JDBC学习笔记（1）

连接数据库

下载mysql-connector-java，这里我是看的这个连接mysql-connector-java下载。
下载后并且导入了Idea中的lib文件下。
在这里插入图片描述
导入成功后，为了验证可以通过CTRL+n来搜索Driver看看有没有添加进来。

随后在MySQL中创建一个数据库，我这里直接使用的是Navicat，创建了一个java_learn的数据库
在这里插入图片描述
在测试类中的测试代码
参考文档元动力

package com;

import org.junit.Test;

import java.sql.Connection;
import java.sql.Driver;
import java.sql.DriverManager;
import java.sql.SQLException;

public class TestJDBC {
    @Test
    public void testConnnection1() throws SQLException {
        //定义要素
        String driverName="com.mysql.cj.jdbc.Driver";
        //后面加上时区
        String url="jdbc:mysql://127.0.0.1:3306/java_learn?serverTimezone=GMT%2B8";
        
        String username="root";
        String password="200625";


        //加载驱动
        Driver driver= new com.mysql.cj.jdbc.Driver();

        //注册驱动
        DriverManager.registerDriver(driver);

        //获取连接
        Connection connection=DriverManager.getConnection(url,username,password);

        System.out.println(connection);

    }
}

//输出
com.mysql.cj.jdbc.ConnectionImpl@1de76cc7

如上，就说明了数据库连接成功。

也可以更加简洁如下

    @Test
    public void testConnnection2() throws SQLException {
        //定义要素
        String driverName="com.mysql.cj.jdbc.Driver";
        //后面加上时区
        String url="jdbc:mysql://127.0.0.1:3306/java_learn?serverTimezone=GMT%2B8";

        String username="root";
        String password="200625";

        //加载驱动
        //new com.mysql.cj.jdbc.Driver();
        //jdk会默认创建，也可以去掉下面的代码
        Class.forName(driverName);

        //获取连接
        Connection connection=DriverManager.getConnection(url,username,password);

        System.out.println(connection);

    }

如果在上面不创建 Class.forName(driverName);这个类的话，会通过SPI自动创建，如下的源码。
在这里插入图片描述
其中会自动读取下面文件中的内容

访问数据库

package com.entity;

import java.util.Date;

public class User {
    private int id;
    private String username;
    private String password;
    private Date data;

    public int getId() {
        return id;
    }

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

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    public Date getData() {
        return data;
    }

    public void setData(Date data) {
        this.data = data;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", username='" + username + '\'' +
                ", password='" + password + '\'' +
                ", data=" + data +
                '}';
    }
}

 @Test
    public void testStatement2() throws SQLException {
        //定义要素
        String driverName="com.mysql.cj.jdbc.Driver";
        //后面加上时区
        String url="jdbc:mysql://127.0.0.1:3306/java_learn?serverTimezone=GMT%2B8";

        String username="root";
        String password="200625";

        //让jvm加载一下就行了
        //Class.forName(driverName);

        //获取连接
        Connection connection= DriverManager.getConnection(url,username,password);

        System.out.println(connection);

        Statement statement=connection.createStatement();

        String sql="select * from user_sql";

        ResultSet resultSet = statement.executeQuery(sql);

        System.out.println(resultSet);
        ArrayList<User> users=new ArrayList<>();
        while(resultSet.next()){
            User user =new User();

            int id = resultSet.getInt(1);
            String name =resultSet.getString(2);
            String password_sql=resultSet.getString(3);
            Date birthday=resultSet.getDate(4);
            user.setId(id);
            user.setUsername(name);
            user.setPassword(password_sql);
            user.setData(birthday);
            users.add(user);
        }
        System.out.println(users);

