How might an application listen for changes in database?

I don't know if this is the right forum, but I had the following question.

I want to build an application that dynamically updates its GUIs (Google Web Toolkit) whenever the underlying relational database is changed. In this application database updates might be done about every 20 seconds.

How might a Java application listen for changes in a database?

I presume that a persistence layer would have to do the listening? Can, for example, a JPA EntityManager listen for changes?

There would not be any way do notify from the database itself, would there be? (Triggers remain solely in SQL, I think). I would not think so.

If you are using hibernate you can use a session interceptor.
You can find more details here: http://docs.jboss.org/hibernate/orm/3.3/reference/en/html/events.html

Thanks, Bogdan. I will have a look at that link.

I wonder if JPA offers similar classes. This hint that
in Hibernate it is an interceptor is useful.