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：

Java代码







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：

Java代码







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;

}

}