[9] RFR (M) 8054888: Runtime: Add Diagnostic Command that prints the class hierarchy

Karen Kinnear karen.kinnear at oracle.com
Mon Jan 12 19:54:37 UTC 2015


I filed JDK-8068830 to capture the request I had to print the super types of
a given class - so you don't have to deal with that as part of this exercise.


On Jan 9, 2015, at 12:53 PM, Karen Kinnear wrote:

> Thanks Frederic for suggesting two different dcmds - they could
> share a lot of the code logic.
> If folks generally prefer these as separate dcmds - I can file an
> rfe to add the inverted one - i.e. start at a given class/interface and
> tell me its supertypes.
> thanks,
> Karen
> On Jan 9, 2015, at 3:53 AM, Frederic Parain wrote:
>> On 01/08/2015 10:29 PM, Chris Plummer wrote:
>>> Hi Karen,
>>> Comments inline.
>>> On 1/8/15 8:07 AM, Karen Kinnear wrote:
>>>> Chris,
>>>> Thank you for doing this. I had a couple of questions/comments.
>>>> I like your idea of being able to start with a specific class to show
>>>> all subclasses of.
>>> Ok. I'll add that.
>>>> I think the way I read the code this shows the superclass hierarchy,
>>>> not the superinterfaces. With the addition
>>>> of default methods in interfaces, I think we have increased the value
>>>> in seeing superinterfaces.
>>> It does include interfaces in the output, but not as part of any class
>>> hierarchy. Interfaces are just shown as regular classes that inherit
>>> from Object. This is the case if one interface extends another, I
>>> suppose because "extends" is just interpreted as "implements" in this case.
>>>> So what I personally would find useful would be to be able to start
>>>> with a specific class and
>>>> find the superclasses and superinterfaces of that class - for the
>>>> debugging I do, I usually am
>>>> trying to look up and need both sets of information. Today if you run
>>>> -XX:+TraceDefaultMethods
>>>> there is one sample way to display all the supertypes of a single
>>>> type, all the way up. I don't know the
>>>> best way to make that consistent with the current output approach,
>>>> e.g. using the |- syntax.
>>>> e.g.
>>>> Class java.util.Arrays$ArrayList requires default method processing
>>>> java/util/Arrays$ArrayList
>>>>  java/util/AbstractList
>>>>    java/util/AbstractCollection
>>>>      java/lang/Object
>>>>      java/util/Collection
>>>>        java/lang/Object
>>>>        java/lang/Iterable
>>>>          java/lang/Object
>>>>    java/util/List
>>>>      java/lang/Object
>>>>      java/util/Collection
>>>>        java/lang/Object
>>>>        java/lang/Iterable
>>>>          java/lang/Object
>>>>  java/util/RandomAccess
>>>>    java/lang/Object
>>>>  java/io/Serializable
>>>>    java/lang/Object
>>>> Do you think there could be value to others in the ability to walk up
>>>> the hierarchy as well as to
>>>> see superclasses and superinterfaces at least from that perspective?
>>> This is a inverted from how my dcmd prints the hierarchy, plus also
>>> includes interfaces. Inverting the hierarchy means a class is listed
>>> with every subclass of the class, which I don't think is desirable.
>>> Including interfaces has the same issue, but introduces a new issue even
>>> when not inverting the hierarchy. The same interface can be in more than
>>> one location in the hierarchy, so there is no avoiding printing it more
>>> than once, so we need to decide how to best include interfaces in the
>>> output.
>> It seems to me that we have two very different use cases here, each one
>> best served with a different output format:
>> 1 - Listing of all classes/interfaces hierarchy when the dcmd is
>>    invoked without arguments:
>>       -> Chris' output format as described below (with interfaces)
>> 2 - Investigation on a particular class or interface when a class
>>    or interface is passed in argument to the dcmd
>>       -> Karen's output format, much easier to work with to
>>          track default methods. Because the output is limited to the
>>          hierarchy from a single class, there's no class duplication
>>          in output (single parent class inheritance) and limited
>>          interfaces duplication.
>> If the implementations of the two features are too different, we could
>> consider having two different dcmds.
>> My 2 cents,
>> Fred
>>> The could just be a simple list right after the class that
>>> implements them:
>>> java.lang.Object
>>> | ...
>>> |--MyBaseClass
>>> | |  implements -> MyInterface1
>>> | |  implements -> MyInterface2
>>> | |--MySubClass
>>> |      implements -> MyInterface1
>>> |      implements -> MyInterface2
>>> | ...
>>> |--MyInterface1
>>> |--MyInterface2
>>> The "implements"  lines could be optional.
>>> I know cvm would distinguish between interfaces the Class declared it
