Hi I m using kotlinx datetime and mockk I would like to mock

Question

Hi I m using kotlinx datetime and mockk I would like to mock `TimeZone currentSystemDefault ` at a below code but error occurs `java lang NoClassDefFoundError kotlinx serialization KSerializer` When `mockkObject TimeZone ` was called the error happened Why happens error Enviroments kotlinx datetime 0 2 1 kotlin stdlib jdk7 1 5 21 mockk 1 12 0 ```class MyTest Before fun setup mockkObject TimeZone After fun tearDown unmockkAll Test fun test every TimeZone currentSystemDefault returns TimeZone of Asia Tokyo assertEquals TimeZone currentSystemDefault TimeZone of Asia Tokyo ``` ```Aug 20 2021 7 43 20 PM io mockk impl log JULLogger warn WARNING Failed to transform class kotlinx datetime TimeZone$Companion java lang NoClassDefFoundError kotlinx serialization KSerializer at java lang Class getDeclaredMethods0 Native Method at java lang Class privateGetDeclaredMethods Class java 2701 at java lang Class getDeclaredMethods Class java 1975 ```

wcaokaze · Answer

Simply just add `org jetbrains kotlinx kotlinx serialization core 1 2 2` to your dependencies

wcaokaze · Answer

<https github com Kotlin kotlinx datetime blob master core build gradle kts L156> kotlinx datetime uses kotlinx serialization but it declared as `compileOnly` This means that `TimeZone` is serializable when our project has kotlinx serialization as a dependency but the jar of kotlinx datetime includes no runtime of kotlinx serialization In most cases this works well since we can not serialize any objects without kotlinx serialization in our dependencies compilation error But unfortunately mockk uses reflection So it can reach `KSerializer` which is normally unreachable And your project doesn t have the runtime of kotlinx serialization at this time so JVM cannot load the class and finally a `NoClassDefFoundError` is thrown This is the reason why it happens

Ippei Mukaida · Answer

< wcaokaze> I added `testRuntimeOnly org jetbrains kotlinx kotlinx serialization core 1 2 2` in `build gradle` so the test was successful Thanks for help kissing heart