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fix: resolve @InjectMockKs initialization order based on dependencies#1500

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Raibaz merged 4 commits intomockk:masterfrom
neungs-2:master
Jan 21, 2026
Merged

fix: resolve @InjectMockKs initialization order based on dependencies#1500
Raibaz merged 4 commits intomockk:masterfrom
neungs-2:master

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@neungs-2
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@neungs-2 neungs-2 commented Jan 15, 2026

Summary

  • Fix @InjectMockKs failing when dependency order differs from field declaration order
  • Use topological sort (Kahn's algorithm) to resolve correct initialization order
  • Add circular dependency detection with clear error message
  • Add comprehensive dependency-order tests for @InjectMockKs

Problem

When multiple @InjectMockKs fields have dependencies on each other, MockK processed them in reflection order (roughly alphabetical), causing "No matching constructors found" errors.

  class ServiceB(val repository: Repository)
  class ServiceA(val b: ServiceB)  // A depends on B

  class MyTest {
      @InjectMockKs lateinit var a: ServiceA  // 'a' < 'b' alphabetically
      @InjectMockKs lateinit var b: ServiceB  // but B must be created first
  }

Solution

Analyze constructor parameter types to build a dependency graph, then use topological sort to determine correct initialization order.

Changes

  • modules/mockk/src/jvmMain/kotlin/io/mockk/impl/annotations/JvmMockInitializer.kt
    • Add dependency-aware ordering for @InjectMockKs via:
      • sortByDependencyOrder(...)
      • buildDependencyGraph(...)
      • topologicalSort(...) (Kahn's algorithm + circular detection)
  • modules/mockk/src/jvmMain/kotlin/io/mockk/impl/annotations/InjectionHelpers.kt
    • Add reflection helpers:
      • KProperty<*>.getReturnTypeKClass()
      • KClass<*>.getConstructorParameterTypes()
      • KType.getKClass()
  • modules/mockk/src/commonTest/kotlin/io/mockk/it/InjectMocksTest.kt
    • Add InjectMocksDependencyOrderTest with cases:
      • twoLevelDependency()
      • threeLevelChainDependency()
      • diamondDependency()
      • misleadingNamesDependency()
      • circularDependencyDetection()

Test

  • Two-level dependency (A → B)
  • Three-level chain (A → B → C)
  • Diamond dependency (D → B,C → A)
  • Misleading field names
  • Circular dependency detection

Closes #1496

@Raibaz
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Raibaz commented Jan 15, 2026

Thanks a lot for putting this together!

I'm a bit worried about the performances, though.

Would it make sense to make the topological sorting of dependencies opt-in and not enabled by default, for instance by adding a flag to @InjectMockKs?

Do you have any data about the impact of topological sorting on the performances?

@neungs-2
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neungs-2 commented Jan 15, 2026

I also wanted to ask you if the flag method would be good, but you just pointed it out!
I'll add the flag and do the performance test.

@neungs-2
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neungs-2 commented Jan 18, 2026

I modified the dependency sorting to be applied via a flag.
This commit failed the ConnectedCheck [api=21] test, so I'll fix that.

For now, I'll add the changes and performance test results.

Changes

  • Added an optional dependency‑ordered initialization for @InjectMockKs using a topological sort. The previous reflection‑order behavior is preserved when the flag is off.
    • Core implementation: modules/mockk/src/jvmMain/kotlin/io/mockk/impl/annotations/JvmMockInitializer.kt
  • Exposed a new flag useDependencyOrder on MockKAnnotations.init() (default false).
    • API entry: modules/mockk/src/commonMain/kotlin/io/mockk/MockK.kt
    • DSL wiring: modules/mockk-dsl/src/commonMain/kotlin/io/mockk/API.kt, modules/mockk-dsl/src/commonMain/kotlin/io/mockk/MockKGateway.kt
    • Updated public API snapshots: modules/mockk/api/mockk.api, modules/mockk-dsl/api/mockk-dsl.api

How to enable the flag

  • MockKAnnotations.init(..., useDependencyOrder = true)

Performance impact of DependencyOrder in Mock initialization

I ran JMH benchmarks comparing initWithDependencyOrder vs initWithoutDependencyOrder across different dependency graph shapes (independent, wide, linear, diamond) and sizes.

Focusing on the more stable and realistic case (size = 20, thrpt mode):
• independent: ~5.3% slower with dependency order
• wide: ~2.9% slower with dependency order
• linear: ~3.9% slower with dependency order
• diamond: ~5.4% slower with dependency order

Overall, applying DependencyOrder consistently introduces a 3~5% throughput regression compared to the unordered version. Therefore, applying the flag seems appropriate.

main summary

Benchmark Variant Shape Size Score (ops/s) Units
initWithDependencyOrder independent 5 9,625.818 ops/s
initWithDependencyOrder independent 20 4,775.431 ops/s
initWithDependencyOrder wide 5 9,533.150 ops/s
initWithDependencyOrder wide 20 2,574.726 ops/s
initWithDependencyOrder linear 5 10,179.793 ops/s
initWithDependencyOrder linear 20 2,679.349 ops/s
initWithDependencyOrder diamond 5 9,670.743 ops/s
initWithDependencyOrder diamond 20 2,595.724 ops/s
initWithoutDependencyOrder independent 5 10,220.903 ops/s
initWithoutDependencyOrder independent 20 5,030.059 ops/s
initWithoutDependencyOrder wide 5 10,223.527 ops/s
initWithoutDependencyOrder wide 20 2,649.183 ops/s
initWithoutDependencyOrder linear 5 10,420.318 ops/s
initWithoutDependencyOrder linear 20 2,783.040 ops/s
initWithoutDependencyOrder diamond 5 10,244.578 ops/s
initWithoutDependencyOrder diamond 20 2,735.884 ops/s

@Raibaz
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Raibaz commented Jan 19, 2026

Thanks a lot for looking into the performances of this change! 🙏

When we release it, let's make sure to document that switching on the useDependencyOrder flag will result in a 3-5% performance hit.

@Raibaz
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Raibaz commented Jan 20, 2026

@neungs-2 this looks good to me.

Can you please add documentation for it in README.md so that we can merge it?

Thanks!

@neungs-2
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neungs-2 commented Jan 20, 2026

@Raibaz Thank you for the quick review!

I've added the @InjectMockKs dependency order section to README.md.
Please feel free to suggest any changes or improvements.

@Raibaz Raibaz merged commit cd84546 into mockk:master Jan 21, 2026
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Dependency of one InjectMockKs on another using lateinit var and constructor injection

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@neungs-2 @Raibaz