>>> implemented, and those it inherited from the interfaces it declared it
>>> implemented. This was necessary for reflection, and I think also to
>>> properly build up interfaces tables. I assume hotspot does something
>>> similar. Is there any need for this information to be conveyed in the
>>> above output, or just list out every interface implemented, and not
>>> worry about how the class acquired it.
>>>> Is there value in printing the defining class loader for each class -
>>>> maybe optionally?
>>> This is already available with GC.class_stats, although not in the
>>> default output. I suppose the reality is that it might be better handled
>>> by this DCMD. The main puprose of GC.class_stats is to print statistics
>>> (counts and sizes), so printing the ClassLoader name there is probably
>>> not appropriate, but then it's not really appropriate for
>>> VM.class_hierarchy either. I'm not sure how best to handle this. One or
>>> both DCMDs possibly should be re-purposed and more clearly define what
>>> type of data it displays.
>>>> If so, I'm wondering if there might be value in future for the jigsaw
>>>> project adding the module for each class - maybe optionally as well?
>>> This relates to my above statement. We need to figure out what type of
>>> data is useful, and which dcmds should handle them.
>>>> Love opinions on that  - especially from the serviceability folks
>>>> thanks,
>>>> Karen
>>> Thanks for the input.
>>> Chris
>>>> On Jan 7, 2015, at 6:29 PM, Chris Plummer wrote:
>>>>> Hi,
>>>>> Please review the following changes for the addition of the
>>>>> VM.class_hierarchy DCMD. Please read the bug first for some
>>>>> background information.
>>>>> Webrev: http://cr.openjdk.java.net/~cjplummer/8054888/webrev.00/
>>>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8054888
>>>>> I expect there will be further restructuring or additional feature
>>>>> work. More discussion on that below. I'm not sure if that additional
>>>>> work will be done later with a separately bug filed or with this
>>>>> initial commit. That's one thing I want to work out with this review.
>>>>> Currently the bulk of the DCMD is implemented in heapInspection.cpp.
>>>>> The  main purpose of this file is to implement the GC.class_stats and
>>>>> GC.class_histogram DCMDs. Both of them require walking the java heap
>>>>> to count live objects of each type, thus the name
>>>>> "heapInspection.cpp". This new VM.class_hierarchy DCMD does not
>>>>> require walking the heap, but is implemented in this file because it
>>>>> leverages the existing KlassInfoTable and related classes
>>>>> (KlassInfoEntry, KlassInfoBucket, and KlassClosure).
>>>>> KlassInfoTable makes it easy to build a database of all loaded
>>>>> classes, save additional info gathered for each class, iterate over
>>>>> them quickly, and also do quick lookups. This exactly what I needed
>>>>> for this DCMD, thus the reuse. There is some downside to this. For
>>>>> starters, heapInspection.cpp is not the proper place for a DCMD that
>>>>> has nothing to do with heap inspection. Also, KlassInfoEntry is being
>>>>> overloaded now to support 3 different DCMDs, as is KlassInfoTable. As
>>>>> a result each has a few fields and methods that are not used for all
>>>>> 3 DCMDs. Some subclassing might be in order here, but I'm not sure if
>>>>> it's worth it. Opinions welcomed. If I am going to refactor, I would
>>>>> prefer that be done as a next step so I'm not disturbing the existing
>>>>> DCMDs with this first implementation.
>>>>> I added some comments to code only used for GC.class_stats and
>>>>> GC.class_histogram. I did this when trying to figure them out so I
>>>>> could better understand how to implement VM.class_hierarchy. I can
>>>>> take them out if you think they are not appropriate for this commit.
>>>>> One other item I like to discuss is whether it is worth adding a
>>>>> class name argument to this DCMD. That would cause just the
>>>>> superclasses and subclasses of the named class to be printed. If you
>>>>> think that is useful, I think it can be added without too much trouble.
>>>>> At the moment not much testing has been done other than running the
>>>>> DCMD and looking at the output. I'll do more once it's clear the code
>>>>> has "settled". I would like to know if there are any existing tests
>>>>> for GC.class_stats and GC.class_histogram (there are none in the
>>>>> "test" directory). If so, possibly one could serve as the basis for a
>>>>> new test for VM.class_hierarchy.
>>>>> thanks,
>>>>> Chris
>> -- 
>> Frederic Parain - Oracle
>> Grenoble Engineering Center - France
>> Phone: +33 4 76 18 81 17
>> Email: Frederic.Parain at oracle.com

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