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