By standing on the shoulders of giants (I’m looking at you, Mark Sailes), I was able to turn around an AWS Lambda Custom Runtime for Java 21 on Java 21 launch day. Instructions to use it are in the repo’s README. All files required to create your own function are in the releases, so no building required.
There are a couple of known issues, particularly around CDS (Class Data Sharing), per the but they appear to be (mostly) cosmetic at first blush. It’ll be fun ironing that out.
I’m also looking forward to releasing a custom image for Java 21, too, as soon as an updated Amazon Corretto image drops.
Happy hacking!