[JavaWeb] JDBC detailed introduction

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One, JDBC principle

JDBC (Java Database Connectivity): Provides a unified access method for a variety of relational database DBMS, and realizes the use of java to operate the database.

2. Main functions of JDBC API:

DriverManager : Manage JDBC drivers

Connection : Connect to the database (generated by DriverManager)

Statement : Add, delete, modify and check the database. (Generated by Connection)

PreparedStatement : used to execute SQL queries and updates containing dynamic parameters (generated through Connection)

CallableStatement : Call a stored procedure/stored function in the database (generated by Connection)

ResultSet : The returned result set (generated by various statements)

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Three, JDBC execution process:

1: Connect to a data source, such as a database.

2: Pass query and update instructions to the database.

3: Process the database response and return the result.

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Four, Statement

Statement operation database:
         add, delete and modify: executeUpdate();
         query: executeQuery();
         getString(Int, etc.): get specific value by field name

Code:

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.Statement;

public class JDBCDemo {
    
    
	private static final  String URL = "jdbc:mysql://127.0.0.1:3306/jdbc?serverTimezone=UTC&characterEncoding=UTF-8";
	private static final  String name = "root"; //账号
	private static final  String pwd = "123456"; //密码
	
	public static void update() {
    
     //增删改操作
		
		try{
    
    
			//导入驱动，加载具体的驱动类(新的要加cj)
			Class.forName("com.mysql.cj.jdbc.Driver");
			
			//与数据库建立连接
			Connection conn = DriverManager.getConnection(URL,name,pwd);
			
			//发送sql语句
			Statement stmt = conn.createStatement();
			String sql = "insert into info values('zhangsan',20)";
			
			//执行sql语句
			int count = stmt.executeUpdate(sql); //返回值为修改的行数
			if(count>0){
    
    
				System.out.println("操作成功！");
			}
			
			//关闭连接
			if(stmt!=null) stmt.close();
			if(conn!=null) conn.close();
			
		}catch(Exception e){
    
    
			e.printStackTrace();
		}
	}
	
	public static void query(){
    
     //查询操作
		
		try{
    
    
			//导入驱动，加载具体的驱动类
			Class.forName("com.mysql.cj.jdbc.Driver");
			
			//连接数据库
			Connection conn = DriverManager.getConnection(URL,name,pwd);
			
			//发送sql语句
			Statement stmt = conn.createStatement();
			String sql = "select * from info";
			
			//执行查询，获取结果集
			ResultSet st = stmt.executeQuery(sql);
			
			//处理结果集
			while(st.next()){
    
    
				String name = st.getString("name");
				int age = st.getInt("age");
				System.out.println("姓名：" + name + "    " + "年龄：" + age);
			}
			
			//关闭连接
			if(st!=null) st.close();
			if(stmt!=null) stmt.close();
			if(conn!=null) conn.close();
			
			}catch(Exception e){
    
    
				e.printStackTrace();
			}
	}
	
	public static void main(String[] args) {
    
    
		//update();
		query();
	}
}

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五、PreparedStatement

public interface PreparedStatement extends Statement

Sub-interface of Statement

PreparedStatement operation database:
         add, delete and modify: executeUpdate();
         query: executeQuery();
         assignment operation: setString(Int, etc.)

Code:

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;

public class JDBCPreparedStatementDemo {
    
    
	private static final String URL = "jdbc:mysql://127.0.0.1:3306/jdbc?serverTimezone=UTC&characterEncoding=UTF-8";
	private static final String name = "root"; //账号
	private static final String pwd = "123456"; //密码
	
	
	public static void update(){
    
    
		try{
    
    
			//加载驱动
			Class.forName("com.mysql.cj.jdbc.Driver");
			
			//连接数据库
			Connection conn = DriverManager.getConnection(URL,name,pwd);
			
			String sql = "insert into info values(?,?)";
			PreparedStatement pstmt = conn.prepareStatement(sql);  ///预编译
			pstmt.setString(1, "lisi"); //设置第一个？的值
			pstmt.setInt(2, 18); //设置第二个？的值
			
			//cnt表示增删改数据的条数
			int cnt = pstmt.executeUpdate();
			
			if(cnt>0){
    
    
				System.out.println("操作成功！");
			}
			
			//关闭连接
			if(pstmt!=null) pstmt.close();
			if(conn!=null) conn.close();
			
		}catch(Exception e){
    
    
			e.printStackTrace();
		}
	}
	
	public static void query(){
    
    
		try{
    
    
			
			//加载驱动
			Class.forName("com.mysql.cj.jdbc.Driver");
			
			//数据库连接
			Connection conn = DriverManager.getConnection(URL,name,pwd);
			
			String sql = "select * from info";
			PreparedStatement pstmt = conn.prepareStatement(sql);
			
			//查询
			ResultSet st = pstmt.executeQuery();
			
			while(st.next()){
    
    
				String name = st.getString("name");
				int age = st.getInt("age");
				System.out.println("姓名： " + name + "  " + "年龄：" + age);
			} 
			
			//关闭连接
			if(st!=null) st.close();
			if(pstmt!=null) pstmt.close();
			if(conn!=null) conn.close();
		}catch(Exception e){
    
    
			e.printStackTrace();
		}
	}
	
	
	
	public static void main(String[] args) {
    
    
		// TODO Auto-generated method stub
		//update();
		query();
	}

}

operation result:

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Six, the difference between Statement and PrepareStatement

1 The difference in use:

1.Statement：

(1) String sql = “”

(2) executeUpdate (sql)

2.PreparedStatement:

(1) String sql = "" (There may be placeholders in the sql statement?)

(2) prepareStatement (sql), when the PreparedStatement is created, the sql statement is pre-compiled,

(3) setString(Int, etc.) Replace placeholders?
For example : setString(1,"zhangsan") Replace the position of the first placeholder with "zhangsan"

(4) executeUpdate()

2 Reasons why PreparedStatement is recommended:

1: The coding is simpler and can avoid complicated string splicing

2: Better performance, faster than statement through pre-compilation (execute only once)

3: Using placeholders, the code is highly readable and easy to maintain

4; More secure, effectively prevent SQL injection, and statement has the risk of being injected

(SQL injection: The code entered by the user is mixed with the SQL statement, quotation marks are paired, etc., so that the wrong information is successfully logged in!)

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