RFR(s): 8203682: Add jcmd "VM.classloaders" command to print out class loader hierarchy, details

Thomas Stüfe thomas.stuefe at gmail.com
Mon May 28 05:59:32 UTC 2018

Hi David,

On Mon, May 28, 2018 at 7:23 AM, David Holmes <david.holmes at oracle.com> wrote:
> Hi Thomas,
> I had a look at this and overall seems okay - the output looks good


> (though I'm not sure how useful the hex values are?).

That is mainly a developer option for us; CLD* and Klass* are useful
if one wishes to dig into core files; Loader oop too (I know that one
is volatile, which is were the jcmd-bundle-commands-at-safepoint idea
comes from.

But yes, this is normally too much noise, therefore disabled by
default. You need to set "verbose" explicitly to see this.

> Can't comment too much on the pretty-printing details - the proof is in the
> output there. (Though have to wonder whether there is any existing
> tree/graph printing logic somewhere in the OpenJDK code?)

None as good as mine :) Seriously, there is similar but not as evolved
printing for class hierarchy. But it does not really print a tree,
just a bunch of '|' dividers.

If this patch gets in, I would in a follow up patch unify tree
printing for these two commands and any other tree-ish structures I

> Two queries:
> 1. Have we previously established whether a CSR request is needed for a new
> Dcmd? (My initial feeling is that it is.)

My feeling is no, since this adds a new command, so there can be no
backward compat issues. What is the general policy for new jcmd
commands, or for that matter anything new added to the outside facing
interface (new options, new Xlog tracing flags, changed output for
existing options)? Do these things require CSR?

My problem with CSR is that it introduces a bottleneck, since it can
only be approved by three very busy people at Oracle - if I understand
the process right. Yes, we need a process to agree e.g. on syntax -
desperately so, since e.g. sub option syntax in jcmd is a mess - but
we seem to be strapped for reviewers even for normal code reviews, so
the effect of creating a CSR in my experience is just a stop-of-work.

> 2. Is ClassLoaderHierarchyVMOperation a safepoint VM-op? I would expect it
> needs to be to be able to walk the CLD hierarchy, unless that is already
> guaranteed to be safely walkable. Either way a comment clearly stating that
> would be useful I think.

According to Coleen, CLDG can be walked outside a safepoint, but I did
not want to risk it so I made it a safepoint operation (like other
commands walking the CLDG, e.g. VM.metaspace).

> Related to #2, is it really possible to encounter a CLD in the process of
> being unloaded? Wouldn't that happen at a safepoint?

Not sure, I am not a GC expert. I see places where this may be called
concurrently, e.g. in the process of

Since a diagnostic command should never endanger the VM it monitors, I
coded defensively.

> Thanks,
> David

Thank you,


> On 28/05/2018 2:50 PM, Thomas Stüfe wrote:
>> All tests passed on jdk-submit.
>> Anyone interested in a review?
>> More output examples for jcmd VM.classloaders :
>> Spring framework, basic tree:
>> http://cr.openjdk.java.net/~stuefe/webrevs/8203682-jcmd-print-classloader-hierarchy/example_spring_short.txt
>> Spring framework, including all classes:
>> http://cr.openjdk.java.net/~stuefe/webrevs/8203682-jcmd-print-classloader-hierarchy/example_spring_long.txt
>> ... Thomas
>> On Wed, May 23, 2018 at 2:46 PM, Thomas Stüfe <thomas.stuefe at gmail.com>
>> wrote:
>>> Dear all,
>>> (not sure if this would be a serviceability or runtime rfe, so sorry
>>> for crossposting)
>>> may I please have feedback/reviews for this small enhancement.
>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8203682
>>> Webrev:
>>> http://cr.openjdk.java.net/~stuefe/webrevs/8203682-jcmd-print-classloader-hierarchy/webrev.00/webrev/
>>> This adds a new command to jcmd, "VM.classloaders". It complements the
>>> existing command "VM.classloader_stats".
>>> This command, in its simplest form, prints the class loader tree. In
>>> addition to that, it optionally prints out loaded classes (both
>>> non-anonymous and anonymous) and various classloader specific
>>> information.
>>> Examples:
>>> http://cr.openjdk.java.net/~stuefe/webrevs/8203682-jcmd-print-classloader-hierarchy/example.txt
>>> http://cr.openjdk.java.net/~stuefe/webrevs/8203682-jcmd-print-classloader-hierarchy/example-with-classes.txt
>>> http://cr.openjdk.java.net/~stuefe/webrevs/8203682-jcmd-print-classloader-hierarchy/example-with-reflection-and-noinflation.txt
>>> Thanks and Best Regards,
>>> Thomas

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