Class files in ByteBuffer

[David Lloyd]

I've opened a bug [1] and pull request [2] incorporating this discussion
(more or less). I've implemented support for both `MemorySegment` and
`ByteBuffer`, but this could be revisited if it doesn't look OK. The
implementation is not terribly invasive for now, only grabbing a few
low-hanging optimizations.

[1] https://bugs.openjdk.org/browse/JDK-8352536
[2] https://github.com/openjdk/jdk/pull/24139

On Mon, Mar 10, 2025 at 12:38 PM David Lloyd <david.lloyd at redhat.com> wrote:

> When defining a class in the JDK, one may either use a byte array or a
> byte buffer to hold the contents of the class. The latter is useful when
> (for example) a JAR file containing uncompressed classes is mapped into
> memory. Thus, some class loaders depend on this form of the API for class
> definition.
>
> If I were to supplement such a class loader with a class transformation
> step based on the class file API, I would have to copy the bytes of each
> class on to the heap as a byte[] before I could begin parsing it. This is
> potentially expensive, and definitely awkward.
>
> After transformation, it doesn't really matter if you have a byte[] or
> ByteBuffer because either way, the class can be defined directly.
>
> It would be nice if the class file parser could accept either a byte[] or
> a ByteBuffer. I did a quick bit of exploratory work and it looks like
> porting the code to read from a ByteBuffer instead of a byte[]  (using
> ByteBuffer.wrap() for the array case) would be largely straightforward
> *except* for the code which parses UTF-8 constants into strings. Also there
> could be some small performance differences (maybe positive, maybe
> negative) depending on how the buffer is accessed.
>
> Is this something that might be considered?
>
> --
> - DML • he/him
>


-- 
- DML • he/him
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