静态代码块加载properties配置文件，注册驱动
静态方法创建连接
静态方法关闭，一个方法关connection,statement，另一个方法多关一个resultSet

package com.aistart.tech.until;

import java.io.IOException;
import java.sql.*;
import java.util.Properties;


public class JDBCUtil {

    public static final Properties info = new Properties();

    static {
        try {
            info.load(JDBCUtil.class.getClassLoader().getResourceAsStream("com/aistart/tech/config/jdbc.properties"));
            Class.forName(info.getProperty("driver"));
        } catch (ClassNotFoundException e) {
            throw new RuntimeException(e);
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

    public static Connection getConnection() throws SQLException {

        //获取配置中的url
        String url = info.getProperty("url");
        //获取用户和密码
        String user = info.getProperty("user");
        String password = info.getProperty("password");


        Connection connection = null;

        if (connection==null){

        connection = DriverManager.getConnection(url,user,password);

        }


        return connection;
    }

    public static void closeAll(Connection connection, Statement statement){
        if (statement != null) {
            try {
                statement.close();
            } catch (SQLException e) {
                throw new RuntimeException(e);
            }
        }

        if (connection != null) {
            try {
                connection.close();
            } catch (SQLException e) {
                throw new RuntimeException(e);
            }
        }
    }

    public static void closeAll(Connection connection, Statement statement, ResultSet resultSet){

        if (resultSet != null) {
            try {
                resultSet.close();
            } catch (SQLException e) {
                throw new RuntimeException(e);
            }
        }
       closeAll(connection,statement);

    }


}

创建Map，键代表字段别名
把id传入占位符中执行sql
获取元数据的列数，根据列数循环获取每列的别名及相应的值，put到Map中

//根据id获取一行内容
public Map<String, Object> getEmpInfoById(int id) {
        Map<String, Object> map = new HashMap<>();
        Connection connection  =null;

        PreparedStatement preparedStatement = null;

        ResultSet resultSet = null;

        try {
           connection = JDBCUtil.getConnection();

            preparedStatement = connection.prepareStatement("select e.last_name as emp_name,job_title from employees e left join jobs j on e.job_id = j.job_id\n" +
                    "where employee_id =? ");

            preparedStatement.setInt(1,id);
             resultSet= preparedStatement.executeQuery();

            //获取结果的列信息,元数据
            ResultSetMetaData metaData = resultSet.getMetaData();

            //获取总共有几列
            int columnCount = metaData.getColumnCount();


            if (resultSet.next()){

            for (int i = 1; i <= columnCount ; i++) {
                //是对应数据库表中原本的列名
                String columnName = metaData.getColumnName(i);
                //返回的table中名字(如果取别名,拿到就是别名)
                String columnLabel = metaData.getColumnLabel(i);


                map.put(columnLabel,resultSet.getString(columnLabel));
//                map.put()

            }

            return map;
            }


        } catch (SQLException e) {
            throw new RuntimeException(e);
        }finally {
            JDBCUtil.closeAll(connection,preparedStatement,resultSet);
        }
        return null;
    }



 @Test
    public void empTest(){
         Map<String, Object> empInfoById = iEmpDao.getEmpInfoById(100);

         System.out.println(empInfoById.get("emp_name"));
         System.out.println(empInfoById.values());
     }