    }

优化代码

@Test
    public void testStatement3() throws SQLException {
        //定义要素
        String driverName="com.mysql.cj.jdbc.Driver";
        //后面加上时区
        String url="jdbc:mysql://127.0.0.1:3306/java_learn?useUnicode=true&characterEncoding=utf8&useSSL=false&serverTimezone=Asia/Shanghai";
        String username="root";
        String password="200625";
        String sql="select * from user_sql";
        Connection conn=null;
        Statement statement =null;
        ResultSet resultSet = null;
        try{
            Driver driver=new com.mysql.cj.jdbc.Driver();
            DriverManager.registerDriver(driver);
            conn = DriverManager.getConnection(url,username,password);
            statement =conn.createStatement();
            resultSet=statement.executeQuery(sql);
            ArrayList<User> users=new ArrayList<>();
            while(resultSet.next()){
                User user =new User();
                int id = resultSet.getInt(1);
                String name =resultSet.getString(2);
                String password_sql=resultSet.getString(3);
                Date birthday=resultSet.getDate(4);
                user.setId(id);
                user.setUsername(name);
                user.setPassword(password_sql);
                user.setData(birthday);
                users.add(user);
            }
            System.out.println(users);
        }catch (Exception exception){
            exception.printStackTrace();
        }finally {
            if(conn!=null){
                try{
                    conn.close();
                }catch (SQLException e){
                    e.printStackTrace();
                }
            }
            if(statement!=null){
                try{
                    statement.close();
                }catch (SQLException e){
                    e.printStackTrace();
                }
            }
            if(resultSet!=null){
                try{
                    resultSet.close();
                }catch (SQLException e){
                    e.printStackTrace();
                }
            }
        }
    }

进一步进行优化
对函数进行封装

package com.util;

import java.sql.*;

public class DBUtil {
    public static Connection getConnection(){
        String url="jdbc:mysql://127.0.0.1:3306/java_learn?useUnicode=true&characterEncoding=utf8&useSSL=false&serverTimezone=Asia/Shanghai";
        String username="root";
        String password="200625";
        String sql="select * from user_sql";
        Connection conn=null;
        Statement statement =null;
        ResultSet resultSet = null;
        try{
            Driver driver = new com.mysql.cj.jdbc.Driver();
            DriverManager.registerDriver(driver);
            return DriverManager.getConnection(url, username, password);
        }  catch (Exception exception){
            exception.printStackTrace();
        }
        return null;
    }

    public static void closeAll(Connection conn,Statement statement,ResultSet resultSet){
        if(conn!=null){
            try{
                conn.close();
            }catch (SQLException e){
                e.printStackTrace();
            }
        }
        if(statement!=null){
            try{
                statement.close();
            }catch (SQLException e){
                e.printStackTrace();
            }
        }
        if(resultSet!=null){
            try{
                resultSet.close();
            }catch (SQLException e){
                e.printStackTrace();
            }
        }
    }

}

 @Test
    public void testStatement3() throws SQLException {
        String sql="select * from user_sql";
        Connection conn=null;
        Statement statement =null;
        ResultSet resultSet = null;
        try{
            conn = DBUtil.getConnection();
            statement =conn.createStatement();
            resultSet=statement.executeQuery(sql);
            List<User> users=new ArrayList<>();
            while(resultSet.next()){
                User user =new User();
                int id = resultSet.getInt(1);
                String name =resultSet.getString(2);
                String password_sql=resultSet.getString(3);
                Date birthday=resultSet.getDate(4);
                user.setId(id);
                user.setUsername(name);
                user.setPassword(password_sql);
                user.setData(birthday);
                users.add(user);
            }
            System.out.println(users);
        }catch (Exception exception){
            exception.printStackTrace();
        }finally {
            DBUtil.closeAll(conn,statement,resultSet);
        }
    }

登录的例子

public static void main(String[] args) {
        //输入用户名和密码
        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();
        String sql = "select * from user_sql where name='"+username+"' and password = '"+password+"'";
        Connection conn = null;
        Statement statement = null;
        ResultSet resultSet = null;
        try{
        	System.out.println("sql------>"+sql);
            conn=DBUtil.getConnection();
            statement=conn.createStatement();
            resultSet=statement.executeQuery(sql);
            if(resultSet.next()){
                System.out.println("登录成功");
                return;
            }else {
                System.out.println("登录失败");
            }

        } catch (SQLException throwables) {
            System.out.println("登录失败");
            throwables.printStackTrace();
        }finally {
            DBUtil.closeAll(conn,statement,resultSet);
        }

