Spring-boot: Cannot mock @Service annotated Kotlin class with member extension function that has generics

[s1monw1]

I was trying to mock a non-final, non-private Kotlin class with a simple Mockito.mock() call and got the following exception. Turns out, the problem is an embedded member extension function in combination with Spring's Service annotation. See below for an example.

org.mockito.exceptions.base.MockitoException: 
Mockito cannot mock this class: class XYService

Mockito can only mock non-private & non-final classes.
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 : 25.181-b13
JVM name           : Java HotSpot(TM) 64-Bit Server VM
JVM version        : 1.8.0_181-b13
JVM info           : mixed mode
OS name            : Mac OS X
OS version         : 10.13.6

Underlying exception : java.lang.IllegalArgumentException: object is not an instance of declaring class

    at XYTest.mockTest(AnotherServiceTests.kt:45)
    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.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:436)
    at org.junit.jupiter.engine.execution.ExecutableInvoker.invoke(ExecutableInvoker.java:115)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.lambda$invokeTestMethod$6(TestMethodTestDescriptor.java:170)
    at org.junit.jupiter.engine.execution.ThrowableCollector.execute(ThrowableCollector.java:40)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.invokeTestMethod(TestMethodTestDescriptor.java:166)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:113)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:58)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor$NodeExecutor.lambda$executeRecursively$3(HierarchicalTestExecutor.java:112)
    at org.junit.platform.engine.support.hierarchical.SingleTestExecutor.executeSafely(SingleTestExecutor.java:66)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor$NodeExecutor.executeRecursively(HierarchicalTestExecutor.java:108)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor$NodeExecutor.execute(HierarchicalTestExecutor.java:79)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor$NodeExecutor.lambda$executeRecursively$2(HierarchicalTestExecutor.java:120)
    at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:184)
    at java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:175)
    at java.util.Iterator.forEachRemaining(Iterator.java:116)
    at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1801)
    at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
    at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
    at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:151)
    at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:174)
    at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
    at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:418)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor$NodeExecutor.lambda$executeRecursively$3(HierarchicalTestExecutor.java:120)
    at org.junit.platform.engine.support.hierarchical.SingleTestExecutor.executeSafely(SingleTestExecutor.java:66)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor$NodeExecutor.executeRecursively(HierarchicalTestExecutor.java:108)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor$NodeExecutor.execute(HierarchicalTestExecutor.java:79)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor$NodeExecutor.lambda$executeRecursively$2(HierarchicalTestExecutor.java:120)
    at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:184)
    at java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:175)
    at java.util.Iterator.forEachRemaining(Iterator.java:116)
    at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1801)
    at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
    at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
    at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:151)
    at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:174)
    at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
    at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:418)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor$NodeExecutor.lambda$executeRecursively$3(HierarchicalTestExecutor.java:120)
    at org.junit.platform.engine.support.hierarchical.SingleTestExecutor.executeSafely(SingleTestExecutor.java:66)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor$NodeExecutor.executeRecursively(HierarchicalTestExecutor.java:108)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor$NodeExecutor.execute(HierarchicalTestExecutor.java:79)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.execute(HierarchicalTestExecutor.java:55)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestEngine.execute(HierarchicalTestEngine.java:43)
    at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:170)
    at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:154)
    at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:90)
    at com.intellij.junit5.JUnit5IdeaTestRunner.startRunnerWithArgs(JUnit5IdeaTestRunner.java:74)
    at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
    at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
    at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
Caused by: java.lang.IllegalArgumentException: object is not an instance of declaring class
    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 net.bytebuddy.description.type.TypeDescription$Generic$AnnotationReader$ForTypeArgument.resolve(TypeDescription.java:3210)
