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.
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.
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?
Is there value in printing the defining class loader for each class - maybe
optionally?
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?
Love opinions on that - especially from the serviceability folks
thanks,
Karen
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
