[External] : Re: Reflective access to bytes[] of loaded class

[Gary Frost]

Sorry Dan.  I missed your question 😉

"Can you expand on the use case for getting the current bytes from the runtime?  As I said in the previous email, they aren't a good candidate for feeding back into the runtime due to the existing JVMTI agent process.  Is this mostly for testing purposes?"

My usecase is 'lifting' program structure from bytecode so that I can target a different runtime.  Specifically, bytecode->OpenCL... or CUDA or SPIRV or PTX or Metal so that we can execute code on SIMD/GPU devices.

Essentially performing a similar function as IntelliJ's  Fernflower based plugin, which maps (quite successfully) bytecode->Java source for files without source available.

Gary



________________________________
From: Dan Heidinga <heidinga at redhat.com>
Sent: Thursday, February 9, 2023 6:24 PM
To: Gary Frost <gary.frost at oracle.com>
Cc: classfile-api-dev at openjdk.org <classfile-api-dev at openjdk.org>
Subject: Re: [External] : Re: Reflective access to bytes[] of loaded class

Thanks for confirming, Gary.

On Thu, Feb 9, 2023 at 11:59 AM Gary Frost <gary.frost at oracle.com<mailto:gary.frost at oracle.com>> wrote:
Dan

I have indeed used JVMTI (and Java) agents for this. In the case of JVMTI by keeping a std::map<std::string,byte[]>, and providing a JNI call to get the bytes.

It works well for cases where we know the set of classes we want bytes for at JVM 'launch time' , or there is something in the class (name match? constant pool entry?) that we can trigger to prune the size of the map.    Otherwise we are forced to retain a map for all classes, just in case.

And of course who knew whether our JVMTI agent was the last 'actor' in the chain of possible mutators to play with the bytes.

Can you expand on the use case for getting the current bytes from the runtime?  As I said in the previous email, they aren't a good candidate for feeding back into the runtime due to the existing JVMTI agent process.  Is this mostly for testing purposes?

--Dan



Hence the desire for runtime help


________________________________
From: Dan Heidinga <heidinga at redhat.com<mailto:heidinga at redhat.com>>
Sent: Thursday, February 9, 2023 4:35 PM
To: Gary Frost <gary.frost at oracle.com<mailto:gary.frost at oracle.com>>
Cc: classfile-api-dev at openjdk.org<mailto:classfile-api-dev at openjdk.org> <classfile-api-dev at openjdk.org<mailto:classfile-api-dev at openjdk.org>>
Subject: [External] : Re: Reflective access to bytes[] of loaded class



On Thu, Feb 9, 2023 at 11:03 AM Gary Frost <gary.frost at oracle.com<mailto:gary.frost at oracle.com>> wrote:
I would like to make a case for adding a reflection API for getting the bytes for class loaded by the VM as part of this Classfile API.

Something akin to

byte[] Class.getClassfileBytes();

I've often wanted the same kind of API when doing one off tests and minor modifications so I'm very sympathetic to the request.  Typically when doing more "serious" class modification, I've used the Instrumentation::retransform API [0] or written a native JVMTI agent [1] and then such an api to get the current classfile bytes isn't required.  Does this match your typical use cases as well?

The JVM does some interesting handling of classfile bytes in these cases - it allows non-retransform-capable agents to do a 1 time modification of the original classfile bytes and then saves those bytes away before passing them on to the retransform-capable agents.  One subsequent retransform events, those saved bytes are reused and only the retransform-capable agents get a chance to modify the bytes.

The reason this matters is that existing retransform-capable agents aren't expecting to see the modifications they've made present in the classfile bytes they get passed.  Providing an easy api to get the currently executing bytecodes will mean that these agents will now "see" their own modifications which may result in incompatibilities.

I'd be interested in feedback from existing major agent providers to see how much of a problem such a change would cause them before pursuing it unless there's a more common pattern of use I'm unaware of.

[0] https://cr.openjdk.java.net/~iris/se/17/latestSpec/api/java.instrument/java/lang/instrument/Instrumentation.html#retransformClasses(java.lang.Class...)
[1] https://cr.openjdk.java.net/~iris/se/17/latestSpec/specs/jvmti.html#RetransformClasses



At present developers usually resort to something like

clazz.getClassLoader().getResourceAsStream(
    clazz.getName().replace(".", "/") + ".class")

Ignoring the fact that we may have just forced an expensive network fetch, to bytes that the JVM clearly already has 'squirrelled away' somewhere...

For me this is hugely problematic as there is no guarentee that the bytes fetched from such a stream will match the bytes that the JVM is using for the class in question....

Java offers all sorts of opportunities (JVMTI agents, Java agents, even custom ClassLoaders) for mutating the incoming classfile's bytes. All of which the stream fetched via getResourceAsStream() completely sidesteps.

See above (or the JVMTI spec) for which bytes get passed through to agents.  The story is more complex than it first appears.

--Dan


This may well be considered 'out of bounds' for the Classfile API, but I think we should consider it, as it seems to be something that users of the Classfile API will need going forward.

Gary
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