JDBC是如何连接数据库的?

jdbc.sql.Driver d =
            (Driver)Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");         jdbc.sql.Connection con =
            DriverManager.getConnection("jdbc:odbc:pubs","sa",""); 

以上的语句您已经很熟了吧 但
 到底是怎么连上数据库的？:-)

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);
...
}             
}
4  结构图：
请点击查看 结构图   http://jdeveloper.myrice.com/doc/jdbc/images/howjdbc.jpg

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