java中Class.forName方法的作用详解

import com.mysql.jdbc.Driver;
 
import java.sql.*;
 
/**
 * @author honway.liu
 * @date 2016/12/8 下午11:07
 * @email gm100861@gmail.com
 * @blog http://linuxsogood.org
 */
public class JdbcDemo {
 
 public static void main(String[] args) throws SQLException, ClassNotFoundException {
  String url = "jdbc:mysql://127.0.0.1:3306/mydb";
  String username = "root";
  String password = "redhat";
  Class.forName("com.mysql.jdbc.Driver");
  Connection connection = DriverManager.getConnection(url, username, password);
  String sql = "SELECT * FROM msg";
  PreparedStatement prepareStatement = connection.prepareStatement(sql);
  ResultSet resultSet = prepareStatement.executeQuery();
  resultSet.next();
  String address = resultSet.getString("address");
  System.out.println(address);
 }
}

 @CallerSensitive
 public static Class<?> forName(String className)
    throws ClassNotFoundException {
  Class<?> caller = Reflection.getCallerClass();
  return forName0(className, true, ClassLoader.getClassLoader(caller), caller);
 }

 private static native Class<?> forName0(String name, boolean initialize,
           ClassLoader loader,
           Class<?> caller)
  throws ClassNotFoundException;

Returns the Class object associated with the class or interface with the given string name,
 using the given class loader. 
Given the fully qualified name for a class or interface (in the same format returned by getName)
 this method attempts to locate, load, and link the class or interface. 
The specified class loader is used to load the class or interface. 
If the parameter loader is null, the class is loaded through the bootstrap class loader.
 The class is initialized only if the initialize parameter is true and if it has not been 
initialized earlier.

public class Driver extends NonRegisteringDriver implements java.sql.Driver {
 
 static {
  try {
   java.sql.DriverManager.registerDriver(new Driver());
  } catch (SQLException E) {
   throw new RuntimeException("Can't register driver!");
  }
 }
 
 
 public Driver() throws SQLException {
  // Required for Class.forName().newInstance()
 }
}

public class JdbcDemo {
 
 public static void main(String[] args) throws SQLException, ClassNotFoundException {
  String url = "jdbc:mysql://127.0.0.1:3306/mydb";
  String username = "root";
  String password = "redhat";
  //Class.forName("com.mysql.jdbc.Driver");
  DriverManager.registerDriver(new Driver());
  Connection connection = DriverManager.getConnection(url, username, password);
  String sql = "SELECT * FROM msg";
  PreparedStatement prepareStatement = connection.prepareStatement(sql);
  ResultSet resultSet = prepareStatement.executeQuery();
  resultSet.next();
  String address = resultSet.getString("address");
  System.out.println(address);
 }
}