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,
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