标getString()。试试这样的：
//Creating ResultSet
ResultSet rs = stmt.executeQuery(sql);
//Displaying database
System.out.println("Displaying records before doing some operations");
if (rs.next()) {
System.out.println(rs.getInt(1) + " "
+ rs.getString(2) + " " + rs.getString(3));
}
如果要遍历整个结果集，请while (rs.next())改用。
您的第二个代码段有效，因为您rs.last()在第一次访问列值之前将光标移动到最后一个位置。
请注意，你总是应该检查返回值rs.next()/ rs.last()/rs.first()访问的列值之前的方法。falsey 返回值表示结果集没有行，并且在调用结果集的任何 getter（rs.getInt()/rs.getString()等）方法时会导致抛出异常。