RFR: 8334714: Class-File API leaves preview [v2]

[Chen Liang]

On Sat, 24 Aug 2024 05:03:33 GMT, xxDark <duke at openjdk.org> wrote:

> If Hotspot now parses these files incorrectly, it might be worth to just throw `UnsupportedClassVersionError` for oak class files?

This would be an important decision; I would wait till Monday when more engineers are back to discuss about the histroy of dropping support for such files, why this format never made its way to the JVMS, and the correct way to handle so as to avoid inconsistency between different versions.

> Edit: I missed the word `preview` in the quote. Does this only apply to *preview* class file features?

Yes, Class-File API is supposed to handle whatever the VM handles. It's actually considered a "JVM API" in Oracle's documentations: https://docs.oracle.com/en/java/javase/22/vm/class-file-api.html

OpenJDK's preview version policy allows shipping the preview features for testing to a wider audience without any implied burden of long-term maintenance. So for example, if the Nullable Types JEP preview adds `~` as a null marker in `Signature` attribute but changes it to `?` in a subsequent preview, the JVM and core libraries will act as if a previous iteration with `~` marker never existed; the policy is similar to that for preview libraries.

Unfortunately, there seems to be some misunderstanding around the role of preview, so some are asking if preview features that is largely unchanged and finalized in a future release can become finalized in an LTS as well. Short answer, no: it's an experiment format enabled by the new 6-month release schedule.

And note that javap uses ClassFile API, and as a result, javap for release N cannot correctly decompile Class Files with version N-1.65535, etc. We are planning to decompile on a best-effort basis, that we try to interpret such as class as if it's N.65535 (maybe emit a warning), trying to salvage the most out of the Class File and printing errors (but continue to output) if we encounter anything we don't expect.

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PR Comment: https://git.openjdk.org/jdk/pull/19826#issuecomment-2308402818