JDBC (detailed explanation)

1.什么是JDBC?

● JDBC（Java DataBase Connectivity)java数据库连接

● 是一种用于执行SQL语句的Java API,可以为多种关系型数据库提供统一访问,它由一组用Java语言编写的类和接口组成.

● 有了JDBC,java开发人员只需要编写一次程序,就可以访问不同的数据库.

JDBC API:

供程序员调用的接口与类,集成在java.sql包中

1. DriverManager类作用:管理各种不同的jDBC驱动

2. Connection 接口与特定数据库的连接

3. Statement 接口执行sql

4. PreparedStatement接口执行sql

5. ResultSet接口 接收查询结果

2.JDBC的搭建

• 注册JDBC驱动程序

• 需要初始化驱动程序,A communication channel with the database can be openedClass.forName(“com.mysql.cj.jdbc.Driver”); //Reflection achieves eitherDriverManager.registerDriver(new Driver());

• 建立与数据库的连接

这需要使用DriverManager.getCOnnection()方法来创建一个

Connection对象,它代表一个物理连接的数据库

Connection conn =

DriverManager.getConnection(URL,USER,PASS);

URL:jdbc:mysql://ip(127.0.0.1):端口(3306)/数据库

名?serverTimezone=Asia/Shanghai

USER:用户名(root)

PASS:密码

• 获得Satement执行sql语句

  Statement st = connection.createStatement();

  Satement中的方法:

  Int executeUpdate(String sql) 用于执行ddl语句和dml(增,删,改)语句 返回

  操作的行数

  用于执行ddl语句返回0

  用于执行dml语句返回操作的行数

  ResultSet executeQuery(String sql); 用于执行查询语句 返回一个

  ResultSet 集合

• 获得PrepareStatement执行sql语句

• 在sql语句中参数位置使用占位符,使用set***方法向sql中设置参数

  PrepareStatement ps = connection.prepareStatement(sql);

  PrepareStatement中的方法:

  Int executeUpdate() 用于执行ddl语句和dml(增,删,改)语句 返回操作的行数

  用于执行ddl语句返回0

  用于执行dml语句返回操作的行数

  ResultSet executeQuery(); 用于执行查询语句 返回一个ResultSet 集合

• 关闭与数据库的链接通道

  每次操作完成后关闭所有与数据库交互的通道

  st.close();

  rs.close();

  conn.close();

  ps.close();

3.PreparedStatement和Statement

1、代码的可读性和可维护性.

虽然用PreparedStatement来代替Statement会使代码多出几行,但这样的代码无论从可读性还是可维护性上来说.都比直接用Statement的代码高很多档次:

stmt.executeUpdate("insert into tb_name (col1,col2,col2,col4) values

('"+var1+"','"+var2+"',"+var3+",'"+var4+"')");

perstmt = con.prepareStatement("insert into tb_name (col1,col2,col2,col4)

values (?,?,?,?)");

perstmt.setString(1,var1);

perstmt.setString(2,var2);

perstmt.setString(3,var3);

perstmt.setString(4,var4);

perstmt.executeUpdate(); //prestmt是 PreparedStatement 对象实例

2、最重要的一点是极大地提高了安全性. 防止sql注入

Stringsql=“

delete from user where id = ”+num;

如果我们把[or 1=1]作为id传入进来?

delete from tb_name where id = 1 or 1 = 1;

因为‘1’=‘1’肯定成立

而如果你使用预编译语句.Anything you pass in will not match the original statement

matching relationship. 预编译模式中每个占位符处,只能插入一个值,而会过滤其他语句.

4.结果集处理

● PreparedStatement和Statement中的executeQuery()方法中会返回一

个ResultSet对象,查询结果就封装在此对象中.

