An agent provides an implementation of this interface in order to transform class files. The transformation occurs before the class is defined by the JVM.
Note the term class file is used as defined in section 3.1 of The Java™ Virtual Machine Specification, to mean a sequence of bytes in class file format, whether or not they reside in a file.
java.lang.instrument.Instrumentation, java.lang.instrument.Instrumentation.addTransformer, java.lang.instrument.Instrumentation.removeTransformer