Kotlin unit test Exception when mocking suspend function java.io.EOFException: Premature end of stream: expected 1 bytes

Issue

I use KTor and Kotlin Serialization library in my android project, along with mockk and junit.jupiter for unit testing. I've encountered some problem when mocking ktor's suspend function readText(). The written unit test tests that initErrorMessage() function returns correct error message.

Test class:

class ErrorTest {

    private val errorMessage = "objectId must be provided."
    private val errorCode = 2689
    private val correctResponseJson = "{\"code\":$errorCode,\"message\":\"$errorMessage\"}"
    // ResponseException class is from ktor library
    private val exceptionMock: ResponseException = mockk(relaxed = true)

    @Test
    fun `initErrorMessage should return correct error message`() = runTest {
        coEvery { exceptionMock.response.readText() } returns correctResponseJson // <-- here is the Error occurs

        val expectedError = errorMessage
        val actualError = initErrorMessage(exceptionMock)

        assertEquals(expectedError, actualError)
    }
}

Method to test:

suspend fun initErrorMessage(cause: ResponseException): String {
    return try {
        val body = cause.response.readText()
        val jsonSerializer = JsonObject.serializer()
        val jsonObj = Json.decodeFromString(jsonSerializer, body)
        jsonObj["message"].toString()
    } catch (e: Exception) {
        ""
    }
}

During execution of the first line in the test method I get an Error:

Premature end of stream: expected 1 bytes
java.io.EOFException: Premature end of stream: expected 1 bytes
    at io.ktor.utils.io.core.StringsKt.prematureEndOfStream(Strings.kt:492)
    at io.ktor.utils.io.core.internal.UnsafeKt.prepareReadHeadFallback(Unsafe.kt:78)
    at io.ktor.utils.io.core.internal.UnsafeKt.prepareReadFirstHead(Unsafe.kt:61)
    at io.ktor.utils.io.charsets.CharsetJVMKt.decode(CharsetJVM.kt:556)
    at io.ktor.utils.io.charsets.EncodingKt.decode(Encoding.kt:103)
    at io.ktor.utils.io.charsets.EncodingKt.decode$default(Encoding.kt:101)
    at io.ktor.client.statement.HttpStatementKt.readText(HttpStatement.kt:173)
    at io.ktor.client.statement.HttpStatementKt.readText$default(HttpStatement.kt:168)
    at com.example.android.http.error.ErrorTest$initErrorMessage should return correct error message$1$1.invokeSuspend(ErrorTest.kt:37)
    at com.example.android.http.error.ErrorTest$initErrorMessage should return correct error message$1$1.invoke(ErrorTest.kt)
    at com.example.android.http.error.ErrorTest$initErrorMessage should return correct error message$1$1.invoke(ErrorTest.kt)
    at io.mockk.impl.eval.RecordedBlockEvaluator$record$block$2$1.invokeSuspend(RecordedBlockEvaluator.kt:28)
    at io.mockk.impl.eval.RecordedBlockEvaluator$record$block$2$1.invoke(RecordedBlockEvaluator.kt)
    at io.mockk.InternalPlatformDsl$runCoroutine$1.invokeSuspend(InternalPlatformDsl.kt:20)

How to mock this suspend method readText() without an Error?

Solution

It turned out that the function readText() hasn't been mocked properly. It is an extension function on HttpResponse and it has to be mocked using mockkStatic function, for example like this:

@BeforeEach
fun setup() {
    mockkStatic(HttpResponse::readText)
}

setup() will be executed before each @Test, because it is marked with @BeforeEach annotation.

Answered By - Sergey