Great news: Lucene 9.4 (optionally) uses mapped MemorySegments when JDK 19 is detected

[Alan Bateman]

On 02/10/2022 11:37, Uwe Schindler wrote:
> At moment there's nothing that ensures the resources are loaded from 
> same folder or are consistent with the classes at runtime. So in my 
> opinion, I would have reduced the whole MR-JAR classloading to "go 
> through the feature version folders backwards starting from current 
> Java version and pick the first class found that is no preview or "has 
> correct major version and is marked as preview".

Class files are located in MR-JARs as resources, e.g. loading p.C will 
attempt to the resource "p/C.class". You can't have a class loader 
loading the class file bytes from one section and the equivalent 
resource lookup locating the class file resource in another section. I 
don't think it feasible to do what you want without updating the MR JAR 
support, e.g. META/versions/19.preview.


> :
>
> Thanks for this feedback. We see it exactly as you described it. 
> Lucene is a library and developers out there should be able to test 
> new JDK preview features with it. After reading the Preview Spec I was 
> able to implement this (the notes at end of the preview spec about 
> reflection were very useful to get it running).
>
> For Lucene, a downstream developer does not need to compile his own 
> classes with --enable-preview (never gets in touch with JDK classes 
> marked as preview), but a downstream developer or a user of 
> Elasticsearch and/or Solr can test our new implementation. This allows 
> users and developers to test the new implementation. To me this is 
> already a success of preview APIs. With the previous incubation this 
> was not "easy doable".

It's good you got it working, a blog post as you mentioned could be 
useful read for others that want to do the same.

-Alan.