Draft JEP: Incubating Language and VM Features (original) (raw)

John Rose john.r.rose at oracle.com
Sat Jan 20 00:24:03 UTC 2018

On Jan 19, 2018, at 3:39 PM, Remi Forax <forax at univ-mlv.fr> wrote:

So let's all agree that this is nice JEP and that the way to mark a class experimental is to add an attribute Experimental to the classfile.

Remi, you are glossing over a big point here: We are going to be experimenting with constant pool formats. You can't read the access_flags or any attributes until you have parsed the constant pool. It is unreasonable to parse the constant pool before you know whether it is allowed to have experimental features in it. This leaves the magic number, major and minor versions, and the CP length itself as channels to carry the experimental bit. Of those, the most reasonable and intuitive is the minor version.

So it's the minor version. Next question: What bit pattern? Best answer, AFAIK is to reserve one bit and concentrate on that. That leaves the other minor version bits untouched and free for other uses.

Next question: Does anybody actually use the minor version bits? The spec suggests they were in play once, but I have never seen anybody use them to carry information in modern versions of Java. If those 16 bits are always zero, then clearly we can steal one of them. And I think we have both the habit and the right of declaring that they must be zero and are reserved for future extensions, such as an experimental flag.

— John

P.S. In fact (separate conversation) I would like to reserve another minor version bit to denote "private use plane", meaning that projects like Valhalla can mark their prototype classfiles using the minor version in such a way that there is guaranteed to be never any standard JVM that will ever load those prototype classfiles.

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