Class.forName 如何加载数据库驱动

一直有一个疑问，Class.forName(driverName)是如何完成加载数据库驱动的，查阅了Oracle驱动的源码之后，大体弄清楚了。

我们知道，Class.forName所做的工作是加载指定的class文件到java虚拟机的内存，加载class文件到内存的时候，该class文件的静态变量和静态初始化块是要执行的，玄机即在此。

看Oracle数据库的驱动代码：

Java代码  

private static OracleDriver defaultDriver = null;   

static  

{   

  try  

  {   

    if (defaultDriver == null)   

    {
      defaultDriver = new oracle.jdbc.OracleDriver();
      DriverManager.registerDriver(defaultDriver);
    }

    AccessController.doPrivileged(new PrivilegedAction()
    {
      public Object run()
      {
        OracleDriver.registerMBeans();
        return null;
      }
    });
    Timestamp localTimestamp = Timestamp.valueOf("2000-01-01 00:00:00.0");
  }
  catch (SQLException localSQLException)
  {
    Logger.getLogger("oracle.jdbc.driver").log(Level.SEVERE, "SQLException in static block.", localSQLException);
  }
  catch (RuntimeException localRuntimeException1)
  {
    Logger.getLogger("oracle.jdbc.driver").log(Level.SEVERE, "RuntimeException in static block.", localRuntimeException1);
  }

  try
  {
    Class localClass = Class.forName("oracle.security.pki.OraclePKIProvider");
    Object localObject = localClass.newInstance();
  }
  catch (RuntimeException localRuntimeException2)
  {
  }
  catch (Exception localException)
  {
  }
  catch (NoClassDefFoundError localNoClassDefFoundError)
  {
  }
  catch (Error localError)
  {
  }
  catch (Throwable localThrowable)
  {
  }

  systemTypeMap = new Hashtable(3);
  try
  {
    systemTypeMap.put("SYS.XMLTYPE", Class.forName("oracle.xdb.XMLTypeFactory"));
  }
  catch (ClassNotFoundException localClassNotFoundException1)
  {
  }

  try
  {
    systemTypeMap.put("SYS.ANYTYPE", Class.forName("oracle.sql.AnyDataFactory"));
    systemTypeMap.put("SYS.ANYDATA", Class.forName("oracle.sql.TypeDescriptorFactory"));
  }
  catch (ClassNotFoundException localClassNotFoundException2)
  {
  }

  _Copyright_2007_Oracle_All_Rights_Reserved_ = null;
}

  private static OracleDriver defaultDriver = null;
  static
  {
    try
    {
      if (defaultDriver == null)
      {
        defaultDriver = new oracle.jdbc.OracleDriver();
        DriverManager.registerDriver(defaultDriver);
      }

      AccessController.doPrivileged(new PrivilegedAction()
      {
        public Object run()
        {
          OracleDriver.registerMBeans();
          return null;
        }
      });
      Timestamp localTimestamp = Timestamp.valueOf("2000-01-01 00:00:00.0");
    }
    catch (SQLException localSQLException)
    {
      Logger.getLogger("oracle.jdbc.driver").log(Level.SEVERE, "SQLException in static block.", localSQLException);
    }
    catch (RuntimeException localRuntimeException1)
    {
      Logger.getLogger("oracle.jdbc.driver").log(Level.SEVERE, "RuntimeException in static block.", localRuntimeException1);
    }

    try
    {
      Class localClass = Class.forName("oracle.security.pki.OraclePKIProvider");
      Object localObject = localClass.newInstance();
    }
    catch (RuntimeException localRuntimeException2)
    {
    }
    catch (Exception localException)
    {
    }
    catch (NoClassDefFoundError localNoClassDefFoundError)
    {
    }
    catch (Error localError)
    {
    }
    catch (Throwable localThrowable)
    {
    }

    systemTypeMap = new Hashtable(3);
    try
    {
      systemTypeMap.put("SYS.XMLTYPE", Class.forName("oracle.xdb.XMLTypeFactory"));
    }
    catch (ClassNotFoundException localClassNotFoundException1)
    {
    }

    try
    {
      systemTypeMap.put("SYS.ANYTYPE", Class.forName("oracle.sql.AnyDataFactory"));
      systemTypeMap.put("SYS.ANYDATA", Class.forName("oracle.sql.TypeDescriptorFactory"));
    }
    catch (ClassNotFoundException localClassNotFoundException2)
    {
    }

    _Copyright_2007_Oracle_All_Rights_Reserved_ = null;
  }

由上面的代码可以看出，在通过Class.forName加载oracle驱动的时候，在静态初始化块中，会完成驱动的注册工作，即创建驱动类的实例，并把实例注册给驱动管理器DriverManager，核心代码如下：

if (defaultDriver == null)
     {
       defaultDriver = new oracle.jdbc.OracleDriver();
       DriverManager.registerDriver(defaultDriver);
     }

 if (defaultDriver == null)
      {
        defaultDriver = new oracle.jdbc.OracleDriver();
        DriverManager.registerDriver(defaultDriver);
      }

如此之后，就不能理解，接下来就可以从驱动管理器中获得到数据库的连接了，如：

public class ConnectionTool {

    static String url = "jdbc:oracle:thin:@10.10.10.100:1521:loushang";
    static String driverName = "oracle.jdbc.driver.OracleDriver";

    public static Connection getCon(){

        try {
            Class.forName(driverName);
            return DriverManager.getConnection(url, "apitest", "apitest");
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return null;
    }
}