MySQL learning notes 3 - JDBC

JDBC brief introduction

Java Database connection ,（Java Database Connectivity, abbreviation JDBC） yes Java Language used to regulate how the client program to access the database application program interface , Provides methods such as querying and updating data in a database .

Database driven

What is database driven ？ Similar to sound card drive 、 Driver.
Our program will be driven by database , Dealing with databases .

JDBC

SUN Companies to simplify the development of （ Unification of databases ） operation , Provides a (Java Operating the database ) standard ,JDBC.

These specifications are implemented by specific manufacturers .

For developers , We just need to master JDBC The interface operation of .

java.sql

javax.sql

You also need to import the database driver package

first JDBC Program

1. Create test database

CREATE DATABASE jdbcStudy CHARACTER SET utf8 COLLATE utf8_general_ci;

USE jdbcStudy;

CREATE TABLE `users`(
id INT PRIMARY KEY,
NAME VARCHAR(40),
PASSWORD VARCHAR(40),
email VARCHAR(60),
birthday DATE
);

INSERT INTO `users`(id,NAME,PASSWORD,email,birthday)
VALUES(1,'zhansan','123456','[email protected]','1980-12-04'),
(2,'lisi','123456','[email protected]','1981-12-04'),
(3,'wangwu','123456','[email protected]','1979-12-04')

2. Create a normal project

3. Import database driver
pom.xml

    <!-- https://mvnrepository.com/artifact/mysql/mysql-connector-java -->
    <dependency>
      <groupId>mysql</groupId>
      <artifactId>mysql-connector-java</artifactId>
      <version>8.0.16</version>
    </dependency>

4. Write test code

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

public class JdbcDemo {
    
    public static void main(String[] args) throws Exception {
    
        //1.  The load driver 
        Class.forName("com.mysql.cj.jdbc.Driver");// Fixed writing  jdbc Driver version 8.0 For before com.mysql.jdbc.Driver

        //2.  User information and url
        //useUnicode=true&characterEncoding=utf8&&useSSL=true
        //useUnicode=true Support Chinese coding ;characterEncoding=utf8 The Chinese character set is set to utf-8;useSSL=true Use secure links 
        //jdbc Driver version 8.0 Then add serverTimezone=UTC
        String url ="jdbc:mysql://localhost:3306/jdbcstudy?serverTimezone=UTC&useUnicode=true&characterEncoding=utf8&&useSSL=true";
        String name = "root";
        String password = "123456";

        //3.  Successful connection , Returns the database object  connection On behalf of the database 
        Connection connection= DriverManager.getConnection(url,name,password);
        //4.  perform SQL The object of  statement  perform SQL The object of 
        Statement statement = connection.createStatement();

        //5.  perform SQL The object of   To carry out SQL  There may be results , View return results 
        String sql="SELECT * FROM users";
        ResultSet resultSet = statement.executeQuery(sql);// The result set returned , The result set encapsulates the results of all our queries 
        while(resultSet.next()){
    
            System.out.println("ID="+resultSet.getObject("id"));
            System.out.println(" full name ="+resultSet.getObject("NAME"));
            System.out.println(" password ="+resultSet.getObject("PASSWORD"));
            System.out.println(" mailbox ="+resultSet.getObject("email"));
            System.out.println(" Birthday ="+resultSet.getObject("birthday"));
            System.out.println("==================================");
        }
        //6.  Release the connection （ Release from back to front ）
        resultSet.close();
        statement.close();
        connection.close();
    }
}

JDBC Detailed explanation of each object in

Step summary ：
1. The load driver

2. Connect to database DriverManager

3. Access to perform SQL The object of Statement

4. Get the returned result set

5. Release the connection

DriverManager

//DriverManager.registerDriver(new com.mysql.jdbc.Driver());
Class.forName("com.mysql.jdbc.Driver");// Fixed writing 
Connection connection= DriverManager.getConnection(url,name,password);

//connection On behalf of the database 
// Database settings auto commit 
// Transaction submission 
// Transaction rollback 
connection.rollback();
connection.commit();
connection.setAutoCommit();

URL

String url ="jdbc:mysql://localhost:3306/jdbcstudy?serverTimezone=UTC&useUnicode=true&characterEncoding=utf8&&useSSL=true";

//mysql  Default port 3306
// agreement :// The host address : Port number / Database name ？ Parameters 1& Parameters 2& Parameters 3...

