java - MySQL Netbeans Java 结果集失败

Question

您好，我在 Netbeans 中安装了 MySql，并且一直在尝试将 Java 与 MySQL 结合使用，但是在运行它时遇到了问题。我的数据库称为“测试”，我的表是“任务”。我拥有的两列是:“id”和“task”(我意识到将变量命名为与表相同的名称可能不是一个好主意)。我在代码区域也有一个附带问题，询问它的作用。这是错误:运行:

May 22, 2015 11:52:25 PM databasetest.DatabaseTest main
SEVERE: Operation not allowed after ResultSet closed
java.sql.SQLException: Operation not allowed after ResultSet closed
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1074)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:988)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:974)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:919)
at com.mysql.jdbc.ResultSetImpl.checkClosed(ResultSetImpl.java:804)
at com.mysql.jdbc.ResultSetImpl.next(ResultSetImpl.java:6986)
at databasetest.DatabaseTest.main(DatabaseTest.java:43)

BUILD SUCCESSFUL (total time: 41 seconds)

这是我的代码:

package databasetest;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.logging.Level;
import java.util.logging.Logger;

public class DatabaseTest {

public static void main(String[] args) {

    Connection con = null;
    Statement st = null;
    ResultSet rs = null;
    PreparedStatement pst = null;

    String url = "jdbc:mysql://localhost:3306/test";
    String user = "root";
    String password = "cinder";

    try {

        String author = "Trygve Gulbranssen";

        con = DriverManager.getConnection(url, user, password);
        st = con.createStatement();
        rs = st.executeQuery("SELECT VERSION()");
          //^^ what is VERSION? What is this supposed to be doing?            

        for (int i=1; i<=1000; i++) {
            String query;
            query = "INSERT INTO task(task) VALUES(" + 2*i + ")";
            st.executeUpdate(query);
        }

        if (rs.next()) {
            System.out.println(rs.getString(1));
        }

    } catch (SQLException ex) {
        Logger lgr = Logger.getLogger(DatabaseTest.class.getName());
        lgr.log(Level.SEVERE, ex.getMessage(), ex);

    } finally {
        try {
            if (rs != null) {
                rs.close();
            }
            if (st != null) {
                st.close();
            }
            if (con != null) {
                con.close();
            }

        } catch (SQLException ex) {
            Logger lgr = Logger.getLogger(DatabaseTest.class.getName());
            lgr.log(Level.WARNING, ex.getMessage(), ex);
        }
    }
}
}

Answer 1

SELECT VERSION() 旨在告诉您您的 MySQL 版本。首先，打印 SELECT 的结果，然后运行其他查询。使用 Statement 运行中间插入查询会隐式关闭 ResultSet，因此会出现错误。移动

if (rs.next()) {
    System.out.println(rs.getString(1));
}

运行前

for (int i=1; i<=1000; i++) {
    // String query;
    String query = "INSERT INTO task(task) VALUES(" + 2*i + ")";
    st.executeUpdate(query);
}