RFR: 8367585: Move Utf8Entry length validation earlier

[John R Rose]

On Mon, 15 Sep 2025 05:43:43 GMT, Chen Liang <liach at openjdk.org> wrote:

> John Rose suggests in https://github.com/openjdk/jdk/pull/26802#issuecomment-3201402304 that ClassFile API should validate Utf8Entry length eagerly upon construction. Currently we validate upon writing to bytes, which avoids validation overhead. However, given that most class file utf8 data are shorter than 1/3 of the max length, which is always an encodable length, the performance impact should be low.
> 
> Preventing the creation of unrepresentable UTF8 entries can prevent passing such invalid instances around, making such problems easier to debug than a failure at building.
> 
> Tier 1-3 seems clear. The performance impact to jdk.classfile.Write or any of the regularly run transformation benchmarks seems neutral, less than 5% perturbations.
> 
> I will update docs to reflect this change, given how widespread this is across JDK - it seems the only exempt classes are Signature, ClassSignature, and MethodSignature.

Early checking is good!

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PR Review: https://git.openjdk.org/jdk/pull/27281#pullrequestreview-3282616530