loom VS valhalla-docs

It's more accurate to say Loom is [a particular type of] continuations in the JVM, which can be used to implement green threads / fibers.

You can see the implementation of VirtualThread here: https://github.com/openjdk/loom/blob/fibers/src/java.base/sh...

This uses the internal 'one-shot delimited continuation': https://github.com/openjdk/loom/blob/fibers/src/java.base/sh...

So, at least in principle, there is scope for other styles of concurrency to be implemented over this.

Loom is actively developed https://github.com/openjdk/loom

Currently the Thread public constructor defaults to platform threads to be fully backwards compatible. So all libraries must be updated to support the new model.

I've been casually following the repo ( https://github.com/openjdk/loom ) and the mailing list ( https://mail.openjdk.java.net/mailman/listinfo/loom-dev ) over the last 18 months or so, and it definitely appears that things are ramping up as of late. The implication (as I see it) is that they are relatively happy with the general design are and are at the point where they want help from experts in the various areas of the JDK (GC, Debugging, etc) to make sure that their pieces play well with Loom.

https://github.com/openjdk/loom/blob/fibers/src/java.base/share/classes/sun/nio/ch/NioSocketImpl.java#L177

[1] https://github.com/openjdk/loom/blob/fibers/src/java.base/share/classes/java/lang/VirtualThread.java#L933

What isn't clear for me from https://github.com/openjdk/valhalla-docs/blob/main/site/design-notes/state-of-valhalla/02-object-model.md is whether or not value types will indeed be headerless.

Regarding type erasure, see https://github.com/openjdk/valhalla-docs/blob/main/site/design-notes/in-defense-of-erasure.md for a justification. Note in particular that not doing erasure would have forced languages like Scala to follow Java's semantics. In cases where erasure is actually a problem, you can work around it with classes like https://fasterxml.github.io/jackson-core/javadoc/2.2.0/com/fasterxml/jackson/core/type/TypeReference.html.

Here's a (relatively recent) post on type erasure from the lead of the value types project at Oracle https://github.com/openjdk/valhalla-docs/blob/main/site/design-notes/in-defense-of-erasure.md

If you want to read something a little more formal and fleshed out, try scanning through this document, and then come back again and tell me that Brian doesn't understand your suggestion.

Source

I haven't been following it incredibly closely, but I have checked in from time to time. It's a hard problem, but there's a lot of real progress (https://github.com/openjdk/valhalla-docs/blob/main/site/desi...).

I'd wager that it will ship by the next LTS, in 2024.

Quotes from here