Agent and sizing

[Michael Rasmussen]

As of Java 9, the attach API isn't allowed to attach to itself anymore.

To circumvent it, you can for instance look at what Byte Buddy does, or
simply depend on byte-buddy-agent to get an instance of Instrumentation by
doing inst = ByteBuddyAgent.install();
For the code, see
https://github.com/raphw/byte-buddy/tree/master/byte-buddy-agent

/Michael



On 12 October 2017 at 22:35, Henri Tremblay <henri.tremblay at gmail.com>
wrote:

> Hi,
>
> Yesterday I was playing with the sizeof <https://github.com/ehcache/sizeof
> >
> library on Java 9. I was expecting bad things to happen. And I was right.
>
> The purpose of this library is the give on heap occupation. We use it for
> ehcache to know the size of the on heap cache and it is used by some other
> frameworks as well.
>
> I noticed two things. First, we load dynamically an agent in the current
> JVM. This is done by reflection so it will be something like:
>
> Class vm = Class.forName("com.sun.tools.attach.VirtualMachine");
>
> Method attach = vm.getMethod("attach");
>
> String name = ManagementFactory.getRuntimeMXBean().getName();
>
> attach.invoke(null, name.substring(0, name.substring(0,
> name.indexof('@')));
>
> It works fine on Java 8 but fails on Java 9 with IOException: Can not
> attach to current VM.
>
> *How can I fix it?* (we then use the Instrumentation to do a getObjectSize.
> It is one of the ways to make it work).
>
> My other question is about the other sizeof implementations. One is using
> reflection to go deep into objects until reaching the primitives. That will
> make the JVM scream warnings all over the place I think.
>
> Another is using Unsafe (objectFieldOffset, arrayBaseOffset,
> arrayIndexScale) to calculate the size. I think this one should not cause
> too much warnings.
>
> *So, I guess there is not much I can do for the ReflectionSizeOf appart
> from adding tons of JVM params?*
>
> Thanks a lot,
> Henri
>