Project Valhalla (Java language)
Project Valhalla is an experimental OpenJDK project to develop major new language features for Java 10 and beyond. The project was announced in July 2014 and is an experimental effort by Oracle, led by engineer Brian Goetz.
Valhalla is incubating Java language features and enhancements in these areas:
- Value Types; highly-efficient small 'objects' without inheritance.
- Generic Specialization;
- Reified Generics; retaining actual type at runtime.
- improved 'volatile' support.
These features will require both syntax and VM-level changes.
The project is organized on the OpenJDK 'Project Valhalla' mailing list. Project Valhalla has begun in the experimental stages, with an inaugural announcement & early draft initial proposals.
Published to date:
- Project Valhalla announcement
- 'Value Types' initial proposal
- 'Generic Specialization' initial proposal
Experimental OpenJDK prototypes can also be built from the Mercurial repositories. These are starting to incorporate many specialization features. Note that these are Gnu Make/ C++ sources, and building the OpenJDK requires technical expertise. See OpenJDK build instructions for details.
Technical benefits and implications
Memory access performance and the efficiency of 'boxed' value access are a major area to be addressed by these features. 'Value Type' features and 'Generic specialization' (when applied to lists or collections) reduce memory usage, but more importantly avoid pointer indirection which typically causes a cache miss.
Instead of a list or array of object references, pointing to data values scattered throughout memory, Project Valhalla enhancements will enable list or array values to potentially be laid out linearly—without indirection—as a consecutive block of memory.
Value Types are envisaged as "Codes like a class, works like an int!" Synchronization and inheritance would be excluded for Value Types. These would no longer require object identity and associated memory/ pointer overheads, though would be able to assume a 'boxed' form for compatibility.
- Goetz, Brian. "Welcome to Valhalla!". OpenJDK mail archive. OpenJDK. Retrieved 12 August 2014.
- Evans, Ben. "Oracle Launches Project Valhalla for Java". InfoQ. InfoWorld. Retrieved 12 August 2014.
- Krill, Paul. "Next generation Project Valhalla proposed; Value types, generic specialization, and enhanced volatiles top the wish-list for Project Valhalla". JavaWorld. InfoWorld. Retrieved 12 August 2014.
- "Value Types & List<int> coming for Java 10 ?". LiterateJava.com. Retrieved 12 August 2014.
- Rose, John; Goetz, Brian; Steele, Guy. "State of the Values". OpenJDK. Retrieved 12 August 2014.