This year the JVM Language Summit 2019 was held on July 29th – 31st at Santa Clara, California. On the first day of the Summit, Oracle Java language architect Brian Goetz gave a talk on updates to OpenJDK Project Valhalla. He shared details on its progress, challenges being faced and what to expect from Project Valhalla in the future. He also talked about the significance of Project Valhalla’s LW2 phase which was released earlier last month.
OpenJDK Project Valhalla is now ready for developers to use for early-adopter experimentation in data structures and language runtimes, concluded Goetz.
The main goal of OpenJDK Project Valhalla is to reboot the Java Virtual Machine (JVM) relationship with data and memory and in particular to enable denser and flatter layouts of object graphs in memory. The major restriction in the development of object layout has been the object identity. Object identity enables mutability, layout polymorphism and locking among others. As all objects do not need object identity and it would be impractical to determine whether an identity is relevant or not, Goetz expects programmers to inform about the whereabouts of a class such that it will make it easier to make a broader range of assumptions about it.
Who cares about Value types?
Goetz believes that value types are important for many applications and writers who desire a better control of memory layout and like to use memory very wisely. He says that library writers would always prefer Value types as it allows them to use all the traditional abstracts without paying the runtime cost of taking an extra indirection every time somebody uses a particular abstraction. Thus library classes like optional or cursors or better numerix do not have to pay the object tax.
Similarly, compiler writers of non-Java languages use Value types as an efficient substrate for language features like tuples, multiple return, built-in numeric types and wrapped native resources. Thus both library writers and compiler writers and their users pay the object tax.
Value types, in a nutshell, can help programmers make their code run faster.
Erased and specialized generics
Currently, OpenJDK Project Valhalla uses erased generics and will eventually have specialized generics. In an erased generics, Valhalla uses the knowable type convention where the erased list of values can be called as Foo<V?>. This will also be moved to specialized generics later on. He also adds that this syntax cannot be used as of now, as the Valhalla team still does not have existing utterances of Foo for spontaneously changing their meaning. Goetz hopes that the migration of generic classes like Array List<T> to specialized generics would be painless.
New top types
Project Valhalla needs new top types RefObject and ValObject for references and values as types are used to indicate a programmer’s intent. It helps the object model reflect the new reality, as everything is an object, but every object does not need an identity. There are many benefits of implementing ref-ness and val-ness into the type system such as:
- Dynamically ask x instanceof Ref object
- Statically constrain method parameters or return values
- Restrict type parameters
- Natural place to hang ref- or val-specific behavior
Nullity is labelled as one of the most controversial issues in Valhalla. As many values use all their bit patterns, Nullity interferes with a number of useful optimizations. On the other hand, if some types are migrated towards values, the existing code will assume nullability. Nullity is expected to be a focus of the L3 investigation.
What to expect next in Project Valhalla
Lastly, Goetz announces that developers building data structures or compiler runtime libraries can start using Project Valhalla. He also adds that the Project Valhalla team is working hard to validate the current programming model by working on quantifying the costs of equality, covariance, etc and is trying to better the user control experience.
Goetz concluded by stating that OpenJDK Project Valhalla is at an inflection point and is trying to figure out Nullity, Migration, specialized generics and support for Graal in the future builds.
You can watch the full talk of Brian Goetz for more details.