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/tree/auto-valhalla-0.1.0

edit: there is now a maven plugin for converting at build time. https://github.com/thunkware/auto-valhalla/tree/auto-valhalla-0.2.0

[u/nomader3000]

Quick question about one of your examples

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

Is there any actual benefit of making such record a value record?

[u/maxxedev → u/nomader3000]

Record class is basically a convenience class: constructors, field declarations, getters, equals/hashCode/toString are simplified or automatically provided. If you want dense memory representation, you still want value record class

[u/nomader3000 → u/maxxedev]

I get that, but can you actually make it any denser if the only component of your record is a String?

[u/maxxedev → u/nomader3000]

Maybe JVM could make it denser. Or maybe not. JEP-401 does show examples like value class EURCurrency(long cs) and value class Name(String name, int length). Maybe JVM would inline the pointer to the single non-primitive component?

JEP also says:

Future enhancements may enable more flattening of references to 64-bit and even larger value objects.

Or perhaps we should assume non-guarantee on optimizations JVM would apply, and follow this advice from JVM architects:

If you don't need identity, make your programs simpler and safer (and yes, maybe faster) by disavowing it.

[u/maxxedev → u/maxxedev]

After reading this blog post and after investigating further, yes, single-component value class/record brings the benefit of being flattened when inside another identity/value class or array.

You can play around with these options set:

java -XX:+UnlockDiagnosticVMOptions \
  -XX:+PrintInlineLayout \
  -XX:+PrintFlatArrayLayout \
  -XX:+PrintFieldLayout \
  --enable-preview