Replace ASM + Gizmo with the Java Class File API (backport)

Goal

Replace all direct ASM and Gizmo usage in the critter code generator with David Lloyd's Class File API backport (io.github.dmlloyd:classfile-backport). The backport runs on Java 17 and is API-identical to the standard java.lang.classfile.* package finalized in JDK 24 (JEP 484) — only the package prefix differs. When the project eventually raises its Java baseline to 24, the backport is dropped and import statements are updated.

Net dependency change in core/pom.xml:

• Remove: org.ow2.asm:asm-tree, org.ow2.asm:asm-util, io.quarkus.gizmo:gizmo
• Add: io.github.dmlloyd:classfile-backport

ASM will no longer appear as a direct dependency. (Confirm exact backport Maven coordinates before starting.)

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Background

Critter currently uses ASM and Gizmo in two distinct roles:

Reading — parse entity .class files into ClassNode / FieldNode / MethodNode / AnnotationNode trees to discover fields, methods, and annotations.

Writing — generate new class bytecode for entity models, property models, and accessor classes. The ASM-level generators (AddFieldAccessorMethods, AddMethodAccessorMethods) inject synthetic methods directly into entity bytecode using raw opcodes. The Gizmo generators (GizmoEntityModelGenerator, PropertyModelGenerator, PropertyAccessorGenerator, VarHandleAccessorGenerator) build new classes via Gizmo's ClassCreator / MethodCreator abstraction.

The Class File API replaces both roles with a single, consistent API: ClassFile.of().parse(bytes) for reading and ClassFile.of().build(classDesc, classBuilder -> ...) / ClassFile.of().transformClass(...) for writing.

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Work items

1. Add backport dependency, update pom files

Add io.github.dmlloyd:classfile-backport to the parent BOM and core/pom.xml. Remove the three outgoing dependencies listed above.

2. New base generator: BaseClassFileGenerator (replaces BaseGizmoGenerator)

BaseGizmoGenerator wraps ClassCreator.Builder and routes bytes to critterClassLoader.register(). Replace with a base that calls ClassFile.of().build(classDesc, classBuilder -> ...) and registers the resulting byte[] directly.

3. Rewrite class reading

Replace new ClassReader(stream).accept(classNode, 0) with ClassFile.of().parse(bytes) returning a ClassModel. Affects:

• CritterGizmoGenerator.generate() (rename class to CritterGenerator)
• PropertyFinder.readClassNode()
• GizmoEntityModelGenerator.registerAnnotations() parent-hierarchy walk

The central data carriers (ClassNode / FieldNode / MethodNode / AnnotationNode) are replaced by ClassModel / FieldModel / MethodModel / java.lang.classfile.attribute.Annotation throughout. Every constructor and method signature that currently takes a FieldNode or MethodNode is updated accordingly.

ASM	Class File API
ClassNode	ClassModel
FieldNode	FieldModel
MethodNode	MethodModel
AnnotationNode	classfile.attribute.Annotation
Type.getType(c)	ClassDesc.of(c.getName())
type.getDescriptor()	classDesc.descriptorString()
type.getInternalName()	classDesc.internalName()
Type.getReturnType(desc)	MethodTypeDesc.ofDescriptor(desc).returnType()
Type.getArgumentTypes(desc)	MethodTypeDesc.ofDescriptor(desc).parameterList()
type.getSort() == ARRAY	classDesc.isArray()

4. Rewrite class writing: AddFieldAccessorMethods + AddMethodAccessorMethods

Replace ClassWriter + MethodVisitor + raw opcodes with ClassFile.of().transformClass(originalBytes, ClassTransform). The idempotency filter that strips pre-existing __read/__write methods moves into a MethodTransform that drops matching method elements. Instruction emission via CodeBuilder (named methods; no opcode constants).

5. Rewrite GizmoEntityModelGenerator

Generated methods are all constant-return or short invocation sequences; they map directly to CodeBuilder. loadProperties() and registerAnnotations() become repeated new + constructor-call + invokevirtual patterns. Depends on Item 8 for annotation bytecode emission.

Note: PR #4268 (Fix CritterMapper lifecycle callbacks) removes the hard-coded hasLifecycle() override and abstract hasLifecycle() from CritterEntityModel, and changes ctor() to use GizmoExtensions.emitClassRef() for non-public entity classes. Review those changes before implementing this item.

6. Rewrite PropertyModelGenerator

Largest single piece (~530 lines). Simple boolean/String-return methods are trivial. Key patterns:

• getLoadNames() — anewarray + aastore loop + invokestatic Arrays.asList()
• emitGetTypeData() — recursive TypeData(Class, TypeData[]) constructor emissions
• registerAnnotations() — depends on Item 8
• emitClassRef() — non-public class fallback via Class.forName(); identical logic, different instruction API

Note: PR #4270 (Consolidate emitTypeData/emitClassRef into GizmoExtensions) moves the private copies of these utilities to GizmoExtensions, making it the sole owner. Review before implementing.