//Oracle 1521
//jdbc:oralce:thin:@localhost:1521:sid

statement perform SQL The object of PrepareStatement perform SQL The object of

String sql="SELECT * FROM users";// To write Sql

statement.executeQuery();// Query operation , return ResultSet
statement.execute();// Carry out any SQL
statement.executeUpdate();// to update , Insert , Delete , Returns the number of rows affected 

ResultSet The result set of the query , Encapsulates all query results

Gets the specified data type

ResultSet resultSet = statement.executeQuery(sql);// The result set returned , The result set encapsulates the results of all our queries 

resultSet.getObject();// Use... Without knowing the column type 

// If you know, use the specified type 
resultSet.getString();
resultSet.getInt();
resultSet.getFloat();
resultSet.getDate();
...       

Traverse , The pointer

        resultSet.next(); // Move to the next 
        resultSet.afterLast();// Move to the end 
        resultSet.beforeFirst();// Move to the front 
        resultSet.previous();// Move to the previous one 
        resultSet.absolute(row);// Move to specified row 

Free memory

//6.  Release the connection （ Release from back to front ） 
        resultSet.close();
        statement.close();
        connection.close();//connection Connection takes up the most resources 

statement object

JDBC Medium statement Object to send... To the database SQL sentence , Want to complete the addition, deletion, modification and query of the database , You only need to send the add, delete, change and query statement to the database through this object .

Statement Object's executeUpdate Method , Used to send add... To the database 、 Delete 、 Changed sq| sentence , executeUpdate After the execution , Will return an integer ( That is to say, the addition, deletion and modification of statements cause several rows of data in the database to change ).

Statement.executeQuery Method is used to generate a query statement to the database ,executeQuery Method to return ResultSet object .

CRUD operation -create
Use executeUpdate(String sql) Method to complete the data adding operation , Example operation ：

 Statement statement = connection.createStatement();
        String sql = "insert into user(...) values(...)";
        int num = statement.executeUpdate(sql);
        if(num>0){
    
            System.out.println(" Insert the success ");
        }

CRUD operation -delete
Use executeUpdate(String sql) Method to delete data , Example operation ：

Statement statement = connection.createStatement();
        String sql = "delete from user where id =1";
        int num = statement.executeUpdate(sql);
        if(num>0){
    
            System.out.println(" Delete successful ");
        }

CURD operation -update
Use executeUpdate(String sql) Method to complete the data modification operation , Example operation ：

Statement statement = connection.createStatement();
        String sql = "update user set name ='' where name = ''";
        int num = statement.executeUpdate(sql);
        if(num>0){
    
            System.out.println(" Modification successful ");
        }

CURD operation -read
Use executeUpdate(String sql) Method to complete data query operation , Example operation ：

Statement statement = connection.createStatement();
        String sql = "select * from user where id =1";
        ResultSet rs= statement.executeQuery(sql);
        if(rs.next()){
    
            System.out.println("");
        }

Packaged as tools

First configuration file db.properties Write a good message

driver=com.mysql.cj.jdbc.Driver
url=jdbc:mysql://localhost:3306/jdbcstudy?serverTimezone=UTC&useUnicode=true&characterEncoding=utf8&&useSSL=true
username=root
password=123456

Tool class

import java.io.FileInputStream;
import java.sql.*;
import java.util.Properties;

public class JdbcUtils {
    

    private static String driver = null;
    private static String url = null;
    private static String username = null;
    private static String password = null;
    static {
    
        try{
    
            FileInputStream in = new FileInputStream("src/main/resources/db.properties");
            Properties properties = new Properties();
            properties.load(in);
            driver=properties.getProperty("driver");
            url=properties.getProperty("url");
            username=properties.getProperty("username");
            password=properties.getProperty("password");

            //1. The driver only loads once 
            Class.forName(driver);

