无法弄清IntelliJ [duplicate]中的JDBC错误

Question

此问题已经在这里有了答案：

Java - Can't use ResultSet after connection close （4个答案）

已关闭19天前。

目前，我正在IntelliJ和MySQL中学习JDBC。这是我在YouTube上从Telusko学到的代码：

package com.company;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Statement;

public class Main {

    public static void main(String[] args) throws Exception {
        String url = "jdbc:mysql://localhost:3306/customers";
        String uname = "root";
        String pass = "791005596";
        String query = "SELECT * FROM customers.registration\n" +
                "where password like '%123'";
        Class.forName("com.mysql.jdbc.Driver");
        Connection con = DriverManager.getConnection(url, uname, pass);
        Statement st = con.createStatement();
        ResultSet rs = st.executeQuery(query);

        String userData = "";
        while (rs.next()) {
            userData = rs.getInt("ID") + rs.getString(2);
            System.out.println(userData);

            st.close();
            con.close();
        }
    }
}

我已经在库中添加了连接器，因为我在IntelliJ上运行它，它以这种方式出现：

"C:\Program Files\Java\jdk-14.0.1\bin\java.exe" "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA Community Edition 2020.1.1\lib\idea_rt.jar=1821:C:\Program Files\JetBrains\IntelliJ IDEA Community Edition 2020.1.1\bin" -Dfile.encoding=UTF-8 -classpath "C:\Users\Jocelyn Huang\IdeaProjects\FSD\out\production\FSD;C:\Users\Jocelyn Huang\IdeaProjects\FSD\lib\mysql-connector-java-8.0.20.jar" com.company.Main
Loading class `com.mysql.jdbc.Driver'. This is deprecated. The new driver class is `com.mysql.cj.jdbc.Driver'. The driver is automatically registered via the SPI and manual loading of the driver class is generally unnecessary.
231245mamamoo123
Exception in thread "main" java.sql.SQLException: Operation not allowed after ResultSet closed
    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:129)
    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:97)
    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:89)
    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:63)
    at com.mysql.cj.jdbc.result.ResultSetImpl.checkClosed(ResultSetImpl.java:445)
    at com.mysql.cj.jdbc.result.ResultSetImpl.next(ResultSetImpl.java:1726)
    at com.company.Main.main(Main.java:22)

Process finished with exit code 1

我不知道这里发生了什么，第一行， 我尝试删除此一个：

Class.forName("com.mysql.jdbc.Driver");

然后再次运行，

"C:\Program Files\Java\jdk-14.0.1\bin\java.exe" "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA Community Edition 2020.1.1\lib\idea_rt.jar=2687:C:\Program Files\JetBrains\IntelliJ IDEA Community Edition 2020.1.1\bin" -Dfile.encoding=UTF-8 -classpath "C:\Users\Jocelyn Huang\IdeaProjects\FSD\out\production\FSD;C:\Users\Jocelyn Huang\IdeaProjects\FSD\lib\mysql-connector-java-8.0.20.jar" com.company.Main
231245mamamoo123
Exception in thread "main" java.sql.SQLException: Operation not allowed after ResultSet closed
    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:129)
    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:97)
    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:89)
    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:63)
    at com.mysql.cj.jdbc.result.ResultSetImpl.checkClosed(ResultSetImpl.java:445)
    at com.mysql.cj.jdbc.result.ResultSetImpl.next(ResultSetImpl.java:1726)
    at com.company.Main.main(Main.java:22)

Process finished with exit code 1

有人可以帮我吗？预先感谢。

Answer 1

发给驾驶员的消息只是警告，导致异常的实际错误是数据的读取。

在读取所有数据之前，请关闭连接和语句。这意味着当再次处理while循环时，无法再读取ResultSet。

您必须在循环后关闭连接和语句。

String userData = "";
while (rs.next()) {
    userData = rs.getInt("ID") + rs.getString(2);
    System.out.println(userData);
}

st.close();
con.close();