    }

//输出
请输入用户名：
root
请输入密码：
123
sql------>select * from user_sql where name=‘root’ and password = ‘123’
登录成功

但是其中也有很严重的问题，比如如下的例子：（sql注入，会绕开逻辑）
请输入用户名：
root
请输入密码：
132432423’ or 1='1
sql------>select * from user_sql where name=‘root’ and password = ‘132432423’ or 1=‘1’
登录成功

因为在输入密码的时候后面加上了or 1='1，使得sql代码执行了1=‘1’，这样就会导致结果是true。

PreparedStatement的使用

mysql的预编译：
通常我们发送一条SQL语句给MySQL服务器时，MySQL服务器每次都需要对这条SQL语句进行校验、解析等操作。
但是有很多情况下，我们的一条SQL语句可能需要反复的执行，每次执行可能仅仅是传递的参数不同而已，类似于这样的SQL语句如果每次都需要进行校验、解析等操作，未免太过于浪费性能了，因此产生了SQL语句的预编译。

所谓预编译就是将一些灵活的参数值以占位符?的形式给代替掉，我们把参数值给抽取出来，把SQL语句进行模板化。让MySQL服务器执行相同的SQL语句时，不需要在校验、解析SQL语句上面花费重复的时间。

在这里插入图片描述

-- 预编译 --
PREPARE statement from 'SELECT * FROM user_sql WHERE id=? and name=?';

set @id=6;
set @name='root';

execute statement using @id,@name;

查看mysql的通用查询日志。

show VARIABLES like '%general_log%'

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发现其中的general_log是OFF，所以这里要增加一个打开的语句。

set GLOBAL general_log=1

如果其中要永久打开通用查询日志，需要在配置文件中进行修改，其中配置文件的路径是C:\ProgramData\MySQL\MySQL Server 8.0
打开其中的my.ini文件，并且将其中的general-log改为1
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改为防止sql注入的方法

    public static void main(String[] args) {
        String sql = "select * from user_sql where id=?";
        Connection conn = null;
        PreparedStatement statement = null;
        ResultSet resultSet = null;
        try{
            System.out.println("sql------>"+sql);
            conn=DBUtil.getConnection();
            statement=conn.prepareStatement(sql);
            statement.setInt(1,3);
            resultSet=statement.executeQuery();
            ArrayList<User> users=new ArrayList<>();
            while(resultSet.next()){
                User user =new User();
                int id = resultSet.getInt(1);
                String name =resultSet.getString(2);
                String password_sql=resultSet.getString(3);
                Date birthday=resultSet.getDate(4);
                user.setId(id);
                user.setUsername(name);
                user.setPassword(password_sql);
                user.setData(birthday);
                users.add(user);
            }
            System.out.println(users);
        } catch (SQLException throwables) {
            throwables.printStackTrace();
        }finally {
            DBUtil.closeAll(conn,statement,resultSet);
        }
    }

改为登录的代码

public static void main(String[] args) {
        String sql = "select * from user_sql where id=? and password =?";
        Connection conn = null;
        PreparedStatement statement = null;
        ResultSet resultSet = null;
        try{
            System.out.println("sql------>"+sql);
            conn=DBUtil.getConnection();
            statement=conn.prepareStatement(sql);

            Scanner sc=new Scanner(System.in);
            System.out.println("请输入账号");
            int id_in=sc.nextInt();
            System.out.println("请输入密码");
            String psw=sc.next();
            statement.setInt(1,id_in);
            statement.setString(2,psw);
            resultSet=statement.executeQuery();
            User user =new User();
            if(resultSet.next()) {
                int id = resultSet.getInt(1);
                String name = resultSet.getString(2);
                String password_sql = resultSet.getString(3);
                Date birthday = resultSet.getDate(4);
                user.setId(id);
                user.setUsername(name);
                user.setPassword(password_sql);
                user.setData(birthday);
                System.out.println(user);
            }
            else {
                System.out.println("登录失败");
            }
        } catch (SQLException throwables) {
            throwables.printStackTrace();
        }finally {
            DBUtil.closeAll(conn,statement,resultSet);
        }
    }