        } catch (Exception e) {
    
            e.printStackTrace();
        }
    }

    //2. Get the connection 
    public static Connection getConnection() throws Exception{
    
        return DriverManager.getConnection(url, username, password);
    }
    //3. Release resources 
    public static void release(Connection conn, Statement st, ResultSet rs) throws SQLException {
    

        if (rs != null) {
    
            rs.close();
        }
        if (st != null){
    
            st.close();
        }
        if(conn != null){
    
            conn.close();
        }

    }
}

Use tool class （ Insert data as an example ）（exectueUpdate You can add, delete and modify three operations ）

import com.cheng.utils.JdbcUtils;
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.Statement;

import static com.cheng.utils.JdbcUtils.getConnection;

public class TestInsert {
    
    public static void main(String[] args){
    
        Connection conn =null;
        Statement st = null;
        ResultSet rs =null;

        try {
    
            conn = getConnection();// Get the connection 
            st = conn.createStatement();// obtain SQL Perform object 
            String sql = "INSERT INTO users(id,`NAME`,`PASSWORD`,`email`,`birthday`)" +
                    "VALUES(5,' Hello ','123456','[email protected]','2020-01-01')";

            int i = st.executeUpdate(sql);
            if(i>0){
    
                System.out.println(" Insert the success ");
            }
            JdbcUtils.release(conn,st,rs);
        } catch (Exception e) {
    
            e.printStackTrace();
        }
    }

}

Use tool class （ Query data as an example ）

import com.cheng.utils.JdbcUtils;

import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

import static com.cheng.utils.JdbcUtils.getConnection;

public class TestQuery {
    
    public static void main(String[] args) throws SQLException {
    
        Connection conn =null;
        Statement st = null;
        ResultSet rs =null;

        try {
    
            conn = getConnection();// Get the connection 
            st = conn.createStatement();// obtain SQL Perform object 
            String sql = "select * from users";
            rs=st.executeQuery(sql);// After the query, the result set is returned 

            while (rs.next()){
    
                System.out.println(rs.getString("NAME"));
            }
            JdbcUtils.release(conn,st,rs);
        } catch (Exception e) {
    
            e.printStackTrace();
        }finally {
    
            JdbcUtils.release(conn,st,rs);
        }
    }
}

SQL Injection problem

sql Vulnerability , Can be attacked and result in data leakage SQL Will be stitched together or

principle , For example, the normal password query is to match the password , But add one or 1=1 Then you will think that the password query passed , because or If there is one on the left and right sides, it is considered to be established , Again because 1=1 Hang up , So the server was cheated , Think the password query passed

Example

import com.cheng.utils.JdbcUtils;

import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

import static com.cheng.utils.JdbcUtils.getConnection;

public class SqlInjection {
    
    public static void main(String[] args) {
    
        //  Normal query 
        login(" Hello ","123456");

        System.out.println("=================");

        // SQL Inject 
        login("' OR '1=1","' OR '1=1");
    }
    public static void login(String name,String password){
    
        Connection conn =null;
        Statement st = null;
        ResultSet rs =null;
        try {
    
            conn = getConnection();// Get the connection 
            st = conn.createStatement();// obtain SQL Perform object 

            //  Normal query ：SELECT * FROM users WHERE `NAME`=' Hello ' AND `PASSWORD`='123456'
            // SQL Inject ：SELECT * FROM users WHERE `NAME`='' OR '1=1' AND `PASSWORD`='' OR '1=1'
            //  That is to say SELECT * FROM users  In this way, you can get all the information of the whole table 
            String sql = "SELECT * FROM users WHERE `NAME`='"+ name +"' AND `PASSWORD`='"+ password +"'" ;

            rs=st.executeQuery(sql);// After the query, the result set is returned 
            while (rs.next()){
    
                System.out.println(rs.getString("NAME"));
            }
                JdbcUtils.release(conn,st,rs);
            } catch (Exception e) {
    
                e.printStackTrace();
            }finally {
    
                try {
    
                    JdbcUtils.release(conn,st,rs);
                } catch (SQLException throwables) {
    
                    throwables.printStackTrace();
                }
            }
    }
}

PreparedStatement object

PreparedStatement Can prevent SQL Inject , More efficient .

public class Test {
    
    public static void main(String[] args) {
    
        Connection connection= null;
        PreparedStatement pstm=null;
        try {
    
            connection = JdbcUtils.getConnection();
            //PreparedStatement prevent SQL Injection essence ： Treat the parameters passed in as characters 
            // Suppose there are escape characters , For example, quotation marks , Will be escaped directly 
            
