前文指路

修改后代码：
所调用的配置文件：jdbc.properties

url = jdbc:mysql:///db3
user = root
password = admin
driver = com.mysql.cj.jdbc.Driver

JDBCUtils.java

package util;

import java.io.FileReader;
import java.io.IOException;
import java.net.URL;
import java.sql.*;
import java.util.Properties;

/**
 * JDBC工具类
 */

public class JDBCUtils {
    private static String url;
    private static String user;
    private static String password;
    private static String driver;
    /**
     * 文件的读取，只需要读取一次即可拿到这些值。使用静态代码块
     */
    static{
        //读取资源，获取值


        try {
            //1.创建Properties集合类
            Properties pro = new Properties();

            //获取src路径下的文件的方式 ----> ClassLoader  类加载器
            /*ClassLoader classLoader = JDBCUtils.class.getClassLoader();
            URL res = classLoader.getResource("jdbc.properties");
            String path = res.getPath();
            System.out.println(path);*/

            //2.加载文件
            pro.load(new FileReader("src/cn/jdbc.properties"));
            //pro.load(new FileReader(path));

            //3.获取数据，赋值
            JDBCUtils.url = pro.getProperty("url");
            user = pro.getProperty("user");
            password = pro.getProperty("password");
            driver = pro.getProperty("driver");
            //4.注册驱动
            Class.forName(driver);
        } catch (IOException e) {
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }

    /**
     * 获取连接
     * @return 连接对象
     */
    public static Connection getConnection() throws SQLException {

        return DriverManager.getConnection(url,user,password);
    }

    /**
     * 释放资源
     * @param stmt
     * @param conn
     */
    public static void close(Statement stmt,Connection conn){
        if(stmt!=null){
            try {
                stmt.close();
            } catch (SQLException throwables) {
                throwables.printStackTrace();
            }
        }
        if(conn!=null){
            try {
                conn.close();
            } catch (SQLException throwables) {
                throwables.printStackTrace();
            }
        }
    }

    public static void close(ResultSet rs,Statement stmt,Connection conn){
        if(rs!=null){
            try {
                rs.close();
            } catch (SQLException throwables) {
                throwables.printStackTrace();
            }
        }
        if(stmt!=null){
            try {
                stmt.close();
            } catch (SQLException throwables) {
                throwables.printStackTrace();
            }
        }
        if(conn!=null){
            try {
                conn.close();
            } catch (SQLException throwables) {
                throwables.printStackTrace();
            }
        }
    }
}

jdbcdemo.java

import cn.itcast.domain.Emp;
import util.JDBCUtils;

import java.sql.*;
import java.util.ArrayList;
import java.util.List;

/**
 * 定义一个方法，查询emp表的数据并将其封装为对象，然后装载集合，返回。
 */

public class jdbcdemo {

    public static void main(String[] args) {
        List<Emp> list = new jdbcdemo().findAll2();
        System.out.println(list);
    }
    /**
     * 查询所有emp对象
     */

    public List<Emp> findAll(){ //方法返回一个List集合，集合里面为Emp对象
        Connection conn =null;
        Statement stmt = null;
        ResultSet rs = null;
        List<Emp> list = null;
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
            conn = DriverManager.getConnection("jdbc:mysql:///db3", "root", "admin");
            String sql = "select * from emp";
            stmt = conn.createStatement();
            rs = stmt.executeQuery(sql);
            Emp emp = null;
            list = new ArrayList<Emp>();
            while(rs.next()){
                int id = rs.getInt(1);
                String ename = rs.getString(2);
                int job_id = rs.getInt(3);
                int mgr = rs.getInt(4);
                Date joindate = rs.getDate(5);
                double salary = rs.getDouble(6);
                double bonus = rs.getDouble(7);
                int dept_id = rs.getInt(8);
                //创建对象，并赋值
                emp = new Emp();
                emp.setId(id);
                emp.setEname(ename);
                emp.setJob_id(job_id);
                emp.setMgr(mgr);
                emp.setJoindate(joindate);
                emp.setSalary(salary);
                emp.setBonus(bonus);
                emp.setDept_id(dept_id);

                //装载集合
                list.add(emp);
            }
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (SQLException throwables) {
            throwables.printStackTrace();
        }finally {
            if(conn!=null){
                try {
                    conn.close();
                } catch (SQLException throwables) {
                    throwables.printStackTrace();
                }
            }
            if(stmt!=null){
                try {
                    stmt.close();
                } catch (SQLException throwables) {
                    throwables.printStackTrace();
                }

            }
            if(rs!=null){
                try {
                    rs.close();
                } catch (SQLException throwables) {
                    throwables.printStackTrace();
                }
            }
        }
        return list;
    }

    public List<Emp> findAll2(){ //方法返回一个List集合，集合里面为Emp对象
        Connection conn =null;
        Statement stmt = null;
        ResultSet rs = null;
        List<Emp> list = null;
        try {
            //Class.forName("com.mysql.cj.jdbc.Driver");
            //conn = DriverManager.getConnection("jdbc:mysql:///db3", "root", "admin");
            conn=JDBCUtils.getConnection();

            String sql = "select * from emp";
            stmt = conn.createStatement();
            rs = stmt.executeQuery(sql);
            Emp emp = null;
            list = new ArrayList<Emp>();
            while(rs.next()){
                int id = rs.getInt(1);
                String ename = rs.getString(2);
                int job_id = rs.getInt(3);
                int mgr = rs.getInt(4);
                Date joindate = rs.getDate(5);
                double salary = rs.getDouble(6);
                double bonus = rs.getDouble(7);
                int dept_id = rs.getInt(8);
                //创建对象，并赋值
                emp = new Emp();
                emp.setId(id);
                emp.setEname(ename);
                emp.setJob_id(job_id);
                emp.setMgr(mgr);
                emp.setJoindate(joindate);
                emp.setSalary(salary);
                emp.setBonus(bonus);
                emp.setDept_id(dept_id);

                //装载集合
                list.add(emp);
            }
        }  catch (SQLException throwables) {
            throwables.printStackTrace();
        }finally {
            /*if(conn!=null){
                try {
                    conn.close();
                } catch (SQLException throwables) {
                    throwables.printStackTrace();
                }
            }
            if(stmt!=null){
                try {
                    stmt.close();
                } catch (SQLException throwables) {
                    throwables.printStackTrace();
                }

            }
            if(rs!=null){
                try {
                    rs.close();
                } catch (SQLException throwables) {
                    throwables.printStackTrace();
                }
            }*/
            JDBCUtils.close(rs,stmt,conn);
        }
        return list;
    }
}

运行结果：