SQLException: 结果集已关闭

Question

我是 Java 使用 NetBeans 的初学者，我试图创建类似于教师注册系统的东西。我使用 SQL Server 2005 创建数据库。在实施过程中，我试图创建一个功能，让学生能够注册他们的科目，因此该功能基本上搜索学生已经完成其先决条件的科目。所以我写了下面的代码：

package GUIs;
import java.sql.*;
import javax.swing.*;


public class AddSubToStd extends javax.swing.JFrame {

final void FillList1(){
    try{
    String url = "jdbc:sqlserver://localhost:1433;databaseName=BIS";
    String username = "sa";
    String password = "*****";
    Connection conn  = DriverManager.getConnection(url,username,password);
    Statement stmt = conn.createStatement();
    DefaultListModel DLM = new DefaultListModel();
    ResultSet res = stmt.executeQuery("SELECT * FROM Students");
    while(res.next()){
        DLM.addElement(res.getString("ID"));
    }
    List1.setModel(DLM);
}
    catch(SQLException e){
        JOptionPane.showMessageDialog(null, e.toString());
    }
}

final void FillList2(){
    try{
    String url = "jdbc:sqlserver://localhost:1433;databaseName=BIS";
    String username = "sa";
    String password = "*****";
    Connection conn  = DriverManager.getConnection(url,username,password);
    Statement stmt = conn.createStatement();
    DefaultListModel DLM = new DefaultListModel();
    String Query = "SELECT * FROM FinishedCourses WHERE ID = '"+List1.getSelectedValue()+"'";
    ResultSet res = stmt.executeQuery(Query);

    ResultSet res1;
    String S_Code;
    String Query1;

    while(res.next()){

         S_Code = res.getString("S_Code");
         Query1 = "SELECT * From Subjects WHERE Prerequisite = '"+S_Code+"'";
         res1 = stmt.executeQuery(Query1);

        while(res1.next()){

            DLM.addElement(res.getString("S_Code"));
        }

    }
    conn.close();
    stmt.close();
    List2.setModel(DLM);
}
    catch(SQLException e){
        JOptionPane.showMessageDialog(null, e.toString());
    }
}

public AddSubToStd() {
    initComponents();
    FillList1();

}

但是当我尝试调用 FillList2()

时，我得到的 SQLException 表明结果集已关闭

 private void UpdateAllowedActionPerformed(java.awt.event.ActionEvent evt) {                                              
    try{
    String url = "jdbc:sqlserver://localhost:1433;databaseName=BIS";
    String username = "sa";
    String password = "*****";
    Connection conn  = DriverManager.getConnection(url,username,password);
    FillList2();    
    }
    catch(SQLException e){
        JOptionPane.showMessageDialog(null, e.toString());
    }

}

有人请帮助。

Answer 1

您正在重复使用同一个 Statement 对象来执行两个查询。当stmt用于执行第二个查询时，关闭前一个语句返回的ResultSet对象。为每个查询创建两个对象。

示例：

Statement stmt = conn.createStatement();
Statement stmt1 = conn.createStatement();
...
ResultSet res = stmt.executeQuery(Query);

...

while(res.next()){

     S_Code = res.getString("S_Code");
     Query1 = "SELECT * From Subjects WHERE Prerequisite = '"+S_Code+"'";
     res1 = stmt1.executeQuery(Query1); // use a separate statement

    while(res1.next()){

        DLM.addElement(res.getString("S_Code"));
    }

}

这在 Statement API docs 的以下引用中有解释：

By default, only one ResultSet object per Statement object can be open at the same time. Therefore, if the reading of one ResultSet object is interleaved with the reading of another, each must have been generated by different Statement objects. All execution methods in the Statement interface implicitly close a statment's current ResultSet object if an open one exists.

同时强烈建议关闭 JDBC 资源以 分配的相反顺序 ，即你应该关闭 Statement 然后关闭Connection，你应该在 finally 块中执行此操作：

catch(SQLException e){
    JOptionPane.showMessageDialog(null, e.toString());
} finally {
    if(res != null) {
        res.close();
    }
    if(res1 != null) {
        res1.close();
    }
    if(stmt != null) {
        stmt.close();
    }
    if(stmt1 != null) {
        stmt1.close();
    }
    if(conn != null) {
         conn.close();
    }
}

如果你使用的是Java7，你可以使用try-with-resources语句来自动关闭这些资源（无需显式调用close()方法）：

// try-with-resources statement declaring two resources
try(Connection conn  = DriverManager.getConnection(url,username,password);
    Statement stmt = conn.createStatement()) {
    ...
} catch(SQLException e){
    JOptionPane.showMessageDialog(null, e.toString());
}

try-with-resources 将确保关闭 Statement 对象，然后在使用它们后 Connection。

SQLException: 结果集已关闭

SQLException: the result set is closed

java

sql

jdbc

resultset