Hi I’m using kotlinx.datetime and mockk. I would like to mock `TimeZone.currentSystemDefault()` at a...

# mockk

[Ippei Mukaida]

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]

Simply, just add org.jetbrains.kotlinx:kotlinx-serialization-core:1.2.2 to your dependencies

[wcaokaze]

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]

@wcaokaze
I added testRuntimeOnly org.jetbrains.kotlinx:kotlinx-serialization-core:1.2.2  in build.gradle , so the test was successful!
Thanks for help😘