# Mocking in the AWS SDK for Kotlin
<a name="mocking"></a>

Developers can use several frameworks to perform mocking in tests with the AWS SDK for Kotlin. This topic documents extra configuration or special considerations that some frameworks require.

## MockK
<a name="mockk-consideration"></a>

When you mock module-wide extension functions using [MockK](https://mockk.io/), you need to do [extra configuration](https://mockk.io/#extension-functions). In the SDK for Kotlin, paginators, waiters, and presigners are examples of extension functions, so when mocking their behavior, you need extra configuration.

You must call `mockkStatic("<MODULE_CLASS_NAME>")` before setting up your mocks. As a general rule, the module class names are: 
+ **Paginators**: `aws.sdk.kotlin.services.<service>.paginators.PaginatorsKt`
+ **Waiters**: `aws.sdk.kotlin.services.<service>.waiters.WaitersKt`
+ **Presigners**: `aws.sdk.kotlin.services.<service>.presigners.PresignersKt`

For example, in the following test that includes mocking `listBucketsPaginated`—a paginator extenstion function—we add `mockkStatic("aws.sdk.kotlin.services.s3.paginators.PaginatorsKt")`:

```
    @Test
    fun testPaginatedListBuckets() = runTest {

        mockkStatic("aws.sdk.kotlin.services.s3.paginators.PaginatorsKt")
        val s3Client: S3Client = mockk()
        val s3BucketLister = S3BucketLister(s3Client)

        val expectedBuckets = listOf(
            Bucket { name = "bucket1" },
            Bucket { name = "bucket2" }
        )

        val response = ListBucketsResponse { buckets = expectedBuckets }
        coEvery { s3Client.listBucketsPaginated() } returns flowOf(response)

        val result = s3BucketLister.getAllBucketNames()

        assertEquals(listOf("bucket1", "bucket2"), result)
    }
```

Without `mockkStatic`, you see the following error:

```
Missing mocked calls inside every { ... } block: make sure the object inside the block is a mock
io.mockk.MockKException: Missing mocked calls inside every { ... } block: make sure the object inside the block is a mock
    at io.mockk.impl.recording.states.StubbingState.checkMissingCalls(StubbingState.kt:14)
    at io.mockk.impl.recording.states.StubbingState.recordingDone(StubbingState.kt:8)
    at io.mockk.impl.recording.CommonCallRecorder.done(CommonCallRecorder.kt:47)
    at io.mockk.impl.eval.RecordedBlockEvaluator.record(RecordedBlockEvaluator.kt:63)
    at io.mockk.impl.eval.EveryBlockEvaluator.every(EveryBlockEvaluator.kt:30)
    at io.mockk.MockKDsl.internalCoEvery(API.kt:100)
    at io.mockk.MockKKt.coEvery(MockK.kt:174)
```

In the case of a presigner extension function without `mockkStatic`, you might see:

```
key is bound to the URI and must not be null
java.lang.IllegalArgumentException: key is bound to the URI and must not be null
    at aws.sdk.kotlin.services.s3.serde.GetObjectOperationSerializer.serialize(GetObjectOperationSerializer.kt:26)
    at aws.sdk.kotlin.services.s3.presigners.PresignersKt.presignGetObject(Presigners.kt:49)
    at aws.sdk.kotlin.services.s3.presigners.PresignersKt.presignGetObject$default(Presigners.kt:40)
    at aws.sdk.kotlin.services.s3.presigners.PresignersKt.presignGetObject-exY8QGI(Presigners.kt:30)
```

