mockk: Bug: ClassCastException when using relaxed mock of generic type

Prerequisites

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Description

I might have run into a MockK limitation trying to use relaxed mock of a generic type (particularly Spring’s JpaRepository). When using such mock object, my test throws a ClassCastException at the point where the test code calls the method of the mock object that returns an object using the type parameter.

For example, if have the following types,

interface BaseRepository<T> {
    fun save(obj: T): T
}

interface BookRepository: BaseRepository<Book>

and I declare a relaxed mock for BookRepository, the test throws a ClassCastException during the invocation of save(book).

Steps to Reproduce

I have prepared a demo code base to illustrate this issue. Please check the last 3-4 commits as I experimented a bit to confirm that this issue doesn’t only affect Spring’s JpaRepository but in general, any type (interface) with similar declaration.

About this issue

Original URL
State: open
Created 5 years ago
Reactions: 39
Comments: 22

Most upvoted comments

I am facing the same issue. I am using a Result wrapper for my network models.

sealed class Result<out R> {
    data class Success<out T>(val data: T) : Result<T>()
    data class Error(val exception: Failure) : Result<Nothing>()
}

However, I am getting;

java.lang.ClassCastException: java.lang.Object cannot be cast to mypath.data.MyResponse

+10

nuhkoca on Mar 6, 2020

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions. If you are sure that this issue is important and should not be marked as stale just ask to put an important label.

stale[bot] on Oct 14, 2019

Not able to mock repository beans, get this error with save calls. Tried to use hint and relaxed, still same problem.

fdhuseynov on Jan 11, 2023

I solved it as below every { repository.save<ReturnTypecClass>(any()) } returns ReturnTypeClass

liquidjoo on May 17, 2021

Any updates on this by any chance? I have hundreds of tests failing because of this issues so unfortunately using hint() on all of them would be an enormous effort. Type inference was somehow working on Java 8 (I only have this issue when switching to Java 11). This is possibly related to https://github.com/mockk/mockk/issues/606 as well?

marcosalis on May 11, 2021

As it is quite common to test with the JpaRepository pattern, this issue is quite impactful.

Jelledb on Apr 28, 2022

same problem here (mockk 1.9.3). Small example code to reproduce the problem:

interface BaseRepository<T> {
    fun save(obj: T): T
}

interface IntRepository: BaseRepository<Int>

fun main(args: Array<String>) {
    val a = mockk<IntRepository>(relaxed = true)
    a.save(1)
}

Results in:

Exception in thread "main" java.lang.ClassCastException: java.lang.Object cannot be cast to java.lang.Integer

robstoll on Aug 15, 2019