注解，其中resultSet.next()的具体解释

boolean java.sql.ResultSet.next() throws SQLException Moves the cursor
forward one row from its current position. A ResultSet cursor is
initially positioned before the first row; the first call to the
method next makes the first row the current row; the second call makes
the second row the current row, and so on. When a call to the next
method returns false, the cursor is positioned after the last row. Any
invocation of a ResultSet method which requires a current row will
result in a SQLException being thrown. If the result set type is
TYPE_FORWARD_ONLY, it is vendor specified whether their JDBC driver
implementation will return false or throw anSQLException on a
subsequent call to next. If an input stream is open for the current
row, a call to the method next will implicitly close it. A ResultSet
object’s warning chain is cleared when a new row is read.

将指针移动到当前位置的下一行。
ResultSet指针的初始位置位于第一行之前；
第一次调用next()方法将会把第一行设置为当前行；
第二次调用next()方法指针移动到第二行，以此类推。
当对next()方法调用返回 false，说明此时指针位于最后一行之后。所有对 ResultSet需要使用当前行的方法[注:如getString()、getInt()等等]的调用都将导致next()方法抛出 SQLException异常。如果返回的 ResultSet 集合的类型被设置为 TYPE_FORWARD_ONLY ，会在随后对next()方法的调用中返回false 或抛出 SQLException 异常，因不同的数据库提供者的 JDBC 驱动实现而异。
如果为当前行打开了一个输入流，对next()方法的调用将会隐式地关闭它。当新的一行读入时，ResultSet对象的警告链将被清空。

小结，PreparedStatement 有一下有点：

1、sql的可读性更强，参数更加灵活更加面向对象，不再是简单的拼接字符串。

2、sql会进行预编译，性能高，可以进行重复利用。

3、sql的预编译同样可以防止sql注入。

增删改查

首先创建如下类
在这里插入图片描述
创建一个用户类

package com.entity;

import java.sql.Date;

public class User {
    private Integer id;
    private String username;
    private String password;
    private Date date;

    public User() {
    }

    public User(Integer id, String username, String password, Date date) {
        this.id = id;
        this.username = username;
        this.password = password;
        this.date = date;
    }

    public Integer getId() {
        return id;
    }

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

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    public Date getDate() {
        return date;
    }

    public void setDate(Date date) {
        this.date = date;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", username='" + username + '\'' +
                ", password='" + password + '\'' +
                ", date=" + date +
                '}';
    }
}

写一个接口

package com.dao;

import com.entity.User;

import java.util.List;

public interface UserDao {
    /**
     * 插入数据
     * @param user 用户实例
     * @return 受影响的行数
     */
    int insertUser(User user);

    /**
     * 根据id删除用户
     * @param id
     * @return 受影响的行数
     */
    int deleterUser(int id);

    /**
     * 修改用户
     * @param user 需要修改的用户
     * @return 修改后的用户
     */
    int updateUser(User user);

    /**
     * 根据id选择用户
     * @param id
     * @return 结果
     */
    User selectUser(int id);

    /**
     * 查询所有的用户
     * @return 用户列表
     */
    List<User> selectAllUsers();

}

对接口中的函数进行重写

package com.dao.impl;

import com.dao.UserDao;
import com.entity.User;
import com.util.DBUtil;

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

public class UserDaoImpl implements UserDao {
    @Override
    public int insertUser(User user) {
        String sql = "insert into user_sql(name,password,birthday) value(?,?,?)";
        Connection conn = null;
        PreparedStatement statement = null;
        ResultSet resultSet = null;
        try{
            System.out.println("sql------>"+sql);
            conn= DBUtil.getConnection();
            statement=conn.prepareStatement(sql);
            statement.setString(1,user.getUsername());
            statement.setString(2,user.getPassword());
            statement.setDate(3, new Date(new java.util.Date().getTime()));
            int rows=statement.executeUpdate();
            return rows;
        } catch (SQLException throwables) {
            throwables.printStackTrace();
            return -1;
        }finally {
            DBUtil.closeAll(conn,statement,null);
        }
    }

