java.sql包中的 java.sql.Driver, jdbc.sql.Connection等提供给程序开发人员统一的开发接口
数据库提供商提供相应的实现，对程序开发人员来讲只要知道这些接口都有哪些方法就可以了
但我们可以深入一些 看看到底这里面都做了那些事， 同时也可以学习其中的编程模式(如Interface模式等)

1 Class.forName(String classname) 的源码为：
public final
class Class implements java.io.Serializable {
...
public static Class forName(String className)
throws ClassNotFoundException {
return forName0(className, true, ClassLoader.getCallerClassLoader());
}
...
}
关于forName0 请自己查看jdk source.
目的是把指定的Class装载到JVM中来。（注意class的装载、初始化过程）
在装载过程中将执行被装载类的static块(如下)
2 sun的JdbcOdbcDriver 源码：
public class JdbcOdbcDriver extends JdbcOdbcObject
implements JdbcOdbcDriverInterface
{
...
/**
* connect to DB
*/
public synchronized Connection connect(String s, Properties properties)
throws SQLException
{
if(JdbcOdbcObject.isTracing())
JdbcOdbcObject.trace("*Driver.connect (" + s + ")");
if(!acceptsURL(s))
return null;
if(hDbc != 0)
{
disconnect(hDbc);
closeConnection(hDbc);
hDbc = 0;
}
if(!initialize())
{
return null;
}
else
{
JdbcOdbcConnection jdbcodbcconnection = new JdbcOdbcConnection(OdbcApi, hEnv, this);
jdbcodbcconnection.initialize(getSubName(s), properties, DriverManager.getLoginTimeout());
jdbcodbcconnection.setURL(s);
return jdbcodbcconnection;
}
}
static
{
if(JdbcOdbcObject.isTracing())
JdbcOdbcObject.trace("JdbcOdbcDriver class loaded");
JdbcOdbcDriver jdbcodbcdriver = new JdbcOdbcDriver();
try
{
DriverManager.registerDriver(jdbcodbcdriver);
}
catch(SQLException sqlexception)
{
if(JdbcOdbcObject.isTracing())
JdbcOdbcObject.trace("Unable to register driver");
}
}
}
public interface JdbcOdbcDriverInterface
extends Driver
{
...
}
3 连接过程
jdbc.sql.Connection con =
DriverManager.getConnection("jdbc:odbc:pubs","sa","");
public class DriverManager {
public static synchronized Connection getConnection(String url,
String user, String password) throws SQLException {
java.util.Properties info = new java.util.Properties();
// Gets the classloader of the code that called this method, may
// be null.
ClassLoader callerCL = DriverManager.getCallerClassLoader();
if (user != null) {
info.put("user", user);
}
if (password != null) {
info.put("password", password);
}
return (getConnection(url, info, callerCL));
}
private static synchronized Connection getConnection(
String url,
java.util.Properties info,
ClassLoader callerCL) throws SQLException
{
...
Connection result = di.driver.connect(url, info);
...
}
}