### All example artifacts
<a name="mockk-full-example"></a>

#### Code under test
<a name="mockk-example-code-under-test"></a>

```
import aws.sdk.kotlin.services.s3.S3Client
import aws.sdk.kotlin.services.s3.paginators.listBucketsPaginated
import kotlinx.coroutines.flow.filter
import kotlinx.coroutines.flow.toList
import kotlinx.coroutines.flow.transform
import kotlinx.coroutines.runBlocking
import org.slf4j.Logger
import org.slf4j.LoggerFactory

fun main() {
    val logger: Logger = LoggerFactory.getLogger(::main.javaClass)

    // Create an S3Client
    S3Client { region = "us-east-1" }.use { s3Client ->
        // Create service instance
        val bucketLister = S3BucketLister(s3Client)
        // Since getAllBucketNames is a suspend function, you'll need to run it in a coroutine scope
        runBlocking {
            val bucketNames = bucketLister.getAllBucketNames()
            logger.info("Found buckets: $bucketNames")
        }
    }
}

class S3BucketLister(private val s3Client: S3Client) {
    suspend fun getAllBucketNames(): List<String> {
        return s3Client.listBucketsPaginated()
            .transform { response ->
                response.buckets?.forEach { bucket ->
                    emit(bucket.name ?: "")
                }
            }
            .filter { it.isNotEmpty() }
            .toList()
    }
}
```

#### Test class
<a name="mockk-example-test-code"></a>

```
import aws.sdk.kotlin.services.s3.S3Client
import aws.sdk.kotlin.services.s3.model.Bucket
import aws.sdk.kotlin.services.s3.model.ListBucketsResponse
import aws.sdk.kotlin.services.s3.paginators.listBucketsPaginated
import io.mockk.coEvery
import io.mockk.mockk
import io.mockk.mockkStatic
import kotlinx.coroutines.flow.flowOf
import kotlinx.coroutines.test.runTest
import org.junit.jupiter.api.Assertions.assertEquals
import org.junit.jupiter.api.Test

class S3BucketListerTest {

    @Test
    fun testPaginatedListBuckets() = runTest {

        mockkStatic("aws.sdk.kotlin.services.s3.paginators.PaginatorsKt")
        val s3Client: S3Client = mockk()
        val s3BucketLister = S3BucketLister(s3Client)

        val expectedBuckets = listOf(
            Bucket { name = "bucket1" },
            Bucket { name = "bucket2" }
        )

        val response = ListBucketsResponse { buckets = expectedBuckets }
        coEvery { s3Client.listBucketsPaginated() } returns flowOf(response)

        val result = s3BucketLister.getAllBucketNames()

        assertEquals(listOf("bucket1", "bucket2"), result)
    }
}
```

#### build.gradle.kts
<a name="mockk-example-gradle-build"></a>

```
plugins {
    kotlin("jvm") version "2.1.20"
    application
}

group = "org.example"
version = "1.0-SNAPSHOT"

repositories {
    mavenCentral()
}

dependencies {
    implementation(platform(awssdk.bom))
    implementation(platform("org.apache.logging.log4j:log4j-bom:2.24.3"))

    implementation(awssdk.services.s3)
    implementation("org.apache.logging.log4j:log4j-slf4j2-impl")

    // Testing Dependencies
    testImplementation(platform("org.junit:junit-bom:5.11.0"))
    testImplementation("org.junit.jupiter:junit-jupiter")
    testImplementation("org.jetbrains.kotlinx:kotlinx-coroutines-test:1.7.3")
    testImplementation("io.mockk:mockk:1.14.0")
}

tasks.test {
    useJUnitPlatform()
}

java {
    toolchain {
        languageVersion = JavaLanguageVersion.of(21)
    }
}

application {
    mainClass = "org.example.S3BucketService"
}
```

#### settings.gradle.kts
<a name="mockk-example-settings-build"></a>

```
plugins {
    id("org.gradle.toolchains.foojay-resolver-convention") version "0.10.0"
}
rootProject.name = "mockK-static"

dependencyResolutionManagement {
    repositories {
        mavenCentral()
    }

    versionCatalogs {
        create("awssdk") {
            from("aws.sdk.kotlin:version-catalog:1.4.69")
        }
    }
}
```