Getting started with JDBC

One 、 What is? JDBC？

Concept ：Java DataBase Connectivity Java Database connection , Java Language operation database

Two 、JDBC The essence

JDBC The essence ： Actually, it's official （sun company ） A set of rules defined to operate all relational databases , Interface . Each database manufacturer to implement this set of interface , Provide database driver jar package . We can use this interface （JDBC） Programming , The code that actually executes is the driver jar The implementation class in the package .

3、 ... and 、JDBC Quick start

1.JDBC Six steps to programming

 First step ： Registration drive （ effect ： tell Java Program , Which brand database will be connected soon ）

 The second step ： Get database connection （ Express JVM The channel between the database process and the database process is opened , This belongs to communication between processes , A heavyweight , Be sure to close the channel after use .）

 The third step ： Get database operation object （ Special execution sql Object of statement ）

 Step four ： perform SQL sentence （DQL DML....）

 Step five ： Process query result set （ Only if the fourth step is select At the time of statement , The fifth step is to process the query result set .）

 Step six ： Release resources （ Be sure to close the resource after using it .Java And database belong to the communication between processes , Be sure to turn it off after you turn it on .）

2.JDBC Quick start

package com.aynu.jdbc;
import cn.aynu.pojo.User;
import java.sql.*;
/** *  Inquire about tab_user surface  */
public class JDBCTest04 {
    
    public static void main(String[] args) {
    
        Connection conn = null;
        Statement stmt = null;
        ResultSet resultSet = null;
        try {
    
            // 1. Registration drive 
            Class.forName("com.mysql.jdbc.Driver");
            // 2. Get the connection 
            conn = DriverManager.getConnection(
            "jdbc:mysql://localhost:3306/db",
             "root",
              "root");
            // 3. Get database operation object 
            stmt = conn.createStatement();
            // 4. perform sql sentence 
            String sql = "select * from tab_user";
            resultSet = stmt.executeQuery(sql);
            // 5. Process query result set 
            User u = new User();
            while (resultSet.next()){
    
                String id = resultSet.getString("id");
                String name = resultSet.getString("name");
                int age = resultSet.getInt("age");
                Date birthday = resultSet.getDate("birthday");
                u.setId(id);
                u.setName(name);
                u.setAge(age);
                u.setBirthday(birthday);
                System.out.println(u);
            }
        } catch (SQLException e) {
    
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
    
            e.printStackTrace();
        } finally {
    
            // 6. Release resources 
            if (resultSet != null){
    
                try {
    
                    resultSet.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();
                }
            }
        }
    }
}

Four 、 Detailed explanation JDBC Each object

1.DriverManager

DriverManager： Drive management objects 
 function ：
1. Registration drive ： Tell the driver of the corresponding database used by the program 
	 Code :Class.forName("com.mysql.jdbc.Driver");
	 Source code analysis ： stay com.mysql.jdbc.Driver Class has static code blocks 
	 static {
	        try {
	            java.sql.DriverManager.registerDriver(new Driver());
	        } catch (SQLException E) {
	            throw new RuntimeException("Can't register driver!");
	        }
		}
	 Be careful ：mysql5 After the drive jar Packages can omit the steps of registering drivers .
2. Get database connection 
	 Method ：static Connection getConnection(String url, String user, String password) 
	 Parameters ：
		url： Specify the path of the connection 
			 grammar ：jdbc:mysql://ip Address ( domain name ): Port number / Database name 
			 Example ：jdbc:mysql://localhost:3306/db
		user： user name （mysql The user name of the database ）
		root： password （mysql Database password ）

2.Connection

Connection： Database connection object 
 Access method ：conn = DriverManager.getConnection(url, user, password);
 function ：
1. Access to perform sql Object of statement 
	conn.createStatement()
	conn.prepareStatement(sql);
2. Manage affairs 
	 Open transaction ：conn.setAutoCommit(false);
	 Commit transaction ：conn.commit();
	 Transaction rollback ：conn.rollback();
	 Be careful ： Here conn Generation refers to Connection object 

3.Statement

Statement： perform sql The object of 
 Access method ：Statement stmt = conn.createStatement();
1.  perform sql
	1. boolean execute(String sql) ：
		 You can do whatever you want sql
	2. int executeUpdate(String sql) ：
		 It can be executed DML（insert、update、delete） sentence 、
		DDL(create,alter、drop) sentence 
		*  Return value ： Number of rows affected , It can be judged by the number of rows affected DML Is the statement executed successfully  
		*  Return value >0 Then the execution is successful , conversely , The failure .
	3. ResultSet executeQuery(String sql)  ：
		 perform DQL（select) sentence 

