Implementation

There is a lot of (not only java-based) implementations, many have a GUI, allowing to graphically design transformation flows.

Clover ETL: http://www.cloveretl.org - open source ETL tool including GUI
Microsoft SQL Server Integration Services: http://www.microsoft.com/sql/technologies/integration/default.mspx
Octopus Java/XML ETL Tool: http://octopus.enhydra.org/
java-etl: http://code.google.com/p/java-etl/
Kettle: http://kettle.pentaho.org/