Difference between redefineClasses and retransformClasses?

[Bastian Helfert]

Hello everybody,

I re-read the API documentation for
*java.lang.instrument.Instrumentation*over and over again, but I could
not figure out what the (main) difference
is between the methods *redefineClasses* and *retransformClasses*.

Imagine I have multiple agents that (re)transform the bytecode of the same
class. By using *retransformClasses* the current definition of the class is
passed to the transform method.
But could not I also read the current class file bytes and instrument them,
e.g. by using a bytecode manipulation library, *before *I pass these bytes
to *redefineClasses*?

Hence, what can I do with *retransformClasses* that I cannot also do with *
redefineClasses*? Or is *retransformClasses* actually "API sugar"? When
would I use what method?

Thank you very much in advance,
Bastian Helfert
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