RFR: 8350938: ResourceParsingClassHierarchyResolver inflates all Utf8 CP entries

[ExE Boss]

On Fri, 21 Nov 2025 16:54:48 GMT, Trevor Bond <duke at openjdk.org> wrote:

> 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 |

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

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PR Comment: https://git.openjdk.org/jdk/pull/28458#issuecomment-3569660591