RFR: JDK-8293156: Dcmd VM.classloaders fails to print the full hierarchy [v2]

[David Holmes]

On Tue, 20 Sep 2022 05:37:44 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> Fixes a bug in the `VM.classloaders` jcmd that causes class loaders to be omitted from the output if a parent class loader never loaded any class and therefore had no associated DCmd.
>> 
>> The fix changes the command to not rely on the existence of a CLD structure for the loader; instead, all information (loader class name, loader name, etc) is pulled via the loader oop, which has to be always there unless its the bootstrap loader.
>> 
>> Also, the tests were expanded to test the display of empty loaders and empty parent loaders.
>> 
>> Thanks to @dholmes-ora for finding this bug.
>
> Thomas Stuefe has updated the pull request incrementally with one additional commit since the last revision:
> 
>   cjplummer feedback

Hi Thomas,

Generally this seems fine - but one issue below.

Thanks.

src/hotspot/share/classfile/classLoaderHierarchyDCmd.cpp line 209:

> 207:         st->print(" \"%s\",", the_loader_name);
> 208:       }
> 209:       st->print(" %s", the_loader_class_name);

`the_loader_class_name` could be NULL here. You need to change `loader_class_name()` t return "??" like the original code uses.

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Changes requested by dholmes (Reviewer).

PR: https://git.openjdk.org/jdk/pull/10312