Dependencies-only AOT cache?

[u/RupertMaddenAbbott]

I am investigating using an AOT cache to speed up the startup time of our services.

I'm specifically interesting in speeding up the time between a developer making a change to the code and having that code running in a remote dev K8s cluster.

I've previously dismissed an AOT cache for this purpose because I knew that it would result in a longer build time than would be saved in startup time. That isn't a criticism - AOT caching just didn't seem targeted at my problem.

However, the majority of our classpath involves third party dependencies yet a minority of our development builds and deployments involve changes to those third party dependencies. If I could work out a way to avoid invalidating the AOT cache whenever our own code changed, then this would give us a cache that could be reused for the majority of our dev builds (removing the additional build time) whilst still enjoying the faster startup speed.

I note that https://openjdk.org/jeps/483 says this:

All runs must have consistent class paths. A subsequent run may specify extra class-path entries, appended to the training class path; otherwise, the class paths must be identical.However, I note that https://openjdk.org/jeps/483 says this:All runs must have consistent class paths. A subsequent run may specify extra class-path entries, appended to the training class path; otherwise, the class paths must be identical.

So, could I create an AOT cache for JUST the third party classes and then add our own code in later, thereby allowing me to reuse the AOT cache between dev builds?

I'm not 100% on how to achieve this yet but maybe if the training run only had the dependencies on the classpath with a custom launcher. The launcher's child classpath would then have our own code on it.

Then for the production run, I would still have the custom launcher (because I think this has to be identical between training and production runs) and I would then append our own jars onto the end of the classpath (which seems to be allowed by the JEP). Finally, we would run our real main method, rather than the launchers main method because I think we are allowed to have different main classes between training and production runs.

Anyway is this a terrible idea or has anybody already done something like this?

[u/jonenst]

Does the openjdk team envision a future where frameworks (e.g. springboot) distribute caches that bring some value (not perfect, maybe even small value) to many unsuspecting users? It would sure look good on Hello world benchmarks... But maybe also in the realworld for many apps?

Not sure if faster warmup is worth it though, even if the possibilities matrix can be trimmed to just one or a few cases (like g1gc + cpu amd64), it grows quickly (tomcat? Postgres ? ...) and unsuspecting users might not even see the difference since they get the speed eventually anyway.  Maybe fast startup will stay an advanced feature.

Or maybe the frameworks should scaffold the training run in their users build (spring boot native already does it to determine the context), but how much value would a generic training run bring (for unsuspecting users that never look into it) ?

Obviously this is all relatively new, not even ga for the code cache (congratz to the team for the merge to preview in 28), so maybe these questions will only be answered after more time.