Agent and sizing

Stephen Felts stephen.felts at
Fri Oct 13 04:05:14 UTC 2017

The short answer is no.

There isn't any programmatic way to know in advance if a warning will be printed. It's easy to test if packages are open to only all unnamed modules but this isn't enough - the reason is that packages can be opened explicitly, via --add-opens`, and there won't be warnings there is an otherwise illegal access to a member of the packages opened this way.


With the current default of --illegal-access=warn, a single warning will be printed.  You can catch the exception and ignore it so the code will be quiet when the default changes to --illegal-access=deny.



From: Henri Tremblay [mailto:henri.tremblay at] 
Sent: Thursday, October 12, 2017 11:54 PM
To: Stephen Felts <stephen.felts at>
Cc: jdk9-dev <jdk9-dev at>
Subject: Re: Agent and sizing


Thanks a lot to you both! That helps a lot.


And so, for the other question: Is there a way to walk through fields on classes the can be anywhere without tons of warnings?


On 12 October 2017 at 15:55, Stephen Felts <HYPERLINK "mailto:stephen.felts at"stephen.felts at> wrote:

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:HYPERLINK "mailto:henri.tremblay at"henri.tremblay at]
Sent: Thursday, October 12, 2017 3:35 PM
To: jdk9-dev <HYPERLINK "mailto:jdk9-dev at"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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