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

[David Holmes]

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?).

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?)

Two queries:

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

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.

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

Thanks,
David

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