Jen
Jen is a lightweight, high-level Java(tm) bytecode manipulation and generation library that permits Java classes to be modified in terms of 'rename this class', 'make a field public', and 'add a new constructor'.
Jen is based on ASM 2.1, and includes full support for J5 annotations and generics. An additional set of utilities is provided to facilitate working with generated classes at runtime (e.g. allowing definition of new classes in the system classloader and so on).
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License
Apache Software License