            //  difference ：
            // 1. Use a question mark placeholder instead of a parameter 
            String sql = "insert into users(id, `NAME`, `PASSWORD`, `email`,`birthday`) values(?, ?, ?, ?, ?)";
            // 2. precompile sql, First write sql Then don't execute 
            pstm = connection.prepareStatement(sql);
            //  Manual assignment 
            pstm.setInt(1,4);// 1 On behalf of the first question mark 
            pstm.setString(2," Zhang San ");
            pstm.setString(3,"123123");
            pstm.setString(4,"[email protected]");
            pstm.setDate(5,new java.sql.Date(new Date().getTime()));//  Be careful to convert to sql Of Date
            // perform 
            int i = pstm.executeUpdate();
            if (i>0){
    
                System.out.println(" Insert the success ");
            }
        } catch (Exception e) {
    
            e.printStackTrace();
        }finally {
    
            try {
    
                JdbcUtils.release(connection,pstm,null);
            } catch (SQLException throwables) {
    
                throwables.printStackTrace();
            }
        }
    }
}

Output ：

Hello
=================
zhansan
lisi
wangwu
Zhang San
Hello

prevent SQL Injection essence , Pass characters with “ ”, Escape characters will be escaped
Improvement examples ：

import com.cheng.utils.JdbcUtils;

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

public class Test {
    
    public static void main(String[] args) {
    
        //  Normal query 
        login(" Hello ","123456");

        System.out.println("=================");
        // SQL Inject 
        login("'' OR 1=1","'' OR 1=1");
    }
    public static void login(String username,String password) {
    
        Connection connection = null;
        PreparedStatement pstm = null;
        ResultSet rs = null;
        try {
    
            connection = JdbcUtils.getConnection();
            //  difference ：
            // 1. Use a question mark placeholder instead of a parameter 
            String sql = "SELECT * FROM users WHERE `NAME`=? AND `PASSWORD`=?";
            // 2. precompile sql, First write sql Then don't execute 
            pstm = connection.prepareStatement(sql);
            //  Manual assignment 
            pstm.setString(1, username);// 1 On behalf of the first question mark 
            pstm.setString(2, password);

            rs = pstm.executeQuery();// Be careful ！！！！ It's not like st.executeQuery(sql); like that , Don't write in brackets sql, Otherwise, the report will be wrong . because PreparedStatement It's precompiled ,PreparedStatement Object already contains SQL Inquire about 
            // After the query, the result set is returned 
            while (rs.next()) {
    
                System.out.println(rs.getString("NAME"));
            }
            JdbcUtils.release(connection, pstm, rs);
        } catch (Exception e) {
    
            e.printStackTrace();
        } finally {
    
            try {
    
                JdbcUtils.release(connection, pstm, rs);
            } catch (SQLException throwables) {
    
                throwables.printStackTrace();
            }
        }
    }
}

Be careful ！！！！rs = pstm.executeQuery(); Unlike rs = st.executeQuery(sql); like that , Don't write in brackets sql, Otherwise, the report will be wrong . because PreparedStatement It's precompiled ,PreparedStatement Object already contains SQL Inquire about

Output ：（SQL Injection failure ）

Hello
=================

Use IDEA Connect to database

Connect to database

add table

Modify data sheet ：
Double click on the data table , modify , Submit

Write a query

JDBC Operational transaction

Either they all succeed , Or they all failed

ACID principle

• Atomicity ： Or it's all done , Or it's not done
• Uniformity ： The total number of results remains the same
• Isolation, ： Multiple processes do not interfere with each other
• persistence ： Once submitted, it's irreversible , Persistent to the database

The problem of isolation ：

• Dirty reading ： One transaction reads another transaction that is not committed
• It can't be read repeatedly ： In the same business , Repeatedly read the data in the table , The watch has changed
• Virtual reading （ Fantasy reading ）： In a business , Read the data inserted by others , The results read before and after are inconsistent

Code implementation

1. Open transaction conn.setAutoCommit(false);
2. A set of business execution completed , Commit transaction
3. Can be in catch Statement , But the default failure will rollback

Create a sample database ：

public class JdbcTransaction{
    
    public static void main(String[] args) {
    

        Connection conn =null;
        PreparedStatement ps = null;
        ResultSet rs = null;

        try {
    
            conn = JdbcUtils.getConnection();
            // Turn off the auto submit function of the database ,  Open transaction 
            conn.setAutoCommit(false);
            // Turn on transactions automatically 
            String sql = "update account set money = money-100 where id = 1";
            ps =conn.prepareStatement(sql);
            ps.executeUpdate();
            String sql2 = "update account set money = money-100 where id = 2";
            ps=conn.prepareStatement(sql2);
            ps.executeUpdate();

