Class files in ByteBuffer

[David Lloyd]

On Wed, Mar 12, 2025 at 11:51 AM Maurizio Cimadamore <
maurizio.cimadamore at oracle.com> wrote:

>
> On 12/03/2025 16:25, Brian Goetz wrote:
>
> That does seem like a more future-proof choice.  (I suspect too it would
> be less intrusive to adapt the internals to MS than BB.)
>
> They are probably similar in spirit -- but at least you would know that
> the MS path is more aggressively/actively optimized.
>
> I do share some of Chen's concerns -- random access on MS (and BB) is not
> comparable to random access on a byte[]. So changing the internals of the
> classfile API to use MS/BB is something that needs to be done carefully
> (and with benchmarks at hands).
>
> One possible area where adopting a "more raw" buffer would be beneficial
> is when writing/reading custom attributes -- since BB/MS will already
> provide the primitives we need to access load/store primitive values
> from/in the buffer. But -- again, something that requires care and
> consideration, it's not a slam dunk.
>
Internally, (on the parsing side at least) it is my expectation that we
would not likely be able to get away with having a single, general access
strategy using the `MemorySegment` API (but we could test that now, even
without the suggested API changes - I would love to be wrong). It seems
more likely that we'd want to keep the current array-based strategy (which
uses `Unsafe` liberally) and add a new direct memory address-based access
strategy (also using `Unsafe` in an equivalent manner), and select the
strategy based on the kind of `MemorySegment` or `ByteBuffer`.

Having three parse and build APIs (one for each of `byte[]`, `ByteBuffer`,
and `MemorySegment`) makes sense to me because there's a use case for each
of them, and they can be implemented in terms of one another to a great
extent which gives a lot of flexibility. Particularly, it seems to me that
as long as `ClassLoader.defineClass(String,ByteBuffer,ProtectionDomain)`
exists, then ByteBuffer should be floated up to the API, even if it ends up
being e.g. `MemorySegment.ofByteBuffer()` on the inside. (That said, I
wouldn't hate it if a new `defineClass` which uses `MemorySegment` could be
defined someday.)

-- 
- DML • he/him
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