Webappication and PostgreSQL login roles

Hi,

I designed a Java web application. The persistence layer is a PostgreSQL
database. The application needs user authentication.
I think it's a good choice to implement this authentication mechanism
via PostgreSQL login roles. So I can create several database login roles
and set the database permissions to this login roles. This is my first
project with the postgres database, so I don't know how I can validate a
login from the website. Is there a best practice to do this or does
PostgreSQL offers a stored procedure like 'authenticateUser(String
username, String password)'?

Thanks for your help.

Bye,
Thorsten