Phase 2: VarHandle accessor generator for runtime mapper mode

[evanchooly]

Summary

• Adds VarHandleAccessorGenerator — a Gizmo-based generator that produces PropertyAccessor<T> implementations using VarHandle (field-based) and MethodHandle (method-based properties) instead of delegating to __readXxx/__writeXxx synthetic methods injected into entity bytecode
• Adds runtimeMode flag to PropertyFinder and CritterGizmoGenerator: when true, entity bytecode is not modified and VarHandle accessors are generated instead
• This is the accessor generation path that CritterMapper (Phase 4) will use at runtime

Closes tasks in #4185.

Key design notes

• Entity class loaded via Thread.currentThread().contextClassLoader in the generated constructor to avoid classloader mismatch with CritterClassLoader (which auto-registers dev.morphia.critter.* classes)
• No checkCast to entity type in get()/set() — VarHandle/MethodHandle performs its own type verification
• Method-based read-only properties (no setter) throw UnsupportedOperationException from set(), matching compile-time behavior

Test plan

• TestVarHandleAccessor — 5 tests: String field, int primitive, Long boxed, entity not modified, all accessors instantiatable
• Full critter-core suite (57 tests) passes unchanged

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds a new runtime-mode accessor generation path for critter-core that avoids modifying entity bytecode, generating PropertyAccessor<T> implementations backed by VarHandle (fields) and MethodHandle (method-based properties). This supports upcoming runtime mapping in CritterMapper.

Changes:

• Introduces VarHandleAccessorGenerator to generate accessor implementations using VarHandle/MethodHandle.
• Adds a runtimeMode flag to PropertyFinder and CritterGizmoGenerator to skip entity bytecode modification and use the new generator.
• Adds TestVarHandleAccessor to validate runtime-mode behavior (basic field accessors + “entity not modified” check).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
critter/core/src/main/kotlin/dev/morphia/critter/parser/gizmo/VarHandleAccessorGenerator.kt	New Gizmo generator producing accessors backed by VarHandle/MethodHandle for runtime mode.
critter/core/src/main/kotlin/dev/morphia/critter/parser/PropertyFinder.kt	Adds runtimeMode to switch between bytecode-modification path vs VarHandle/MethodHandle accessors.
critter/core/src/main/kotlin/dev/morphia/critter/parser/gizmo/CritterGizmoGenerator.kt	Threads runtimeMode through generator entrypoint and exposes varHandleAccessor(...) helpers.
critter/core/src/test/kotlin/dev/morphia/critter/parser/TestVarHandleAccessor.kt	New tests for runtime-mode field accessor generation and ensuring the entity class isn’t modified.

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[Copilot]

testEntityNotModified() uses Kotlin's assert(...) for its checks. JVM assertions are often disabled in test runs unless the JVM is started with -ea, so this test can become a no-op and silently pass even when the entity is modified. Use TestNG assertions (e.g., assertTrue(...) / assertEquals(...)) so the test always enforces the expectation.

[Copilot]

hasSetter uses Class.forName(propertyType) and entity.getDeclaredMethod(...). This can (a) load the parameter type from the wrong classloader in runtime mode, and (b) incorrectly treat inherited/public setters as missing (since getDeclaredMethod doesn’t search superclasses). Consider resolving the parameter Class via the entity’s classloader/TCCL (and handling arrays), and use getMethod(...) or entity.methods-based detection to match the existing ASM path’s behavior.

[Copilot]

The generated constructor calls Class.forName(entity.name, true, tccl) with initialize=true. This can trigger entity static initialization as a side effect of merely constructing an accessor, which may be undesirable in runtime mapper mode. Consider using initialize=false unless class initialization is required for privateLookupIn/handle resolution.

Suggested change

	tryBlock.load(true),
	tryBlock.load(false),

[Copilot]

Runtime-mode method-based accessors (using MethodHandle) and the read-only UnsupportedOperationException path aren’t covered by the new tests (current tests exercise only field-based accessors on Example). Adding a runtime-mode test using MethodExample (including the read-only getComputedValue() property) would help prevent regressions in the MethodHandle path.

Suggested change

	return
	}
	// Do NOT checkCast to entity type — same classloader mismatch concern as in get().
	val castModel = method.getMethodParam(0)
	val castValue =
	if (isPrimitive) {
	val boxed = method.checkCast(method.getMethodParam(1), wrapperType)
	method.smartCast(boxed, propertyType)
	} else {
	method.checkCast(method.getMethodParam(1), propertyType)
	}
	val handleRef =
	if (isFieldBased) method.readInstanceField(handleDesc, method.`this`)
	else method.readInstanceField(setterHandleDesc!!, method.`this`)
	if (isFieldBased) {
	method.invokeVirtualMethod(
	ofMethod(
	VarHandle::class.java,
	"set",
	"void",
	Object::class.java.name,
	Object::class.java.name,
	),
	handleRef,
	castModel,
	castValue,
	)
	} else {
	method.invokeVirtualMethod(
	ofMethod(
	MethodHandle::class.java,
	"invoke",
	"void",
	Object::class.java.name,
	Object::class.java.name,
	),
	handleRef,
	castModel,
	castValue,
	)
	} else {
	// Do NOT checkCast to entity type — same classloader mismatch concern as in get().
	val castModel = method.getMethodParam(0)
	val castValue =
	if (isPrimitive) {
	val boxed = method.checkCast(method.getMethodParam(1), wrapperType)
	method.smartCast(boxed, propertyType)
	} else {
	method.checkCast(method.getMethodParam(1), propertyType)
	}
	val handleRef =
	if (isFieldBased) method.readInstanceField(handleDesc, method.`this`)
	else method.readInstanceField(setterHandleDesc!!, method.`this`)
	if (isFieldBased) {
	method.invokeVirtualMethod(
	ofMethod(
	VarHandle::class.java,
	"set",
	"void",
	Object::class.java.name,
	Object::class.java.name,
	),
	handleRef,
	castModel,
	castValue,
	)
	} else {
	method.invokeVirtualMethod(
	ofMethod(
	MethodHandle::class.java,
	"invoke",
	"void",
	Object::class.java.name,
	Object::class.java.name,
	),
	handleRef,
	castModel,
	castValue,
	)
	}