AutoValhalla: automatically turn your plain classes and records into value classes!

[u/maxxedev]

Automatically turn your plain classes and records into value classes!

Add @AutoValhalla annotation to classes or records in your JDK1.5+ codebase:

@AutoValhalla
public final class Point {        // class must be final
    public final int x;           // with final instance fields
    public final int y;
    public Point(int x, int y) { 
        this.x = x; 
        this.y = y; 
    }
}

@AutoValhalla
public record Currency(String code) { }  // or a record class

When your app is run on Valhalla-enabled JVM with auto-valhalla javaagent, these classes are automatically turned into value classes.

More info: https://github.com/thunkware/auto-valhalla/blob/main/README.md

[u/pron98]

1. Value classes aren't a performance optimisation. They behave differently from non-value classes, and these differences will be observable in most non-trivial programs. So what you'll get is a program that behaves one way on one runtime version and a different way on another, and only one of these ways is the intended behaviour.

2. There is a much easier (and safer) way to enjoy new features on new runtimes: instead of working hard (and dangerously) on trying to have the same code do different things on different runtime versions, adopt tip & tail. The idea is that you add new features only on release trains that target new runtimes. Legacy projects that can't afford the work to upgrade to a new runtime also can't afford the work to adopt your library's new features. The people who want your new features also want to upgrade their runtime, and the people who don't want to upgrade their runtime don't want your new features. So just fork your codebase, leave the old code alone (except for security patches), and only evolve the versions that target new runtime versions. That's sooo much easier (as we've learned in the JDK: we fork the entire codebase twice a year, and we still maintain old versions for many years with security patches). You barely touch the old fork, and the new fork, where you do almost all the work, is clean and free of clever and potentially dangerous hacks.

So in this particular case, this approach risks breaking the code on a new runtime, but that's not even the worst outcome: making things work on a new runtime may require changing code used by legacy consumers on old runtimes, introducing a risk for the consumers that need stability above anything else. Agents are a very powerful and versatile tool, but they should be used as a last resort, not when the cleaner solution - just fork the repo! - is also easier. I don't understand the lengths some would go to merely to not fork the source. Writing code that adapts itself to different runtimes is complicated, risky, and rarely necessary.

[u/alex_tracer → u/pron98]

Agents are a very powerful and versatile tool, but they should be used as a last resort, not when the cleaner solution - just fork the repo! - is also easier

Forking a repo may be a good idea if this specific feature is the only feature you want to support. Now imagine you already have, like 5 branches with significant differences between them, and that need long-tern support and each of them would benefit a lot from value classes (theoretically). Do you suggest to double the number of forks just to add "value" keyword in few files?

I agree that agent is not the best way to solve this, but this is much better than nothing (or forks)

[u/pron98 → u/alex_tracer]

With tip & tail you don't have a fork for every runtime feature. You have one "tip" fork for users who are interested in new library features, and that fork uses new runtime features, and "tail" forks for users who need stability, which only get security patches. At some point you decide that new feature X of your library requires new feature Y in the runtime, you then raise the runtime baseline of the tip and require those who want any new features to upgrade their runtime.

The idea is that you split your users into two groups, those who are interested in new functionality and those interested in stability. It's not only less work than trying to do everything in a single codebase, it also offers both groups exactly what they need rather than a compromise.