8350938: ResourceParsingClassHierarchyResolver inflates all Utf8 CP entries

[trevorkbond]

Enhance ResourceParsingClassHierarchyResolver.getClassInfo to use ClassReaderImpl to improve performance. Previously this method inflated and stored all UTF-8 entries in the constant pool and later accessed the array of strings to try finding the name of the superclass. ClassReaderImpl instead stores constant pool offsets and later lazily reads/inflates UTF8 entries as needed.

I’ve ran all tier 1 tests and tests within test/jdk/jdk/classfile on the latest version of this change, and they all pass.

I ran some informal performance testing to see if these changes led to any improvement. I created a .class file with several thousand unique strings in a String array to deliberately enlarge the number of UTF-8 entries in the constant pool. I then benchmarked the performance of running a process nearly identical to ClassHierarchyInfoTest.testClassLoaderParsingResolver on this custom class over 200 runs using JMH. The results are as follows.

Version	Avg Time (ns/op)	Δ vs Before
Before	1,483,671.539 ± 3,477.744	—
After	1,380,064.517 ± 3,482.434	≈ 7.0% faster

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Issue

• JDK-8350938: ResourceParsingClassHierarchyResolver inflates all Utf8 CP entries (Enhancement - P5)

Reviewers

• Chen Liang (@liach - Reviewer)

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[bridgekeeper]

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[openjdk]

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8350938: ResourceParsingClassHierarchyResolver inflates all Utf8 CP entries

Reviewed-by: liach

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At the time when this comment was updated there had been 45 new commits pushed to the master branch:

• d9b6c31: 8372324: jdk/jfr/event/gc/detailed/TestStringDeduplicationEvent.java#Parallel failed
• b36b694: 8371259: ML-DSA AVX2 and AVX512 intrinsics and improvements
• c0abecd: 8372441: JFR: Improve logging of TestBackToBackSensitive
• ... and 42 more: https://git.openjdk.org/jdk/compare/5f806e7f8b66c14954a07005be3bceb2f812fbff...master

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[mlbridge]

Webrevs

• 01: Full - Incremental (a990cd30)
• 00: Full (c906c061)

[dansmithcode]

I'm available as a sponsor when we get this reviewed

[ExE-Boss]

It might be better to make the previous version lazy, as the new version requires pre‑parsing the whole input, whereas the old version would only read the classfile header.

[jdlib]

The new version would always read all classfile bytes, the old one in chunks of 8192 bytes, both stop parsing at this_class position (with or without inflating constant pool entries).
Maybe the benchmark would not show much difference for typical classfiles (not having thousands of strings).
But in my eyes removing the logic to parse the constant pool and instead reuse ClassReader is a good thing.

[ExE-Boss]

Maybe ClassReader could be updated to have a version which lazily uses an InputStream.

[liach]

Looks good in principle. Even though people wonder about the buffering policy, I think it is negligible considering that we have caching mechanism. And the constant pool is flexible sized, so "only read the classfile header" might be an optimisitic assumption.

[liach]

Suggested change

	ClassReader reader = new ClassReaderImpl(ci.readAllBytes(), (ClassFileImpl) ClassFile.of());
	var reader = new ClassReaderImpl(ci.readAllBytes(), ClassFileImpl.DEFAULT_CONTEXT);

And you can remove the 3 imports you have added.

[trevorkbond]

Thank you for this. I adjusted that line and the imports accordingly. I still found a need to import java.lang.classfile.constantpool.ClassEntry for the line further down converting the ClassEntry to a ClassDesc. If that can also be avoided I'm happy to fix. I also removed a few imports previously needed but that can be removed with these changes.

[liach]

Also, the key to this patch is not performance - it is about the maintainability from a consistent way to parse class files, so in the future, we don't need to update this site any more if we have new constant pool entry type or other format expansions.

[trevorkbond]

/integrate

[openjdk]

@trevorkbond
Your change (at version a990cd3) is now ready to be sponsored by a Committer.

[jaikiran]

Hello Trevor, please update the copyright year on ClassHierarchyImpl.java from 2022, 2024, to 2022, 2025,.