Re: JVMTI_ERROR_INVALID_SLOT error after using AdviceAdapter.onMethodEnter

On 2/8/2018 5:31 AM, Haasip Satang wrote:
> Found the problem. It is indeed caused by the wrong start for the local variable "this" at slot 0. 
> As you also pointed out the original byte code now starts at position 3.
> 
> So it seems that JVMTI does consider those values as well, which makes sense I guess as debuggers 
> usually are build on top of JVMTI (at least I wouldn't know another way).
> 
> The problem in my app is that the jvmti agent sets a breakpoint to the start at the method --> 
> position 0. Since it is an instance method usually the "this" variable should be available at slot 0 
> at this point already. After the instrumentation, however, the visibility for "this" only starts at 
> instruction 3. Therefore the agent cannot access "this" yet although being in the right stack frame.
> 
> I worked around this in a hacky way now by patching the offset (the "position" field of the label) 
> to 0 in visitLocalVariable if the name is "this" and offset != 0. This way it works.
> 
> Am just wondering if there isn't a cleaner way to do this. While the byte code is perfectly valid 
> for the JVM in Java we would never get an instance method where "this" is not available at positon 0 
> already.
> What do you think?

I believe you, though it surprises me that JVMTI would look at that information.
I would have to dig deeply back into the ASM documentation / code, but I suspect
there is a way to produce the Local Variable Table you want without having to
"hack" it.  But good work finding the problem!

EM


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