Technology

JDK 28 preview of value classes highlights JVM optimizations and performance trade-offs

While Java's Valhalla milestone reduces memory overhead by removing object identity, erased megamorphic calls can cause unexpected heap allocations.

Updated Aug 26, 2026 · 10:31 UTC1 reports

The short version

JDK 28 JEP 401 introduces value classes to the Java Virtual Machine, allowing the runtime to flatten data structures and eliminate object identity overhead.
Immutable fields enable the JVM to store multi-word values without reference pointers, whereas mutable fields must retain reference layouts to prevent data tearing during multi-threaded updates.
Megamorphic virtual calls involving generic interfaces can degrade performance by forcing scalarized values to be repeatedly allocated on the heap at method boundaries.

JDK 28 JEP 401 integrates value classes as a preview feature, enabling the JVM to flatten values and represent components in registers or on the stack by eliminating object identity.[Hacker News]
Under JEP 539, strictly initialized final fields allow the JVM to use a non-atomic flattened layout because the enclosing object cannot be observed prior to field initialization.[Hacker News]
Mutable fields exceeding atomic update limits revert to reference layouts to comply with the Java Memory Model and prevent torn assignments during concurrent thread operations.[Hacker News]
At megamorphic call sites, type erasure in Java generics forces scalarized value objects to be converted back into standard heap references to pass through erased method signatures.[Hacker News]
Engineers on the C2 compiler team are actively investigating potential compiler solutions to address performance regressions caused by value class materialization in generic code.[Hacker News]

The timeline and specific implementation details for how the C2 compiler team will resolve allocation overhead at megamorphic call sites remain under active investigation.[Hacker News]