Allow suppressing "Failed to set backing field" OR fail test #1291

cgm-aw · 2024-08-29T07:53:23Z

Introduction

Hi everyone,

I encountered a behavior of mockk, where, if the backing field has a different type than the getter, a warning is printed to the logs but the test continues and passes.
This seems weird to me - I would either expect the test to fail or no warning message.
My suggestion:
Add a parameter to the mockk() function to be able to suppress the warning, and if suppression is not active, the test will fail (or the other way around).

I'm aware of this: https://mockk.io/#property-backing-fields
With the solution presented there, everything works fine. However, if you do not use this mechanism, you will just get confusing error messages and probably an inconsistent behavior.

Prerequisites

I am running the latest version
I checked the documentation and found no answer
I checked to make sure that this issue has not already been filed

Expected Behavior

Either a broken test or no log message

Current Behavior

A warning message is printed, including a stack trace, but the test passes

Steps to Reproduce

Create this class (this is a Java class!)

import java.util.Optional;

public class ContainerClass {
    private Optional<String> someField;
    public String getSomeField() {
        return someField.orElse("empty");
    }
}

Create this Kotlin test:

    @kotlin.test.Test
    fun reproduceFailedToSetBackingField() {
        val testObject = mockk<ContainerClass>()

        every { testObject.someField } returns "mockValue"

    }

You will see this log entry:

2024-08-29 07:37:00,610 [main] WARN  i.m.i.r.states.StubbingAwaitingAnswerState - Failed to set backing field (skipping)
java.lang.IllegalArgumentException: Can not set java.util.Optional field test.ContainerClass.someField to java.lang.String
[...]

Using propertyType yields the correct behavior:
every { testObject.someField } propertyType Optional::class answers { "mockValue" }

Context

MockK version: 1.13.12
OS: MacOS
Kotlin version:2.0.20
JDK version: 21
JUnit version: 5.10.1
Type of test: unit test

Stack trace

// -----------------------[ YOUR STACK STARTS HERE ] -----------------------
2024-08-29 07:37:00,610 [main] WARN  i.m.i.r.states.StubbingAwaitingAnswerState - Failed to set backing field (skipping)
java.lang.IllegalArgumentException: Can not set java.util.Optional field test.ContainerClass.someField to java.lang.String
	at java.base/jdk.internal.reflect.FieldAccessorImpl.throwSetIllegalArgumentException(FieldAccessorImpl.java:228)
	at java.base/jdk.internal.reflect.FieldAccessorImpl.throwSetIllegalArgumentException(FieldAccessorImpl.java:232)
	at java.base/jdk.internal.reflect.MethodHandleObjectFieldAccessorImpl.set(MethodHandleObjectFieldAccessorImpl.java:115)
	at java.base/java.lang.reflect.Field.set(Field.java:836)
	at io.mockk.InternalPlatformDsl.dynamicSetField(InternalPlatformDsl.kt:182)
	at io.mockk.impl.recording.states.StubbingAwaitingAnswerState.assignFieldIfMockingProperty(StubbingAwaitingAnswerState.kt:68)
	at io.mockk.impl.recording.states.StubbingAwaitingAnswerState.access$assignFieldIfMockingProperty(StubbingAwaitingAnswerState.kt:9)
	at io.mockk.impl.recording.states.StubbingAwaitingAnswerState$assignFieldIfMockingProperty$1.invoke(StubbingAwaitingAnswerState.kt:52)
	at io.mockk.impl.recording.states.StubbingAwaitingAnswerState$assignFieldIfMockingProperty$1.invoke(StubbingAwaitingAnswerState.kt:51)
	at io.mockk.impl.stub.AnswerAnsweringOpportunity.notifyFirstAnswerHandlers(AnswerAnsweringOpportunity.kt:30)
	at io.mockk.impl.stub.AnswerAnsweringOpportunity.provideAnswer(AnswerAnsweringOpportunity.kt:21)
	at io.mockk.MockKStubScope.answers(API.kt:2292)
	at io.mockk.MockKStubScope.returns(API.kt:2296)
	at test.CustomersPojoTest.reproduceFailedToSetBackingField(CustomersPojoTest.kt:102)
	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
	at java.base/java.lang.reflect.Method.invoke(Method.java:580)
	at org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:728)
	at org.junit.jupiter.engine.execution.MethodInvocation.proceed(MethodInvocation.java:60)
	at org.junit.jupiter.engine.execution.InvocationInterceptorChain$ValidatingInvocation.proceed(InvocationInterceptorChain.java:131)
	at org.junit.jupiter.engine.extension.TimeoutExtension.intercept(TimeoutExtension.java:156)
	at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestableMethod(TimeoutExtension.java:147)
	at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestMethod(TimeoutExtension.java:86)
	at org.junit.jupiter.engine.execution.InterceptingExecutableInvoker$ReflectiveInterceptorCall.lambda$ofVoidMethod$0(InterceptingExecutableInvoker.java:103)
	at org.junit.jupiter.engine.execution.InterceptingExecutableInvoker.lambda$invoke$0(InterceptingExecutableInvoker.java:93)
	at org.junit.jupiter.engine.execution.InvocationInterceptorChain$InterceptedInvocation.proceed(InvocationInterceptorChain.java:106)
	at org.junit.jupiter.engine.execution.InvocationInterceptorChain.proceed(InvocationInterceptorChain.java:64)
	at org.junit.jupiter.engine.execution.InvocationInterceptorChain.chainAndInvoke(InvocationInterceptorChain.java:45)
	at org.junit.jupiter.engine.execution.InvocationInterceptorChain.invoke(InvocationInterceptorChain.java:37)
	at org.junit.jupiter.engine.execution.InterceptingExecutableInvoker.invoke(InterceptingExecutableInvoker.java:92)
	at org.junit.jupiter.engine.execution.InterceptingExecutableInvoker.invoke(InterceptingExecutableInvoker.java:86)
	at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.lambda$invokeTestMethod$7(TestMethodTestDescriptor.java:218)
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
	at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.invokeTestMethod(TestMethodTestDescriptor.java:214)
	at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:139)
	at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:69)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$6(NodeTestTask.java:151)
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:141)
	at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:137)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$9(NodeTestTask.java:139)
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:138)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:95)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)
	at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:41)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$6(NodeTestTask.java:155)
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:141)
	at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:137)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$9(NodeTestTask.java:139)
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:138)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:95)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)
	at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:41)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$6(NodeTestTask.java:155)
	at org.junit.platform.e
8000
ngine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:141)
	at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:137)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$9(NodeTestTask.java:139)
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:138)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:95)
	at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.submit(SameThreadHierarchicalTestExecutorService.java:35)
	at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.execute(HierarchicalTestExecutor.java:57)
	at org.junit.platform.engine.support.hierarchical.HierarchicalTestEngine.execute(HierarchicalTestEngine.java:54)
	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.execute(EngineExecutionOrchestrator.java:198)
	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.execute(EngineExecutionOrchestrator.java:169)
	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.execute(EngineExecutionOrchestrator.java:93)
	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.lambda$execute$0(EngineExecutionOrchestrator.java:58)
	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.withInterceptedStreams(EngineExecutionOrchestrator.java:141)
	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.execute(EngineExecutionOrchestrator.java:57)
	at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:103)
	at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:85)
	at org.junit.platform.launcher.core.DelegatingLauncher.execute(DelegatingLauncher.java:47)
	at org.junit.platform.launcher.core.SessionPerRequestLauncher.execute(SessionPerRequestLauncher.java:63)
	at com.intellij.junit5.JUnit5IdeaTestRunner.startRunnerWithArgs(JUnit5IdeaTestRunner.java:57)
	at com.intellij.rt.junit.IdeaTestRunner$Repeater$1.execute(IdeaTestRunner.java:38)
	at com.intellij.rt.execution.junit.TestsRepeater.repeat(TestsRepeater.java:11)
	at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:35)
	at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:232)
	at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:55)
// -----------------------[ YOUR STACK TRACE ENDS HERE ] -----------------------

