Agent and sizing

[Henri Tremblay]

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