Spoon JDT
Spoon is a Java program processor that fully supports Java 5 and 6. It provides a complete and fine-grained Java metamodel where any program element (classes, methods, fields, statements, expressions...) can be accessed both for reading and modification.
Spoon provides an Eclipse plugin (SpoonJDT) that allows for tight integration of Spoon within the Eclipse environment. With SpoonJDT, a set of validations and transformations packaged as a Spoonlet can occur on the fly and the reports are well-integrated into the Eclipse JDT.
Spoon is Free and Open Source (CeCILL-C license - French equivalent to LGPL). SpoonJDT is not Open Source (yet).
Click here to know more about: http://spoon.gforge.inria.fr/