java library to maintain database structure

Try DBDeploy. Although I haven't used it in the past, it sounds like this project would help in your case. DBDeploy is a database refactoring manager that:

"Automates the process of establishing which database refactorings need to be run against a specific database in order to migrate it to a particular build."

It is known to integrate with both Ant and Maven.

Try Liquibase.

Liquibase is an open source (Apache 2.0 Licensed), database-independent library for tracking, managing and applying database changes. It is built on a simple premise: All database changes are stored in a human readable yet trackable form and checked into source control.

Supported features:

• Extensibility
• Merging changes from multiple developers
• Code branches
• Multiple Databases
• Managing production data as well as various test datasets
• Cluster-safe database upgrades
• Automated updates or generation of SQL scripts that can be approved and applied by a DBA
• Update rollbacks
• Database ”diff“s
• Generating starting change logs from existing databases
• Generating database change documentation

We use a piece of software called Liquibase for this. It's very flexible and you can set it up pretty much however you want it. We have it integrated with Maven so our database is always up to date.