Java中的Class.forName

在做JAVA EE开发的过程中,更多的是使用框架来提高开发效率.越来越发现,之前很基础的一些东西,都忘记的差不多了.从今天开始慢慢的复习一下基础.今天在看JDBC的时候,就有一个有趣的地方,之前学的时候,也没在意.这个Class.forName究竟是什么鬼.

连接数据库几大步.看以下代码

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);
    }
}

1	`其中第一步,搞的我有点想不通.为啥Class.forName传入了一段字符串之后,就知道我连接的数据库是mysql? 有点不科学啊.Class.forName到底做了啥.下面就开始到源码中,一探究竟.`

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

发现它调用了forName0方法,继续跟踪再看看

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

native方法,源码也只能到此结束了.看下官方文档,怎么说吧.

https://docs.oracle.com/javase/7/docs/api/java/lang/Class.html#forName(java.lang.String)

发现官方文档,还是描述的很清楚的.

ReturnstheClass object associatedwiththeclass or interface withthegivenstring name,
 usingthegivenclass loader. 
Giventhefullyqualifiednamefor a class or interface (in thesameformatreturnedbygetName)
 this methodattemptsto locate, load, and linktheclass or interface. 
Thespecifiedclass loaderis usedto loadtheclass or interface. 
If theparameterloaderis null, theclass is loadedthroughthebootstrapclass loader.
 Theclass is initializedonlyif theinitializeparameteris true and if ithasnot been
initializedearlier.

嗯,描述的还算是很清楚.返回一个给定类或者接口的一个Class对象,如果没有给定classloader,那么会使用根类加载器.如果initalize这个参数传了true,那么给定的类如果之前没有被初始化过,那么会被初始化.我们在JDBC第一步的时候,传入的参数是com.mysql.jdbc.Driver. 也就是说这个类会被初始化.我们看一下这个类里面的内容.

public class Driverextends NonRegisteringDriverimplements 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()
    }
}

我们发现这个类也是超级简单的.一个构造函数和一个静态代码块.我们知道,类在初始化的时候,静态代码块的内容会被执行的.也就是说我们Class.forName和直接写DriverManager.registerDriver(new Driver)两者功能是等同的.我们换成这种写法.再试试看.

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);
    }
}