JDBC-select练习&jdbc工具类&数据库登录案例

一、select练习

1、说明

练习：
        * 定义一个方法，查询emp表的数据将其封装为对象，然后装载集合，返回。
            1. 定义Emp类
            2. 定义方法 public List<Emp> findAll(){}
            3. 实现方法 select * from emp;

2、建表

create table emp(
id int primary key not null,
ename varchar(50),
job_id int,
mgr int,
joindate date,
salary decimal(7,2),
bonus decimal(7,2),
dept_id int
);

insert into emp values(1001,'孙悟空',4,1004,'2000-12-17',8000.00,null,20);
insert into emp values(1002,'卢俊义',3,1006,'2001-02-20',16000.00,3000.00,30);
insert into emp values(1003,'林冲',3,1006,'2001-02-22',12500.00,5000.00,30);

3、代码

Emp类

package cn.itcast.domain;

import java.util.Date;

public class Emp {
    private int id;
    private String ename;
    private int job_id;
    private int mgr;
    private Date joindate;
    private double salary;
    private double bonus;
    private int dept_id;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getEname() {
        return ename;
    }

    public void setEname(String ename) {
        this.ename = ename;
    }

    public int getJob_id() {
        return job_id;
    }

    public void setJob_id(int job_id) {
        this.job_id = job_id;
    }

    public int getMgr() {
        return mgr;
    }

    public void setMgr(int mgr) {
        this.mgr = mgr;
    }

    public Date getJoindate() {
        return joindate;
    }

    public void setJoindate(Date joindate) {
        this.joindate = joindate;
    }

    public double getSalary() {
        return salary;
    }

    public void setSalary(double salary) {
        this.salary = salary;
    }

    public double getBonus() {
        return bonus;
    }

    public void setBonus(double bonus) {
        this.bonus = bonus;
    }

    public int getDept_id() {
        return dept_id;
    }

    public void setDept_id(int dept_id) {
        this.dept_id = dept_id;
    }

    @Override
    public String toString() {
        return "Emp{" +
                "id=" + id +
                ", ename='" + ename + '\'' +
                ", job_id=" + job_id +
                ", mgr=" + mgr +
                ", joindate=" + joindate +
                ", salary=" + salary +
                ", bonus=" + bonus +
                ", dept_id=" + dept_id +
                '}';
    }
}

实现：

package cn.itcast.jdbc;

import cn.itcast.domain.Emp;

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

/*
 * *定义一个方法，查询emp表的数据将其封装为对象，然后装载集合，返回。
 */
public class JDBCDemo8 {
    public static void main(String[] args) {
        List<Emp> list = new JDBCDemo8().findall();
        System.out.println(list);
    }
    /**
     * 查询所有对象
     *
     */
    public List<Emp> findall() {
        Connection conn = null;
        Statement stmt = null;
        ResultSet rs = null;
        ArrayList<Emp> list = null;
        try {
            //1.注册驱动
            Class.forName("com.mysql.jdbc.Driver");
            //2.获取连接
            conn = DriverManager.getConnection("jdbc:mysql:///ceshi", "root", "123456");
            //3.定义SQL
            String sql = "select * from emp";
            //4.获取执行SQL的对象
            stmt = conn.createStatement();
            //5.执行SQL
            rs = stmt.executeQuery(sql);
            //6.遍历结果集，封装对象，装载集合
            Emp emp = null;
            list = new ArrayList<>();
            while (rs.next()) {
                //获取数据
                int id = rs.getInt("id");
                String ename = rs.getString("ename");
                int job_id = rs.getInt("job_id");
                int mgr = rs.getInt("mgr");
                Date joindate = rs.getDate("joindate");
                double salary = rs.getDouble("salary");
                double bonus = rs.getDouble("bonus");
                int dept_id = rs.getInt("dept_id");
                //创建emp对象
                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 e) {
            e.printStackTrace();
        } finally {
            if (rs != null) {
                try {
                    rs.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }

            if (stmt != null) {
                try {
                    stmt.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }

