Attempt to mock a Kotlin-sealed class contained in another class fails when using mock-maker-inline

This issue is raised when attempting to mock a sealed class that is contained in another class. Sealed classes are used in Kotlin to restrict class hierarchies.

class MyTest {

    abstract class AbstractClass class private constructor()

    sealed class SealedClass

    @Test
    fun mockAbstract() {
        Mockito.mock(AbstractClass::class.java) // Succeeds
    }

    @Test
    fun mockSealedClass() {
        Mockito.mock(SealedClass::class.java) // Fails
    }
}

Now the generated byte code for both classes is similar, except for the access flags:

// ================MyTest$AbstractClass.class =================
// class version 50.0 (50)
// access flags 0x421
public abstract class MyTest$AbstractClass {


  // access flags 0x2
  private <init>()V
   L0
    LINENUMBER 3 L0
    ALOAD 0
    INVOKESPECIAL java/lang/Object.<init> ()V
    RETURN
   L1
    LOCALVARIABLE this LMyTest$AbstractClass; L0 L1 0
    MAXSTACK = 1
    MAXLOCALS = 1

  @Lkotlin/Metadata;(mv={1, 1, 2}, bv={1, 0, 1}, k=1, d1={"\u0000\u000c\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n\u0002\u0008\u0002\u0008&\u0018\u00002\u00020\u0001B\u0007\u0008\u0002\u00a2\u0006\u0002\u0010\u0002\u00a8\u0006\u0003"}, d2={"LMyTest$AbstractClass;", "", "()V", "test sources for module ui"})
- // access flags 0x409
- public static abstract INNERCLASS MyTest$AbstractClass MyTest AbstractClass
  // compiled from: Test.kt
}

// ================MyTest$SealedClass.class =================
// class version 50.0 (50)
// access flags 0x421
public abstract class MyTest$SealedClass {


  // access flags 0x2
  private <init>()V
   L0
    LINENUMBER 5 L0
    ALOAD 0
    INVOKESPECIAL java/lang/Object.<init> ()V
    RETURN
   L1
    LOCALVARIABLE this LMyTest$SealedClass; L0 L1 0
    MAXSTACK = 1
    MAXLOCALS = 1

  @Lkotlin/Metadata;(mv={1, 1, 2}, bv={1, 0, 1}, k=1, d1={"\u0000\u000c\n\u0002\u0018\u0002\n\u0002\u0010\u0000\n\u0002\u0008\u0002\u00086\u0018\u00002\u00020\u0001B\u0007\u0008\u0002\u00a2\u0006\u0002\u0010\u0002\u00a8\u0006\u0003"}, d2={"LMyTest$SealedClass;", "", "()V", "test sources for module ui"})
+ // access flags 0x9
+ public static INNERCLASS MyTest$SealedClass MyTest SealedClass
  // compiled from: Test.kt
}

When turning off mock-maker-inline, both cases succeed. It also does not occur when the sealed class is not nested.

The stacktrace:

org.mockito.exceptions.base.MockitoException: 
Mockito cannot mock this class: class MyTest$SealedClass.

If you're not sure why you're getting this error, please report to the mailing list.


Java               : 1.8
JVM vendor name    : Oracle Corporation
JVM vendor version : 25.76-b03
JVM name           : OpenJDK 64-Bit Server VM
JVM version        : 1.8.0_76-release-b03
JVM info           : mixed mode
OS name            : Linux
OS version         : 4.4.0-45-generic


You are seeing this disclaimer because Mockito is configured to create inlined mocks.
You can learn about inline mocks and their limitations under item #39 of the Mockito class javadoc.

Underlying exception : java.lang.UnsupportedOperationException: class redefinition failed: attempted to change the class modifiers

	at MyTest.mockSealedClass(Test.kt:17)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
	at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:119)
	at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:42)
	at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:234)
	at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:74)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)
Caused by: java.lang.UnsupportedOperationException: class redefinition failed: attempted to change the class modifiers
	at sun.instrument.InstrumentationImpl.retransformClasses0(Native Method)
	at sun.instrument.InstrumentationImpl.retransformClasses(InstrumentationImpl.java:144)
	at org.mockito.internal.creation.bytebuddy.InlineBytecodeGenerator.triggerRetransformation(InlineBytecodeGenerator.java:98)
	at org.mockito.internal.creation.bytebuddy.InlineBytecodeGenerator.mockClass(InlineBytecodeGenerator.java:78)
	at org.mockito.internal.creation.bytebuddy.TypeCachingBytecodeGenerator$CachedBytecodeGenerator.getOrGenerateMockClass(TypeCachingBytecodeGenerator.java:87)
	at org.mockito.internal.creation.bytebuddy.TypeCachingBytecodeGenerator.mockClass(TypeCachingBytecodeGenerator.java:34)
	at org.mockito.internal.creation.bytebuddy.InlineByteBuddyMockMaker.createMockType(InlineByteBuddyMockMaker.java:164)
	at org.mockito.internal.creation.bytebuddy.InlineByteBuddyMockMaker.createMock(InlineByteBuddyMockMaker.java:145)
	at org.mockito.internal.util.MockUtil.createMock(MockUtil.java:35)
	at org.mockito.internal.MockitoCore.mock(MockitoCore.java:63)
	at org.mockito.Mockito.mock(Mockito.java:1632)
	at org.mockito.Mockito.mock(Mockito.java:1545)
	... 28 more

Using Mockito version 2.2.9.

• The mockito message in the stacktrace have useful information, but it didn’t help
• The problematic code (if that’s possible) is copied here; Note that some configuration are impossible to mock via Mockito
• Provide versions (mockito / jdk / os / any other relevant information)
• Provide a Short, Self Contained, Correct (Compilable), Example of the issue (same as any question on stackoverflow.com)
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[raphw]

I filed an issue at Kotlin: https://youtrack.jetbrains.com/issue/KT-14774

There is not much we can do as the Java reflection API does not correctly expose that the Bar class is abstract which is information we rely on. As said, Kotlin corrupts the class file’s inner class modifier which is ignored by the JVM’s runtime but applied by the reflection API. This puts our mocking engine off track.

[nhaarman]

Kotlin 1.0.6 has been released with a fix for this.