Minimal reproducible code (the gist of this issue)

See above

The text was updated successfully, but these errors were encountered:

PavlosTze · 2024-10-31T14:01:27Z

I'm also encountering the same issue. @Raibaz @zkkv @ any ideas?

bjpe · 2024-11-05T10:51:12Z

Hi, I also got the same problem today. @cgm-aw : Could you elaborate how you circumvented this issue using https://mockk.io/#property-backing-fields? Maybe you could just provide the adjusted test code. My current attempts do not work, unfortunately.

cgm-aw · 2024-11-05T14:26:14Z

Hi @bjpe

yes I added it to the description. Use this:
every { testObject.someField } propertyType Optional::class answers { "mockValue" }

I also noticed that formatting in my post was a mess, I fixed that.

bjpe · 2024-11-06T08:44:16Z

Hi @cgm-aw , thank you very much! I also tried this, but it failed in my case because the field is not accessible (the mocked class is located in a library). Anyway, thanks a lot!

p4ulor · 2024-12-28T19:07:39Z

I have a similar issue. I'm in Android, using RectF from android.graphics
Mockk doesn't seem to access java fields

val rect = mockk<RectF>(relaxed = true) {
    every { width() } returns 1600f
    every { height() } returns 900f

    every { left } returns 0f // error: Missing mocked calls inside every { ... } block: make sure the object inside the block is a mock
    every { getProperty("left") } propertyType java.lang.Float::class answers { 0f } // error: can't find property left for dynamic property get

}

cgm-aw · 2025-02-17T16:19:15Z

I'm not really happy with the solution - in my mind, this is a bug because the developer wanted to mock something which just does not work. I added a PR with a proposal for a fix: #1349

p4ulor mentioned this issue Feb 11, 2025

Update README.md - Clarify that private fields cannot be mocked #1347

Merged

Raibaz closed this as completed Feb 11, 2025

cgm-aw mentioned this issue Feb 17, 2025

Added new property "failOnSetBackingFieldException" to fail test if a backing field could not be set #1349

Merged

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Allow suppressing "Failed to set backing field" OR fail test #1291

Allow suppressing "Failed to set backing field" OR fail test #1291

Allow suppressing "Failed to set backing field" OR fail test #1291

Allow suppressing "Failed to set backing field" OR fail test #1291

Comments

Introduction

Prerequisites

Expected Behavior

Current Behavior

Steps to Reproduce

Context

Stack trace

Minimal reproducible code (the gist of this issue)