Welcome to the Minimal Value Types early adopter's project !

What is the  Minimal Value Types (MVT) project?

• The Minimal Value Types project is an early prototype for Value Types
  • provides initial subset of Value Type functionality
  • provides a new type which is:
    • immutable, identity-agnostic, non-nullable, non-synchronizable, final
    • does not inherit from java.lang.Object

• • Arrays of value types are flattened
  • Can contain primitives or references

Target Audience

• Power users  - Java/JVM Language, Framework, Library authors/exports
  • who are comfortable with early experimental software
  • who recognize that everything in the experiment - the model, the classfile extensions, the byte codes is likely to change
  • who want to contribute to early exploration of Value Types
  • who will not build any products based on these prototypes
• Who are willing to provide feedback to the developers on a subset of Value Type features
• Who will provide use cases for the development team to experiment with optimizations

How to Try Minimal Value Types:

Repository and Build Instructions

To create a new mvt branch:

hg clone http://hg.openjdk.java.net/valhalla/valhalla valhalla

cd valhalla

sh get_source.sh

sh common/bin/hgforest.sh update -r mvt

To update repository: 

cd valhalla

sh common/bin/hgforest.sh pull

sh common/bin/hgforest.sh update -r mvt

To build repository

bash configure

make images

http://cr.openjdk.java.net/~chegar/docs/sandbox.html // instructions for working with branch repositories

Note: Valhalla is a child of the jdk10/hs repository, to keep current with latest hotspot development.

Programming Model

• Create a POJO using an experimental javac, with an annotation ValueCapableClass (jvm.internal.value.ValueCapableClass)
  • JVM will derive a Derived Value Class (DVC) from this defined "box" which we call the Value Capable Class (VCC)
  • DVC contains an immutable copy of the instance fields from the VCC
• Work with MethodHandles and experimental reflection package ValueType (jdk.experimental.value.ValueType)
• Or spin your own byte codes
  
  • experimental MethodBuilder (jdk/experimental/value.MethodHandleBuilder)
  • future: ASM support is planned
• Code samples
  • See directory jdk/test/valhalla/mvt in your repository
    • e.g. MethodHandlesTest

Run Experimental MVT

• java -Xverify:none -XX:+EnableMVT <Test>
• Command-line flags: Minimal Value Types Command-line Options

Filing Bugs

• File openjdk bugs with synopsis starting with "[MVT]" and labels: "mvt"

Limitations

• platforms: x64 Linux, x64 Mac OS X
• no verifier (work-in-progress)
• no VarHandles
• no support for atomic fields containing value types
• no JNI, unsafe, redefineclasses, reflection
• -Xint and C2 only, no C1, no tiered-compilation
• interpreter is not optimized, focus is on JIT optimization
• Need additional USE CASES for optimizations
• Working on additional test cases

References

• Experimental Appendix for Java Virtual Machine Specification for Value Classes
• http://cr.openjdk.java.net/~chegar/docs/sandbox.html  // instructions for working with branch repositories

• Minimal Value Types Command-line Options
• Youtube JVMLS 2017: Minimal Value Types: Origins and Programming Model
  
• Youtube JVMLS 2017: Minimal Value Types: Under the Hood