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

[Chris Plummer]

On Fri, 16 Sep 2022 14:55:42 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.

src/hotspot/share/classfile/classLoaderHierarchyDCmd.cpp line 187:

> 185:   }
> 186: 
> 187:   void print_with_childs(outputStream* st, BranchTracker& branchtracker,

I know this isn't part of your changes, but the plural of child is children, not childs. However, I think child_nodes would probably be best here.

src/hotspot/share/classfile/classLoaderHierarchyDCmd.cpp line 320:

> 318:     return (loader_klass() != NULL) && (loader_klass() == target_node->loader_klass()) &&
> 319:            ((loader_name_oop() == NULL && target_node->loader_name_oop() == NULL) ||
> 320:              (::strcmp(loader_name(), target_node->loader_name()) == 0));

This is hard to parse. I think using if/else statements would be cleaner.

test/hotspot/jtreg/serviceability/dcmd/vm/ClassLoaderHierarchyTest.java line 124:

> 122:         output.shouldContain("      +-- \"Kevin\", ClassLoaderHierarchyTest$TestClassLoader");
> 123:         output.shouldContain("      +-- ClassLoaderHierarchyTest$TestClassLoader");
> 124:         output.shouldContain("      |     +-- \"Bill\", ClassLoaderHierarchyTest$TestClassLoader");

Can you test using one long string, and do the same for each section below? I think embedding \\R as the newline character should work on all platforms. If you run into unforeseen issues, then don't worry about.

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