Testing Http Client (MockEngine)

Estimated reading time: 1 minute

Ktor exposes a MockEngine for the HttpClient. This engine allows simulating HTTP calls without actually connecting to the endpoint. It allows to set a code block, that can handle the request and generates a response.

This engine is defined in the class io.ktor.client.engine.mock.MockEngine in the artifact io.ktor:ktor-client-mock:$ktor_version,io.ktor:ktor-client-mock-jvm:$ktor_version,io.ktor:ktor-client-mock-js:$ktor_version,io.ktor:ktor-client-mock-native:$ktor_version.
dependencies { api "io.ktor:ktor-client-mock:$ktor_version" api "io.ktor:ktor-client-mock-jvm:$ktor_version" api "io.ktor:