How to detect if the VM is running with compact refs from within the VM (no agent)?

Uwe Schindler uschindler at
Wed Mar 14 06:06:19 PDT 2012



I tried this code as platform (means JVM independent) detection for
compressed Oops:


boolean compressedOops = false;

if (JRE_IS_64BIT) try {

  final Class<?> beanClazz =

  final Object hotSpotBean = ManagementFactory.newPlatformMXBeanProxy(





  final Object vmOption = hotSpotBean.getClass().getMethod("getVMOption",

    .invoke(hotSpotBean, "UseCompressedOops");

  compressedOops = Boolean.valueOf(




} catch (Exception e) {

  // if anything fails before, we assume that oops are not compressed

  compressedOops = false;



If anybody finds this useful, here it is,



Uwe Schindler

uschindler at 

Apache Lucene PMC Member / Committer

Bremen, Germany


From: hotspot-compiler-dev-bounces at
[mailto:hotspot-compiler-dev-bounces at] On Behalf Of Krystal
Sent: Wednesday, March 14, 2012 10:42 AM
To: Dawid Weiss
Cc: hotspot compiler
Subject: Re: How to detect if the VM is running with compact refs from
within the VM (no agent)?


Hi Dawid,


For product releases of Oracle/Sun JDK:


The option -XX:+UseCompressedOops became available from around JDK6u14 [1].

Before 6u23, the option has to be explicitly set on the command line or from
a config file (.hotspotrc) in order to use compressed oops.

>From 6u23 and on, the ergonomics in HotSpot VM will automatically enable
UseCompressedOops when the whole GC heap is less than around 32GB and G1GC
is not in use. 7017008 [2] removed the G1GC limitation, and was delivered in
JDK7 (but not in any of the released JDK6 versions).


- Kris



On Wed, Mar 14, 2012 at 4:44 PM, Dawid Weiss <dawid.weiss at> wrote:

Thanks Kris!

I thought about parsing command-line but I wasn't sure about the
runtime -- can it switch to compressed OOPS on its own (or rather: is
there a heuristic to switch to compressed OOPS or is it always full
refs in 64 bit mode unless an explicit command line option's been


> You could use HotSpotDiagnosticMXBean via JMX to get VM options. An
> here [1].
> If you're trying to get this information from the command line, use:
>   jinfo -flag UseCompressedOops <pid>
> HTH,
> - Kris
> [1]:
> On Wed, Mar 14, 2012 at 4:30 PM, Dawid Weiss <dawid.weiss at>
>> Hi everyone,
>> This has been asked on Apache Lucene JIRA and I admit I don't know the
>> answer:
>> Is there a way to detect the reference size (which boils down to
>> compact vs. full reference on 64 bit machines) from within Java?
>> I assume the VM starts without an agent so Instrumentation is not
>> available. I'd appreciate a comment if somebody knows right away (even
>> if it's a: "no"), otherwise I'll go through the sources and try to
>> figure it out on my own.
>> Thanks,
>> Dawid


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