Any source that uses JDBC needs to import the java.sql package, using:
Important: Do not import the org.postgresql package. If you do, your source will not compile, as javac will get confused.
Before you can connect to a database, you need to load the driver. There are two methods available, and it depends on your code which is 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("org.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 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 system in the future, and you do not use any Postgres-specific 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=org.postgresql.Driver example.ImageViewerIn this example, the JVM will attempt to load the driver as part of its initialization. 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 database packages without recompiling the code. The only thing that would also change is the connection 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 class path, or the value in the parameter not being correct.
With JDBC, a database is represented by a URL (Uniform Resource Locator). With Postgres, this takes one of the following forms:
The host name of the server. Defaults to localhost.
The port number the server is listening on. Defaults to the Postgres standard port number (5432).
The database name.
To connect, you need to get a Connection instance from JDBC. To do this, you would use the DriverManager.getConnection() method:
Connection db = DriverManager.getConnection(url, username, password);
To close the database connection, simply call the close() method to the Connection: