RFR 8026985: Rewrite SystemDictionary::classes_do and Dictionary::classes_do to use KlassClosure

David Holmes david.holmes at oracle.com
Tue Mar 14 00:30:57 UTC 2017

Hi Coleen,

On 11/03/2017 12:39 AM, coleen.phillimore at oracle.com wrote:
> David, Thank you for reviewing.
> On 3/10/17 12:18 AM, David Holmes wrote:
>> Hi Coleen,
>> On 9/03/2017 2:24 AM, coleen.phillimore at oracle.com wrote:
>>> Summary: Clean up and examine uses of classes_do for the
>>> SystemDictionary
>>> See bug comments for more details.  I wanted to clean this up while
>>> examining the idea of having system dictionary information added per
>>> ClassLoaderData, rather than a global table.
>>> open webrev at http://cr.openjdk.java.net/~coleenp/8026985.01/webrev
>>> bug link https://bugs.openjdk.java.net/browse/JDK-8026985
>> The bulk of the deletion looks good! :)
>> I guess my main query is how ClassLoaderDataGraph::classes_do /
>> loaded_classes_do relate to SystemDictionary::classes_do? I would
>> presume SD::classes_do can only act on loaded classes - by definition.
>> So then it is unclear how you can replace it with either of the CLDG
>> methods?
> True, loaded_classes_do only walks loaded classes, which is probably
> what we want most of the time.  I was trying to figure this out and the
> differences which led to this change.  I am trying to make it less
> confusing.  At least for me!
> The ClassLoaderData::classes_do includes anonymous, array and allocated
> classes.   Most of the time we want the first two.  For GC we want the
> last (because it has a mirror to walk).  The SystemDictionary only has
> loaded InstanceKlasses, both with defined loader and initiating
> loader.   Except for one place in the code, we ignore the entries with
> initiating loader.

I'm still very unclear as to why it is now okay to expand the set of 
klasses being walked at a given point. For example:


    static void compute() {
-    SystemDictionary::classes_do(do_class);
+    ClassLoaderDataGraph::classes_do(do_class);

IIUC SD::classes_do does not see anonymous classes, but CLDG::classes_do 
does. Why is this okay?

You mentioned bugs like JDK-8024423 "missing anonymous classes" but my 
limited understanding of that bug suggests to me that the fix was to 
completely hide anonymous classes not to expose them! They should not be 
visible to JVM TI as I understand the intent of VM anonymous classes!


> The only place where methods_do() is called for all the methods is for
> print_method_data_histogram and print_method_profiling_data. These
> should only use loaded classes, so I've made the change below:
> void ClassLoaderData::methods_do(void f(Method*)) {
>   // Lock-free access requires load_ptr_acquire
>   for (Klass* k = load_ptr_acquire(&_klasses); k != NULL; k =
> k->next_link()) {
>     if (k->is_instance_klass() && InstanceKlass::cast(k)->is_loaded()) {
>       InstanceKlass::cast(k)->methods_do(f);
>     }
>   }
> }
>> It also seems a little odd to switch from SD to CLDG for classes_do,
>> but go the other way, from CLDG to SD for methods_do ? I would
>> expect/hope to have a single "entry point" for this kind of iteration.
> I know.  Unfortunately SD::methods_do includes the methods added for
> MethodHandle intrinsics, which is owned by SystemDictionary.
> void SystemDictionary::methods_do(void f(Method*)) {
>   ClassLoaderDataGraph::methods_do(f);
>   invoke_method_table()->methods_do(f);
> }
> I don't like this either.   This invoke_method_table() really has
> nothing to do with the Dictionary, it's just where it ended up.  I could
> expand the change to find a better place for it.  I don't know where
> would be good though.
> Aside, I'd forgotten about invoke_method_table - it seems to be a
> mechanism for isolated methods.
> Coleen
>> Thanks,
>> David
>>> Tested with java.lang.instrument, sun.com.jdi, tonga colocated (closed)
>>> tests, and JPRT, because of difference in which classes_do is called for
>>> heap dumping.
>>> Note, will update copyrights on commit.
>>> Thanks,
>>> Coleen

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