如何解决jdbc连接错误？ 为什么我无法从 mysql 服务器获取表数据？

Question

import com.mysql.jdbc.Driver;

import java.sql.*;

public class JdbcDemo {
    public static class jdbcTest
    {
        public static void main(String[] args) throws Exception
        {
            String url = "jdbc:mysql://localhost:3306/coffee_db";
            String uname = "Jeep";
            String pass = "random";
            String query = "select * from customers;";

            Class.forName("com.mysql.jdbc.Driver");
            Connection con = DriverManager.getConnection(url,uname,pass);
            Statement st = con.createStatement();
            ResultSet rs = st.executeQuery("query");

            while (rs.next()) {
                String name = rs.getString("first_name");
                System.out.println(name);
            }
            st.close();
            con.close();
        }
    }

Error:

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.
Exception in thread "main" java.sql.SQLException: Statement.executeQuery() cannot issue statements that do not produce result sets.
    at enter code herecom.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.StatementImpl.executeQuery(StatementImpl.java:1135)
    at com.company.JdbcDemo$jdbcTest.main(JdbcDemo.java:22)

Answer 1

Can you try and replace

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

with

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

I think that the name of the class that implements java.sql.Driver in MySQL Connector/J has changed from com.mysql.jdbc.Driver to com.mysql.cj.jdbc.Driver.

Answer 2

There are two issues:

1. Class.forName("com.mysql.jdbc.Driver"); - this JDBC driver is old and has been replaced by the stated new one. This is only a warning. This is not the cause of your problem.

You do not need to use Class.forName . Just remove that line. The correct driver will be used when you call DriverManager.getConnection , as long as you have the correct driver library in your classpath.

2. This statement:

   ResultSet rs = st.executeQuery("query");

is trying to execute the query "query". But "query" is not a valid SQL query. You need to use the variable name instead:

ResultSet rs = st.executeQuery(query);