Summary

Enhance the JVM with value classes and objects.

Problem

The JEP provides a high-level discussion motivating the feature.

The JVM, in particular, needs enhancement in --enable-preview mode, doing all of the following:

• Distinguish between value classes (which do not require/assume identity) and identity classes (which do)
• Modify the instance creation mechanism to ensure that value objects are created with final fields that can never be observed to mutate
• Automatically configure loading of modified JDK classes when the --enable-preview flag is set
• Allow for and signal the loading of the value classes used by a particular class early enough to influence the class's layout and method calling conventions
• Provide identity-free semantics for the if_acmpeq, if_acmpne, and monitorenter instructions

Solution

Our solution enhances the JVM as follows:

• Repurpose the ACC_SUPER flag to be interpreted (in preview class files) as ACC_IDENTITY; identity classes set this flag, while value classes and interfaces do not. Validate that value classes conform to various constraints: do not extend identity classes, do not declare inappropriate instance fields, do not declare synchronized methods
• Ensure that all instance fields of a value class are strictly-initialized: they must be marked with the ACC_STRICT flag (formerly used to express floating-point properties on methods), which instructs verification to prevent any putfield operations after the super() constructor call. Field initialization is only allowed before that point.
• Have the launcher reconfigure the java.base module to load special preview class files when preview features are enabled
• Introduce a new attribute, LoadableDescriptors, to indicate classes mentioned in descriptors that can be speculatively loaded during class loading, and might be loaded earlier than usual during linking
• Update if_acmpeq and if_acmpne to compare instances of value classes in terms of their field values; update monitorenter to reject value class instances

A more comprehensive outline of the solution can be found in the JDK-8317278 description.

Specification

JVMS changes are included in the attached spec change document bundle. See the file specs/value-objects-jvms.html.