3.1.1 Keeping Track of Available Drivers

The DriverManager class maintains a list of Driver classes that have registered themselves by calling the method DriverManager.registerDriver. All Driver classes should be written with a static section (a static initializer) that creates an instance of the class and then registers it with the DriverManager class when it is loaded. Thus, a user would not normally call DriverManager.registerDriver directly; it should be called automatically by a Driver class when it is loaded. A Driver class is loaded, and therefore automatically registered with the DriverManager, in one of two ways:

4. by calling the method Class.forName. This explicitly loads the driver class. Since it does not depend on any external setup, this way of loading a driver is the recommended one for using the DriverManager framework. The following code loads the class acme.db.Driver:
Class.forName("acme.db.Driver");
If acme.db.Driver has been written so that loading it causes an instance to be created and also calls DriverManager.registerDriver with that instance as the parameter (as it should do), then it is in the DriverManager's list of drivers and available for creating a connection.
5. by adding the Driver class to the java.lang.System property jdbc.drivers. This is a list of driver classnames, separated by colons, that the DriverManager class loads. When the DriverManager class is initialized, it looks for the system property "jdbc.drivers," and if the user has entered one or more drivers, the DriverManager class attempts to load them. The following code illustrates how a programmer might enter three driver classes in ~/.hotjava/properties (HotJava loads these into the system properties list on startup):
jdbc.drivers=foo.bah.Driver:wombat.sql.Driver:bad.test.ourDriver

The first call to a DriverManager method will automatically cause these driver classes to be loaded.

Note that this second way of loading drivers requires a preset environment that is persistent. If there is any doubt about that being the case, it is safer to call the method Class.forName to explicitly load each driver. This is also the right method to use to bring in a particular driver since once the DriverManager class has been initialized, it will never recheck the jdbc.drivers property list.
In both of these cases, it is the responsibility of the newly-loaded Driver class to register itself by calling DriverManager.registerDriver. As mentioned, this should be done automatically when the class is loaded.
For security reasons, the JDBC management layer will keep track of which class loader provided which driver. Then when the DriverManager class is opening a connection, it will use only drivers that come from the local file system or from the same class loader as the code issuing the request for a connection.