Agent and sizing

Stephen Felts stephen.felts at
Thu Oct 12 19:55:17 UTC 2017

One way is to set the environment variable JDK_JAVA_OPTIONS="-Djdk.attach.allowAttachSelf=true" to turn off the self-attach error.

Another way is to use ProcessBuilder to create another process and have the new process attach.  This follows all of the current rules.  Several projects are using this approach.

-----Original Message-----
From: Henri Tremblay [mailto:henri.tremblay at] 
Sent: Thursday, October 12, 2017 3:35 PM
To: jdk9-dev <jdk9-dev at>
Subject: Agent and sizing


Yesterday I was playing with the 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("");

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,

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