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

    }

}

二、JDBC工具类

1、说明

在写jdbc时，有很多重复代码，可以写一个工具类，来简化书写；

2、分析

分析：
    1. 注册驱动也抽取

    2. 抽取一个方法获取连接对象
        * 需求：不想传递参数（麻烦），还得保证工具类的通用性。
        * 解决：配置文件
            jdbc.properties
                url=
                user=
                password=

    3. 抽取一个方法释放资源

3、jdbc工具类

package cn.itcast.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{
        //读取资源文件，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);///D:/IdeaProjects/itcast/out/production/day04_jdbc/jdbc.properties
            //2. 加载文件
           // pro.load(new FileReader("D:\\IdeaProjects\\itcast\\day04_jdbc\\src\\jdbc.properties"));
            pro.load(new FileReader(path));

            //3. 获取数据，赋值
            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 e) {
                e.printStackTrace();
            }
        }

        if( conn != null){
            try {
                conn.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

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

        if( stmt != null){
            try {
                stmt.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }

        if( conn != null){
            try {
                conn.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

}

配置文件

--jdbc.properties--

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

使用JDBC工具类

public List<Emp> findAll2(){
        Connection conn = null;
        Statement stmt = null;
        ResultSet rs = null;
        List<Emp> list = null;
        try {
            conn = JDBCUtils.getConnection();
            //3.定义sql
            String sql = "select * from emp";
            //4.获取执行sql的对象
            stmt = conn.createStatement();
            //5.执行sql
            rs = stmt.executeQuery(sql);
            //6.遍历结果集，封装对象，装载集合
            Emp emp = null;
            list = new ArrayList<Emp>();
            while (rs.next()) {
                // 获取数据
                int id = rs.getInt("id");
                String ename = rs.getString("ename");
                int job_id = rs.getInt("job_id");
                int mgr = rs.getInt("mgr");
                Date joindate = rs.getDate("joindate");
                double salary = rs.getDouble("salary");
                double bonus = rs.getDouble("bonus");
                int dept_id = rs.getInt("dept_id");
                // 创建emp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 e) {
            e.printStackTrace();
        }finally {

            JDBCUtils.close(rs,stmt,conn);
        }
        return list;
    }

}

三、JDBC登录案例

package cn.itcast.jdbc;

import cn.itcast.util.JDBCUtils;

import java.sql.*;
import java.util.Scanner;

/**
 * 练习：
 *         * 需求：
 *             1. 通过键盘录入用户名和密码
 *             2. 判断用户是否登录成功
 */
public class JDBCDemo9 {

    public static void main(String[] args) {
        //1.键盘录入，接受用户名和密码
        Scanner sc = new Scanner(System.in);
        System.out.println("请输入用户名：");
        String username = sc.nextLine();
        System.out.println("请输入密码：");
        String password = sc.nextLine();
        //2.调用方法
        boolean flag = new JDBCDemo9().login2(username, password);
        //3.判断结果，输出不同语句
        if(flag){
            //登录成功
            System.out.println("登录成功！");
        }else{
            System.out.println("用户名或密码错误！");
        }

    }

    /**
     * 登录方法
     */
    public boolean login(String username ,String password){
        if(username == null || password == null){
            return false;
        }
        //连接数据库判断是否登录成功
        Connection conn = null;
        Statement stmt =  null;
        ResultSet rs = null;
        //1.获取连接
        try {
            conn =  JDBCUtils.getConnection();
            //2.定义sql
            String sql = "select * from user where username = '"+username+"' and password = '"+password+"' ";
            System.out.println(sql);
            //3.获取执行sql的对象
            stmt = conn.createStatement();
            //4.执行查询
            rs = stmt.executeQuery(sql);
            //5.判断
           /* if(rs.next()){//如果有下一行，则返回true
                return true;
            }else{
                return false;
            }*/
           return rs.next();//如果有下一行，则返回true

        } catch (SQLException e) {
            e.printStackTrace();
        }finally {
            JDBCUtils.close(rs,stmt,conn);
        }

        return false;
    }

建表语句

create table USER (
    id INT PRIMARY KEY AUTO_INCREMENT,
    username VARCHAR(32),
    PASSWORD VARCHAR(32)
);

insert into user values(null,'zhangsan','123');
insert into user values(null,'lisi','234');