JDBC-select练习&jdbc工具类
Posted weiyiming007
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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; } /** * 登录方法,使用PreparedStatement实现 */ public boolean login2(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; PreparedStatement pstmt = null; ResultSet rs = null; //1.获取连接 try { conn = JDBCUtils.getConnection(); //2.定义sql String sql = "select * from user where username = ? and password = ?"; //3.获取执行sql的对象 pstmt = conn.prepareStatement(sql); //给?赋值 pstmt.setString(1,username); pstmt.setString(2,password); //4.执行查询,不需要传递sql rs = pstmt.executeQuery(); //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,pstmt,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‘);
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