            // Business finished , Commit transaction 
            conn.commit();
            System.out.println(" Successful operation ");
        } catch (Exception e) {
    
            try {
    
                // If you fail , Rollback by default 
                conn.rollback();// If you fail , Roll back 
                System.out.println(" Failure ");
            } catch (SQLException throwables) {
    
                throwables.printStackTrace();
            }
            e.printStackTrace();
        }finally {
    
            try {
    
                JdbcUtils.release(conn,ps,rs);
            } catch (SQLException throwables) {
    
                throwables.printStackTrace();
            }
        }
    }
}

Database connection pool

Traditional connection ： Database connection – completion of enforcement – Release

The server repeatedly connects – Releasing will be a waste of resources

Pool technology ： Prepare some resources in advance , Come and connect with the prepared

• Number of common connections ：10
• Minimum connections ：10（ It is generally equal to the number of commonly used connections ）
• maximum connection ： 100（ The maximum carrying capacity of the service ）
• If it is greater than the maximum number of connections , Waiting in line ,
• Waiting for timeout ：100ms（100ms After waiting for the link to disconnect ）

Write connection pools ： Implement an interface DateSource

Common open source data source implementations ( Use immediately )
The following are common in the market DateSource Implementation class of interface

• DBCP
• C3P0
• Druid: Alibaba

After using these database connection pools , We don't need to write the code to connect to the database in the project development

DBCP

needed JAR package

（ Use maven Import ）
pom.xml

    <!-- https://mvnrepository.com/artifact/org.apache.commons/commons-dbcp2 -->
    <dependency>
      <groupId>org.apache.commons</groupId>
      <artifactId>commons-dbcp2</artifactId>
      <version>2.7.0</version>
    </dependency>

DBCP The configuration file

dbcp-config.properties

# connections setting up 
driverClassName=com.mysql.cj.jdbc.Driver
url=jdbc:mysql://localhost:3306/jdbcstudy?serverTimezone=UTC&useUnicode=true&characterEncoding=utf8&&useSSL=true
username=root
password=123456

#<!--  Initialize connection  -->
initialSize=10

# Maximum number of connections 
maxActive=50

#<!--  Maximum free connection  -->
maxIdle=20

#<!--  Minimum free connection  -->
minIdle=5

#<!--  Timeout wait time in milliseconds  6000 millisecond /1000 be equal to 60 second  -->
maxWait=60000
#JDBC The format of the connection attribute attribute attached to the driver when establishing the connection must be as follows ：【 Property name =property;】
# Be careful ："user"  And  "password"  Two attributes are explicitly passed , So there's no need to include them .
connectionProperties=useUnicode=true;characterEncoding=UTF8

# Specifies the automatic commit of connections created by the connection pool （auto-commit） state .
defaultAutoCommit=true

#driver default  Specifies the read-only... Of the connection created by the connection pool （read-only） state .
# If the value is not set , be “setReadOnly” Method will not be called .（ Some drivers do not support read-only mode , Such as ：Informix）
defaultReadOnly=

#driver default  Specify the transaction level of the connection created by the connection pool （TransactionIsolation）.
# The available values are one of the following ：（ Details visible javadoc.）NONE,READ_UNCOMMITTED, READ_COMMITTED, REPEATABLE_READ, SERIALIZABLE
defaultTransactionIsolation=READ_UNCOMMITTED

Tool class

import org.apache.commons.dbcp2.BasicDataSourceFactory;
import javax.sql.DataSource;
import java.io.FileInputStream;
import java.sql.*;
import java.util.Properties;

public class Dbcp {
    
    private static DataSource dataSource = null;
    static {
    
        try{
    
            FileInputStream in = new FileInputStream("src/main/resources/dbcpconfig.properties");
            Properties properties = new Properties();
            properties.load(in);

            // 1. create data source   Factory mode ——> establish 
            dataSource = BasicDataSourceFactory.createDataSource(properties);