● 使用ResultSet中的next()方法获得下一行数据

● 使用getXXX(String name)方法获得值

public static void main(String[] args) throws SQLException, ClassNotFoundException {
//        DriverManager类作用：管理各种不同的jDBC驱动
//        Connection 接口 与特定数据库的连接
//        Statement 接口 执行sql
//        PreparedStatement接口 执行sql
//        ResultSet接口 接收查询结果

        String cname="admin";
        int age=10;
        int num=34;
        DriverManager.registerDriver(new Driver());
        Class.forName("com.mysql.cj.jdbc.Driver");
        Connection connection=DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/schoolms?serverTimezone=Asia/Shanghai","root","root");
        Statement st=connection.createStatement();//执行SQL语句
        st.executeUpdate("INSERT INTO students(cname,age,num)\n" +
                "VALUES('刘子文',20,34)");
//        st.executeUpdate("INSERT INTO students(cname,age,num)" +"VALUE('"+cname+",'"+age+",'"+num+"',noe()");
        st.close();
        connection.close();
    }

 

public static void main(String[] args) throws ClassNotFoundException, SQLException {
        DriverManager.registerDriver(new Driver());
        Class.forName("com.mysql.cj.jdbc.Driver");
        Connection connection=DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/schoolms?serverTimezone=Asia/Shanghai","root","root");
        PreparedStatement ps=connection.prepareStatement("insert into students(cname,age,num)values(?,?,?) ");
        ps.setObject(1,"刘子文");
        ps.setObject(2,22);
        ps.setObject(3,34);
        ps.executeLargeUpdate();
        ps.close();
        connection.close();

    }

import java.sql.*;
//Invoke the element with a specific non-value
public class Demo3 {
    public static void main(String[] args) {
        int num=24;
        User user= null;
        try {
            try {
                user = new Demo3().getUserById(num);//调用Demo4的方法
            } catch (SQLException throwables) {
                throwables.printStackTrace();
            }
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
        System.out.println(user);//输出元素

    }

    public User getUserById(int num) throws ClassNotFoundException, SQLException {
        User user= null;
        PreparedStatement ps=null;
        Connection connection=null;
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");//初始化驱动程序,Open a communication channel with the database
            connection= DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/schoolms?serverTimezone=Asia/Shanghai","root","root");//创建一个对象,它代表一个物理连接的数据库
            ps=connection.prepareStatement("select*from students where num=?");//在SQL语句中参数位置使用占位符,使用set**向SQLParameters are set in the statement
            ps.setObject(1,num);
            ResultSet rs= ps.executeQuery();//查询语句,执行查询操作,The result is packaged into ResultSet
            //从ResultSetencapsulates data into objects we define ourselves
            while(rs.next()){
                user=new User();
                user.setNum(rs.getInt("num"));
                user.setAge(rs.getInt("age"));
                user.setCname(rs.getString("cname"));

            }




        } finally{
            //Close the channel connected to the database after each operation
            ps.close();
            connection.close();
        }
        return user;

    }
}

import java.sql.*;
import java.util.ArrayList;
//All elements in the query table,Print the elements in the table one by one
public class Demo4 {
    public static void main(String[] args) {

        try {
            try {
            ArrayList<User> users = new Demo4().getUsers();
                System.out.println(users);
            } catch (SQLException throwables) {
                throwables.printStackTrace();
            }
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }

    public ArrayList<User>getUsers() throws ClassNotFoundException, SQLException {
       ArrayList<User> users=new ArrayList<>();
        PreparedStatement ps=null;
        Connection connection=null;
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
            connection= DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/schoolms?serverTimezone=Asia/Shanghai","root","root");
            ps=connection.prepareStatement("select*from students");
            ResultSet rs= ps.executeQuery();//查询语句,执行查询操作,The result is packaged into ResultSet
            //从ResultSetencapsulates data into objects we define ourselves
            while(rs.next()){
             User  user=new User();
                user.setNum(rs.getInt("num"));
                user.setAge(rs.getInt("age"));
                user.setCname(rs.getString("cname"));
users.add(user);
            }




        } finally{
            ps.close();
            connection.close();
        }
        return users;

    }
}

import java.util.ArrayList;

public class User extends ArrayList<User> {
    private  int num;
    private int age;
    private  String cname;


    public int getNum() {
        return num;
    }

    public void setNum(int num) {
        this.num = num;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getCname() {
        return cname;
    }

    public void setCname(String cname) {
        this.cname = cname;
    }

    @Override
    public String toString() {
        return "User{" +
                "num=" + num +
                ", age=" + age +
                ", cname='" + cname + '\'' +
                '}';
    }


}