    at net.bytebuddy.description.type.TypeDescription$Generic$AnnotationReader$Delegator$Chained.resolve(TypeDescription.java:2995)
    at net.bytebuddy.description.type.TypeDescription$Generic$AnnotationReader$ForTypeArgument.resolve(TypeDescription.java:3183)
    at net.bytebuddy.description.type.TypeDescription$Generic$AnnotationReader$Delegator.asList(TypeDescription.java:2949)
    at net.bytebuddy.description.type.TypeDescription$Generic$OfWildcardType$ForLoadedType.getDeclaredAnnotations(TypeDescription.java:4164)
    at net.bytebuddy.description.type.TypeDescription$Generic$OfWildcardType$Latent.getDeclaredAnnotations(TypeDescription.java:4319)
    at net.bytebuddy.description.type.TypeDescription$Generic$OfWildcardType$Latent.getDeclaredAnnotations(TypeDescription.java:4319)
    at net.bytebuddy.implementation.attribute.AnnotationAppender$ForTypeAnnotations.apply(AnnotationAppender.java:639)
    at net.bytebuddy.implementation.attribute.AnnotationAppender$ForTypeAnnotations.onWildcard(AnnotationAppender.java:582)
    at net.bytebuddy.implementation.attribute.AnnotationAppender$ForTypeAnnotations.onWildcard(AnnotationAppender.java:333)
    at net.bytebuddy.description.type.TypeDescription$Generic$OfWildcardType.accept(TypeDescription.java:4072)
    at net.bytebuddy.implementation.attribute.AnnotationAppender$ForTypeAnnotations.onParameterizedType(AnnotationAppender.java:601)
    at net.bytebuddy.implementation.attribute.AnnotationAppender$ForTypeAnnotations.onParameterizedType(AnnotationAppender.java:333)
    at net.bytebuddy.description.type.TypeDescription$Generic$OfParameterizedType.accept(TypeDescription.java:4429)
    at net.bytebuddy.implementation.attribute.MethodAttributeAppender$ForInstrumentedMethod.apply(MethodAttributeAppender.java:183)
    at net.bytebuddy.implementation.attribute.MethodAttributeAppender$Compound.apply(MethodAttributeAppender.java:425)
    at net.bytebuddy.dynamic.scaffold.TypeWriter$MethodPool$Record$ForDefinedMethod$WithBody.applyAttributes(TypeWriter.java:615)
    at net.bytebuddy.dynamic.scaffold.TypeWriter$MethodPool$Record$ForDefinedMethod$WithBody.applyBody(TypeWriter.java:607)
    at net.bytebuddy.dynamic.scaffold.TypeWriter$MethodPool$Record$ForDefinedMethod.apply(TypeWriter.java:526)
    at net.bytebuddy.dynamic.scaffold.TypeWriter$Default$ForCreation.create(TypeWriter.java:4170)
    at net.bytebuddy.dynamic.scaffold.TypeWriter$Default.make(TypeWriter.java:1633)
    at net.bytebuddy.dynamic.scaffold.subclass.SubclassDynamicTypeBuilder.make(SubclassDynamicTypeBuilder.java:174)
    at net.bytebuddy.dynamic.scaffold.subclass.SubclassDynamicTypeBuilder.make(SubclassDynamicTypeBuilder.java:155)
    at net.bytebuddy.dynamic.DynamicType$Builder$AbstractBase.make(DynamicType.java:2639)
    at net.bytebuddy.dynamic.DynamicType$Builder$AbstractBase$Delegator.make(DynamicType.java:2741)
    at org.mockito.internal.creation.bytebuddy.SubclassBytecodeGenerator.mockClass(SubclassBytecodeGenerator.java:120)
    at org.mockito.internal.creation.bytebuddy.TypeCachingBytecodeGenerator$1.call(TypeCachingBytecodeGenerator.java:37)
    at org.mockito.internal.creation.bytebuddy.TypeCachingBytecodeGenerator$1.call(TypeCachingBytecodeGenerator.java:34)
    at net.bytebuddy.TypeCache.findOrInsert(TypeCache.java:138)
    at net.bytebuddy.TypeCache$WithInlineExpunction.findOrInsert(TypeCache.java:346)
    at net.bytebuddy.TypeCache.findOrInsert(TypeCache.java:161)
    at net.bytebuddy.TypeCache$WithInlineExpunction.findOrInsert(TypeCache.java:355)
    at org.mockito.internal.creation.bytebuddy.TypeCachingBytecodeGenerator.mockClass(TypeCachingBytecodeGenerator.java:32)
    at org.mockito.internal.creation.bytebuddy.SubclassByteBuddyMockMaker.createMockType(SubclassByteBuddyMockMaker.java:71)
    at org.mockito.internal.creation.bytebuddy.SubclassByteBuddyMockMaker.createMock(SubclassByteBuddyMockMaker.java:42)
    at org.mockito.internal.creation.bytebuddy.ByteBuddyMockMaker.createMock(ByteBuddyMockMaker.java:25)
    at org.mockito.internal.util.MockUtil.createMock(MockUtil.java:35)
    at org.mockito.internal.MockitoCore.mock(MockitoCore.java:65)
    at org.mockito.Mockito.mock(Mockito.java:1855)
    at org.mockito.Mockito.mock(Mockito.java:1768)
    ... 55 more

Reproduce it with this simple Test:

@Service
    class TestClass {
        fun <T> List<T>.x() = "x"
    }

    @Test
    fun mockTest() {
        Mockito.mock(TestClass::class.java)
    }

Two ways to make it work:
1) Remove @Service and make the class open or
2) Remove the generics from the member extension method, e.g. fun String.x = "x"

Thanks in advance.

kotlin: '1.2.51'
springBoot: '2.0.3.RELEASE'

[snicoll]

Thanks but none of this relates to Spring Boot. @Service is part of Spring Framework so please use the appropriate issue tracker.

[s1monw1]

@snicoll so that's how you collaborate with the other team? You don't care about Spring Framework related issues? Feels a bit weird, but here we go: https://jira.spring.io/browse/SPR-17128

Thanks

[snicoll]

Thanks for creating the issue. I have no idea how you came to such conclusion: I am merely asking that issues are handled in the proper place and there is no way for me to move this issue to Jira.

[s1monw1]

Well, I don't believe everybody who uses Spring Boot knows much about other projects that are used in the background (i.e. SpringFramework) and especially that there's another issue tracker for them. Of course, out of your position, that totally makes sense.