        } catch (Exception e) {
    
            e.printStackTrace();
        }
    }

    // 2. Get the connection 
    public static Connection getConnection() throws Exception{
    
        return dataSource.getConnection();  //  Get the connection from the data source 
    }
    // 3. Release resources 
    public static void release(Connection conn, Statement st, ResultSet rs) throws SQLException {
    

        if (rs != null) {
    
            rs.close();
        }
        if (st != null){
    
            st.close();
        }
        if(conn != null){
    
            conn.close();
        }

    }
}

Test code

import com.cheng.utils.Dbcp;
import java.sql.*;

public class DbcpTest {
    
    public static void main(String[] args) throws SQLException {
    
        Connection conn =null;
        PreparedStatement ps = null;
        ResultSet rs =null;
        try {
    
            conn = Dbcp.getConnection();
            String sql = "select * from users";
            ps=conn.prepareStatement(sql);
            rs=ps.executeQuery();
            while (rs.next()){
    
                System.out.println(rs.getString("NAME"));
            }
        } catch (Exception e) {
    
            e.printStackTrace();
        }finally {
    
            Dbcp.release(conn,ps,rs);
        }
    }
}

C3P0

needed JAR package

（ Use maven Import ）
pom.xml

    <!-- https://mvnrepository.com/artifact/com.mchange/c3p0 -->
    <dependency>
      <groupId>com.mchange</groupId>
      <artifactId>c3p0</artifactId>
      <version>0.9.5.4</version>
    </dependency>

C3P0 The configuration file

c3p0-config.xml

<?xml version="1.0" encoding="UTF-8"?>
<c3p0-config>
    <!--  If in code "ComboPooledDataSource ds=new ComboPooledDataSource();" This means that you are using c3p0 Default （ Default ）-->
    <!-- c3p0 Default （ Default ） To configure -->
    <default-config>
        <property name="driverClass">com.mysql.cj.jdbc.Driver</property>
        <property name="jdbcUrl">jdbc:mysql://localhost:3306/jdbcstudy?serverTimezone=UTC&amp;useUnicode=true&amp;characterEncoding=utf8&amp;&amp;useSSL=true</property>
        <property name="user">root</property>
        <property name="password">123456</property>

        <property name="acquireIncrement">5</property>
        <property name="initialPoolSize">10</property>
        <property name="minPoolSize">5</property>
        <property name="maxPoolSize">20</property>
    </default-config>
    <!--  If in code "ComboPooledDataSource ds=new ComboPooledDataSource("s1");" This means that you are using s1 Configuration parameters ）-->
    <named-config name="s1">
        
    </named-config>
    
</c3p0-config>

in addition , You can use code to configure parameters

//  You can use code to configure parameters 
	dataSource = new ComboPooledDataSource();
	dataSource.setDriverClass("");
	dataSource.setJdbcUrl("");
	dataSource.setUser("");
	dataSource.setPassword("");
	dataSource.setMaxPoolSize(100);

Tool class

import com.mchange.v2.c3p0.ComboPooledDataSource;
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;


public class C3p0 {
    
    private static ComboPooledDataSource dataSource = null;
    static {
    
        try{
    
            //  create data source 
            dataSource = new ComboPooledDataSource();

        } catch (Exception e) {
    
            e.printStackTrace();
        }
    }

    // 2. Get the connection   because getConnection() It's the interface method   So the following is DBCP Nothing has to change 
    public static Connection getConnection() throws Exception{
    
        return dataSource.getConnection();  //  Get the connection from the data source 
    }
    // 3. Release resources 
    public static void release(Connection conn, Statement st, ResultSet rs) throws SQLException {
    

        if (rs != null) {
    
            rs.close();
        }
        if (st != null){
    
            st.close();
        }
        if(conn != null){
    
            conn.close();
        }
    }
}

Test code

import com.cheng.utils.C3p0;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

public class C3p0Test {
    
    public static void main(String[] args) throws SQLException {
    
        Connection conn =null;
        PreparedStatement ps = null;
        ResultSet rs =null;
        try {
    
            conn = C3p0.getConnection();
            String sql = "select * from users";
            ps=conn.prepareStatement(sql);
            rs=ps.executeQuery();
            while (rs.next()){
    
                System.out.println(rs.getString("NAME"));
            }
        } catch (Exception e) {
    
            e.printStackTrace();
        }finally {
    
            C3p0.release(conn,ps,rs);
        }
    }
}

Conclusion

No matter what data source is used , The essence is unchangeable ,DateSource The interface will not change , The method won't change , Because these open source data sources are DateSource Interface implementation .