Spoon
Spoon is a Java program processor that fully supports Java 5. 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 can be used on validation purpose, to ensure that your programs respect some programming conventions or guidelines, or for program transformation, by using a pure-Java template engine.
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License
GNU Library or Lesser General Public License (LGPL)