How the jdbc driver is loaded?

If you are communicating to SQL databases using JDBC, the typical steps are like below.

Step 1: import sql package

import java.sql.*;

Step 2: Register jdbc driver. By using below statement, we are loading main driver class into the JVM.

Ex: Class.forName("com.mysql.jdbc.Driver");

Step 3: Open connection to database, using DriverManager class.

Ex:

String url = "jdbc:mysql://localhost/sample";

String userName = "root";

String pasword = "tiger";

Connection con = DriverManager.getConnection(url, userName, pasword);

jdbc:mysql://localhost/sample

Above string is a database connection string.

jdbc:mysql:// = java database connection to a mysql database

localhost = mysql database is on local machine

sample = name of database being connected to

Many people wonder, how come DriverManager class knows about the "com.mysql.jdbc.Driver" class.?

All the magic done by Class.forName method. When you load a class using Class.forName method, it calls the static block of that class. "com.mysql.jdbc.Driver" class register itself to the DriverManager class in its static block.

Let us see with an example.

Connection.java

package com.sample.app;

public class Connection {

}

Driver.java

package com.sample.app;

public interface Driver {
 public Connection getConnection();
}

DriverManager.java

package com.sample.app;

public class DriverManager {

 private static Driver driver;
 
 public static void registerDriver(Driver driver) {
  DriverManager.driver = driver;
 }
 
 public static Connection getConnection() {
  return driver.getConnection();
 }
}

MyDriver.java

package com.sample.app;

public class MyDriver implements Driver {

 static {
  System.out.println("Registering MyDriver to DriverManager");
  DriverManager.registerDriver(new MyDriver());
 }

 @Override
 public Connection getConnection() {
  return new Connection();
 }

}

Application.java

package com.sample.app;

public class Application {

 public static void main(String args[]) throws Exception {
  System.out.println("Loading MyDriver class");
  Class.forName(MyDriver.class.getName());

  DriverManager.getConnection();

 }
}

Output

Loading MyDriver class

Registering MyDriver to DriverManager

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