    @Override
    public int deleterUser(int id) {
        String sql = "delete from user_sql where id=?";
        Connection conn = null;
        PreparedStatement statement = null;
        ResultSet resultSet = null;
        try{
            System.out.println("sql------>"+sql);
            conn= DBUtil.getConnection();
            statement=conn.prepareStatement(sql);
            statement.setInt(1, id);
            int rows=statement.executeUpdate();
            return rows;
        } catch (SQLException throwables) {
            throwables.printStackTrace();
            return -1;
        }finally {
            DBUtil.closeAll(conn,statement,null);
        }
    }

    @Override
    public int updateUser(User user) {
        String sql = "update user_sql set name =?,password=?,birthday=? where id=? ";
        Connection conn = null;
        PreparedStatement statement = null;
        ResultSet resultSet = null;
        try{
            System.out.println("sql------>"+sql);
            conn= DBUtil.getConnection();
            statement=conn.prepareStatement(sql);
            statement.setString(1,user.getUsername());
            statement.setString(2,user.getPassword());
            statement.setDate(3,  user.getDate());
            statement.setInt(4,user.getId());
            return statement.executeUpdate();
        } catch (SQLException throwables) {
            throwables.printStackTrace();
            return 0;
        }finally {
            DBUtil.closeAll(conn,statement,null);
        }

    }

    @Override
    public User selectUser(int id) {
        String sql = "select * from user_sql where id=?";
        Connection conn = null;
        PreparedStatement statement = null;
        ResultSet resultSet = null;
        try{
            System.out.println("sql------>"+sql);
            conn= DBUtil.getConnection();
            statement=conn.prepareStatement(sql);
            statement.setInt(1,id);
            resultSet=statement.executeQuery();
            User user =new User();
            if(resultSet.next()) {
                String name = resultSet.getString("name");
                String password_sql = resultSet.getString("password");
                Date birthday = resultSet.getDate("birthday");
                user.setId(id);
                user.setUsername(name);
                user.setPassword(password_sql);
                user.setDate(birthday);
                return user;
            }
            else {
                return null;
            }
        } catch (SQLException throwables) {
            throwables.printStackTrace();
            return null;
        }finally {
            DBUtil.closeAll(conn,statement,resultSet);
        }

    }

    @Override
    public List<User> selectAllUsers() {
        String sql = "select id,name,password,birthday from user_sql";
        Connection conn = null;
        PreparedStatement statement = null;
        ResultSet resultSet = null;
        List<User> L=new ArrayList<>();
        try{
            System.out.println("sql------>"+sql);
            conn= DBUtil.getConnection();
            statement=conn.prepareStatement(sql);
            resultSet=statement.executeQuery();
            while (resultSet.next()) {
                User user =new User();
                int id=resultSet.getInt(1);
                String name = resultSet.getString(2);
                String password_sql = resultSet.getString(3);
                Date birthday = resultSet.getDate(4);
                user.setId(id);
                user.setUsername(name);
                user.setPassword(password_sql);
                user.setDate(birthday);
                L.add(user);
            }
        } catch (SQLException throwables) {
            throwables.printStackTrace();
        }finally {
            DBUtil.closeAll(conn,statement,resultSet);
        }
        return L;
    }
}

增删改查测试

    @Test
    public  void TestInsert() {
        User user =new User(null,"lym","123",new Date(new java.util.Date().getTime()));
        UserDao userDao=new UserDaoImpl();
        userDao.insertUser(user);
    }
    @Test
    public  void TestDelete() {
        int id=11;//打算删除的用户
        UserDao userDao=new UserDaoImpl();
        userDao.deleterUser(11);
    }
    @Test
    public  void TestUpdate() {
        User user =new User(8,"lym","123",new Date(new java.util.Date().getTime()));
        UserDao userDao=new UserDaoImpl();
        userDao.updateUser(user);
    }

    @Test
    public  void TestSelect() {
        UserDao userDao=new UserDaoImpl();
        User user=userDao.selectUser(3);
        System.out.println(user);
    }
    @Test
    public  void TestSelectAll() {
        UserDao userDao=new UserDaoImpl();
        List<User> L=userDao.selectAllUsers();
        System.out.println(L);
    }