Before you can connect to a database, you need to load the driver. There are two methods available, and it depends on your code to the best one to use.
In the first method, your code implicitly loads the driver using the Class.forName() method. For Postgres, you would use:
Class.forName("postgresql.Driver");
This will load the driver, and while loading, the driver will
automatically register itself with JDBC.
Note: The forName() method can throw a ClassNotFoundException, so you will need to catch it if the driver is not available.
This is the most common method to use, but restricts your code to use just Postgres. If your code may access another database in the future, and you don't use our extensions, then the second method is advisable.
The second method passes the driver as a parameter to the JVM as it starts, using the -D argument.
Example:
% java -Djdbc.drivers=postgresql.Driver example.ImageViewer
In this example, the JVM will attempt to load the driver as part of it's initialisation. Once done, the ImageViewer is started.
Now, this method is the better one to use because it allows your code to be used with other databases, without recompiling the code. The only thing that would also change is the URL, which is covered next.
One last thing. When your code then tries to open a Connection, and you get a No driver available SQLException being thrown, this is probably caused by the driver not being in the classpath, or the value in the parameter not being correct.