Note: PR #4271 (Replace hand-rolled signature parser with ASM SignatureReader) replaces the manual typeData() / balanced() string-parsing with SignatureReader + SignatureVisitor. If merged before this work starts, the target is to replace SignatureReader/SignatureVisitor (ASM) with the Class File API's Signature type hierarchy (ClassTypeSig, TypeVarSig, etc.), which provides the same structured parse tree without ASM.

7. Rewrite PropertyAccessorGenerator

Short (~160 lines). get() and set() become CodeBuilder sequences: checkcast, invokevirtual __readXxx, typed return. Gizmo's smartCast for primitives becomes explicit box/unbox instructions — a small per-primitive helper covers all eight cases.

8. Rewrite VarHandleAccessorGenerator

Most structurally complex due to exception handling and conditional branching.

• TryBlock / addCatch → CodeBuilder.trying(tryHandler, catchesHandler)
• BranchResult / ifNull → CodeBuilder.ifThen() / ifThenElse()
• SignatureBuilder.forClass().addInterface(parameterizedType(...)) → ClassSignature.of(...) from the backport's Signature API

The VarHandle/MethodHandle runtime logic (MethodHandles.privateLookupIn, findVarHandle, findVirtual) is unchanged — those are runtime calls being emitted, not compile-time API choices.

Note: PR #4267 (Refactor VarHandleAccessorGenerator.ctor() into private helpers) splits the 76-line ctor() into emitPrivateLookup(), emitVarHandleLookup(), emitGetterHandleLookup(), and emitSetterHandleLookup(). Review that structure before implementing.

9. Rewrite GizmoExtensions

With both ASM and Gizmo gone this class becomes a pure Class File API utility:

• annotationBuilder(classfile.Annotation, CodeBuilder) — emits bytecode that calls the annotation's builder
• load(CodeBuilder, Type, Object) — emits constant loads for various value types
• emitTypeData(CodeBuilder, TypeData<?>) — emits TypeData constructor calls
• asClass(ClassDesc, ClassLoader) — primitive type detection via ClassDesc.isPrimitive() / .isArray()
• rawType() / attributeType() — pure reflection utilities; no API dependency; unchanged
• Drop the AnnotationNode branch in load()

10. Update build-plugins/AnnotationNodeExtensions.kt (the code generator)

This Kotlin Maven Mojo generates AnnotationNodeExtensions.java at build time. The generated file currently hard-codes both ASM and Gizmo types in every per-annotation method:

// currently generated:
void setBuilderValues(AnnotationNode, MethodCreator, ResultHandle)
<T> T toMorphiaAnnotation(AnnotationNode)

These need to change to:

void setBuilderValues(classfile.attribute.Annotation, CodeBuilder)
<T> T toMorphiaAnnotation(classfile.attribute.Annotation)

The processTypeJava() and processArrayTypeJava() Kotlin helpers must be updated for the different annotation value access patterns of the Class File API:

Value kind	ASM (AnnotationNode.values flat list)	Class File API (AnnotationElement)
Enum	((String[]) val)[1]	((AnnotationValue.OfEnum) val).constantName()
Class	((org.objectweb.asm.Type) val).getClassName()	((AnnotationValue.OfClass) val).classSymbol().displayName()
Nested annotation	(AnnotationNode) val	((AnnotationValue.OfAnnotation) val).annotation()
String/int/boolean	same cast	same cast
String array	(List<String>) val	((AnnotationValue.OfArray) val).values() then cast each element

setBuilderValues currently generates Gizmo invokeVirtualMethod(MethodDescriptor, ...) calls; it needs to generate CodeBuilder.invokevirtual(ClassDesc, String, MethodTypeDesc) calls instead.

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Suggested sequencing

Item 1 (deps)
  → Item 2 (base generator)
    → Item 3 (class reading + data model swap)
      → Item 10 (code generator for AnnotationNodeExtensions)
        → Items 4, 5, 6, 7, 8, 9 (can be done in any order)

Items 4–9 can be parallelised once Item 10 is done, since they depend on the generated AnnotationNodeExtensions signature but are otherwise independent.

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Acceptance criterion

All critter tests pass with -Dmorphia.mapper=critter:

./mvnw test -pl :morphia-core -Dmorphia.mapper=critter -Ddeploy.skip=true

The generated bytecode contracts are identical to today's — only the generators change.

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Pending PRs that affect this scope

The following open PRs touch files in scope. Review their changes before implementing the corresponding items above:

PR	Affects
#4267 — Refactor VarHandleAccessorGenerator.ctor() into helpers	Item 8
#4268 — Fix CritterMapper lifecycle callbacks	Item 5
#4269 — Fix PropertyFinder inherited getter discovery	Item 3
#4270 — Consolidate emitTypeData/emitClassRef into GizmoExtensions	Items 6, 9
#4271 — Replace hand-rolled signature parser with ASM SignatureReader	Item 6
#4272 — Add explicit tests for critter coverage gaps	Testing baseline

Start with the pending PRs listed in the issue, then inspect core/pom.xml, BaseGizmoGenerator, CritterGizmoGenerator, and the named generator classes to map the dependency and API migration. Run ./mvnw test -pl :morphia-core -Dmorphia.mapper=critter -Ddeploy.skip=true as a baseline. Done means ASM and Gizmo are removed from the critter generators and all critter tests pass with equivalent generated bytecode contracts.