Encoding Ownership Types in Java Nicholas Cameron James Noble Victoria University of Wellington, New Zealand

Ownership types for real life Ownership types are great! – (More later...)

Ownership types for real life But ownership type systems are big and complex – And writing compilers is hard – And the type systems are not well-understood

Ownership types for real life There is another way...

Ownership Types Are a facilitating type system: – Effects Parallelisation Optimisation – Concurrency – Memory management – Security –...

Ownership Types When the heap gets large, reasoning gets hard Solution: break it up into smaller regions – BUT, we don’t program this way Nest the regions – Welcome to ownership types!

Ownership Types owner:ClassName – this:C – world:D owner keyword names the owner of this – owner:C Context parameters add flexibility

Java Generics – List

Java Wildcards – List

End of Background...

Basic idea We use type parameters to mimic ownership parameters (OGJ)

An object’s owner (and the ‘world’ context) class C {...} world:C class C {...} C – class World {}

Context parameters Become type parameters

Bounds

The ‘this’ context This* is where it gets interesting We depart from OGJ – (OGJ does this with magic) Must correspond with the this variable *no pun intended

The ‘this’ context Kind of like another context parameter – class C {... } We can name This within the class

The ‘this’ context But this cannot be named outside the class – So neither should This Use a wildcard to hide This

The ‘this’ context class E world:E class E E

The ‘this’ context But, what about nesting?

The ‘this’ context Use bounds – class C Wildcards inherit declared bounds – C

The ‘this’ context class E world:E class E<C1, C2, Owner, This extends Owner> E

The ‘this’ context class E world:E class E<C1, C2, Owner, This extends Owner> E (E ) new

The ‘this’ context

The type system thinks there is a hierarchy –  X inside  Y inside  Z inside... But in reality all owners are World

Nice...

Existential Owners and variant ownership Use wildcards

Inner Classes Require inner classes to be able to name surrounding This parameter – Comes naturally with Java generics

Type Parameters Work alongside translated context parameters class F {... } world:F class F {... } F

Universes rep CC peer CC any CC

and... Ownership Domains Context-parametric methods Dynamic aliases Fields as contexts Existential downcasting

Owners-as-Dominators Most of the work is done by the hiding of This using wildcards Must ensure it cannot be named indirectly Works with the extensions too – Including inner classes

Owners-as-Dominators Cannot be enforced by translating compiler Requires enforcing well-formedness of intermediate types

Contributions Prototype compilers – Ownership types++ – Universes How to leverage existing compiler technology for OTs Formalisation of OTs in Java – Proved sound – Ownership hierarchy is preserved and enforced at runtime Better understanding of OTs

Thank you!