Difference between redefineClasses and retransformClasses?
Alan.Bateman at Sun.COM
Fri May 23 05:31:17 PDT 2008
Bastian Helfert wrote:
> 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?
The main recommendation is to use retransformClasses if you are doing
instrumentation. The redefineClasses method is more intended for
fix-and-continue and other cases where the class definition needs to be
The main motivation and benefit of retransformClasses is that it allows
multiple instrumentation agents to co-exist. Each agent's transformer
gets to instrument the class in turn. Removing instrumentation becomes
significantly easier as the retransformClasses starts with the initial
Does does help?
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