4.ResultSet

ResultSet： Result set object , Encapsulate query results 
 Access method ：ResultSet resultSet = stmt.executeQuery(sql);
	boolean next():  Move the cursor down one line , Determine whether the current line is the end of the last line ( Is there any data ),
	 If it is , Then return to false, If not, return true
* getXxx( Parameters ): get data 
	* Xxx： Representative data type     Such as ： int getInt() ,	String getString()
	*  Parameters ：
		1. int： Represents the number of the column , from 1 Start     Such as ： getString(1)
		2. String： Name of representative column .  Such as ： getDouble("balance")

while (resultSet.next()){
    
	//  Get the data of this row in the database 
    String id = resultSet.getString("id");
    String name = resultSet.getString("name");
    int age = resultSet.getInt("age");
    Date birthday = resultSet.getDate("birthday");
}

5. PreparedStatement

PreparedStatement： perform sql The object of （ Precompiled objects ）
 Access method ：PrepareStatement pstmt = conn.prepareStatement(sql);
1. SQL Injection problem ： In splicing sql when , Somewhat sql Special keywords participate in string splicing .
 Can cause security problems 
2  solve sql Injection problem ： Use PreparedStatement Object to solve 
2.  precompiled SQL： Parameters use ? As placeholder 

package com.aynu.jdbc;
import java.sql.*;
import java.util.Scanner;

/** *  Practice using PreparedStatement *  Implement login function id name */
public class JDBCTest05 {
    
    public static void main(String[] args) {
    
        Connection conn = null;
        PreparedStatement pstmt = null;
        ResultSet resultSet = null;
        try {
    
            // 1. Registration drive 
            Class.forName("com.mysql.jdbc.Driver");
            // 2. Get the connection 
            conn = DriverManager.getConnection(
            "jdbc:mysql://localhost:3306/day04", 
            "root", "root");
            // 3. Get database operation object 
            //  Definition sql sentence 
            String sql = "select * from tab_user where id = ? and name = ?";
            pstmt = conn.prepareStatement(sql);
            System.out.print("id：");
            String id = new Scanner(System.in).nextLine();
            System.out.print("name：");
            String name = new Scanner(System.in).nextLine();
            //  to ？ assignment 
            pstmt.setString(1, id);
            pstmt.setString(2, name);
            // 4. perform sql sentence 
            resultSet = pstmt.executeQuery();
            // 5. Process query result set 
            if (resultSet.next()){
    
                System.out.println(" Landing successful ");
            } else{
    
                System.out.println(" Login failed ");
            }
        } catch (ClassNotFoundException e) {
    
            e.printStackTrace();
        } catch (SQLException e) {
    
            e.printStackTrace();
        } finally {
    
            // 6. Release resources 
            if (resultSet != null){
    
                try {
    
                    resultSet.close();
                } catch (SQLException e) {
    
                    e.printStackTrace();
                }
            }
            if (pstmt != null){
    
                try {
    
                    pstmt.close();
                } catch (SQLException e) {
    
                    e.printStackTrace();
                }
            }
            if (conn != null){
    
                try {
    
                    conn.close();
                } catch (SQLException e) {
    
                    e.printStackTrace();
                }
            }
        }
    }
}

5、 ... and 、 Extract tool class

It is obvious from the above two code cases ,JDBC Code is not complicated , however Duplicate code A lot , So we need to extract a tool class , The following is a tool class I extracted in this stage of study , And configuration files （ take driver,url,user,password Extract a properties in ）, You can also use it .

driver=com.mysql.jdbc.Driver
url=jdbc:mysql://localhost:3306/db?useUnicode=true&characterEncoding=utf-8
user=root
password=root

 Be careful ： In daily study 
?useUnicode=true&characterEncoding=utf-8user=root
 To deal with the problem of Chinese garbled code 

package com.aynu.util;

import java.io.FileInputStream;
import java.io.FileReader;
import java.io.IOException;
import java.net.URL;
import java.sql.*;
import java.util.Properties;

/** * JDBC Tool class  */
public class JDBCUtils {
    
    private static String url;
    private static String user;
    private static String password;
    private static String driver;

    /** *  Static code block , Used to load configuration files  */
    static {
    
        try {
    
            Properties pro = new Properties();
            String path = Thread.currentThread().getContextClassLoader().getResource("jdbc.properties").getPath();
            pro.load(new FileInputStream(path));
            url = pro.getProperty("url");
            user = pro.getProperty("user");
            password = pro.getProperty("password");
            driver = pro.getProperty("driver");
            //  Registration drive 
            Class.forName(driver);
        } catch (IOException e) {
    
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
    
            e.printStackTrace();
        }
    }

    /** *  Get the connection  * @return  Connection object  */
    public static Connection getConnection() throws SQLException {
    
        return DriverManager.getConnection(url, user, password);
    }

    /** *  Release resources  * @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();
            }
        }
    }

    /** *  Release resources  * @param rs * @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();
            }
        }
    }
}