What's the status of / relation between "JEP 169: Value Objects" / "Value Types for Java" / "Object Layout"

[Brian Goetz]

> Question: is JEP 169 still under active development or has it been
> merged into the more general "Value types for Java" proposal below?

It has been merged into the more general Value Types for Java proposal.

> The "Value types for Java" approach clearly seems to be the most
> general but also the most complex proposal.

For some meanings of "complex".  It is certainly the most intrusive and 
large; new bytecodes, new type signatures.  But from a user-model 
perspective, value types are actually fairly simple.

> It's out of scope for Java
> 9 and still questionable for Java 10 and above. The "PackedObject" and
> "ObjectLayout" approaches are clearly simpler and more limited in
> scope as they only concentrate on better object layout.

To your list, I'd add: Project Panama, the sister project to Valhalla. 
Panama focuses on interop with native code and data, including layout 
specification.  A key goal of Packed was to be able to access off-heap 
native data in its native format, rather than marshalling it across the 
JNI boundary.  Panama is focused on this problem as well, but aims to 
treat it as a separate problem from Java object layout, resulting in 
what we believe to be a cleaner decomposition of the two concerns.

Packed is an interesting mix of memory density (object embedding and 
packed arrays) and native interop.  But mixing the two goals also has 
costs; our approach is to separate them into orthogonal concerns, and we 
think that Valhalla and Panama do just that.  So in many ways, while a 
larger project, the combination of Valhalla+Panama addresses the problem 
that Packed did, in a cleaner way.

> Question: is there a chance to get a some sort of Java-only but
> transparently optimizable structure package like "ObjectLayout" into
> Java early (i.e. Java 9)?

It would depend on a lot of things -- including the level of readiness 
of the design and implementation, and the overlap with anticipated 
future features.  We've reviewed some of the early design of 
ObjectLayout and provided feedback to the projects architects; 
currently, I think it's in the "promising exploration" stage, but I 
think multiple rounds of simplification are needed before it is ready to 
be considered for "everybody's Java."  But if the choice is to push 
something that's not ready into 9, or to wait longer -- there's not 
actually a choice to be made there.

I appreciate the desire to "get something you can use now", but we have 
to be prepared to support whatever we push into Java for the next 20 
years, and deal with the additional constraints it generates -- which 
can be an enormous cost.  (Even thought the direct cost is mostly borne 
by Oracle, the indirect cost is borne by everyone, in the form of slower 
progress on everything else.)  So I am very wary of the motivation of 
"well, something better is coming, but this works now, so can we push it 
in?"  I'd prefer to focus on answering whether this is right thing for 
Java for the next 20 years.

> In my eyes this wouldn't contradict with a more general solution like
> the one proposed in the "Value types for Java" approach while still
> offering quite significant performance improvements for quite a big
> range of problems.

The goals of the ObjectLayout effort has overlap with, but also differs 
from, the goals of Valhalla.  And herein is the problem; neither 
generalizes the other, and I don't think we do the user base a great 
favor by pursuing two separate neither-coincident-nor-orthogonal 
approaches.  I suspect, though, that after a few rounds of 
simplification, ObjectLayout could morph into something that fit either 
coincidently or orthogonally with the Valhalla work -- which would be 
great.  But, as you know, our resources are limited, so we (Oracle) 
can't really afford to invest in both.  And such simplification takes 
time -- getting to that "aha" moment when you realize you can simplify 
something is generally an incompressible process.

> Question: what would be the right place to propose something like the
> "ObjectLayout" library for Java 9/10? Would that fit within the
> umbrella of the Valhalla project or would it be done within its own
> project / under it's own JEP?

Suggesting a version number at this point would be putting the cart 
before the horse (you'll note that we've not even proposed a version 
number for Valhalla; the closest we've gotten to that is "after 9".)

OpenJDK Projects are a tool for building a community around a body of 
work; JEPs are a project-management tool for defining, scoping, and 
tracking the progress of a feature.  Given where OL is, it would be 
reasonable to start a Project, which would become the nexus of 
collaboration that could eventually produce a JEP.

Hope this helps,
-Brian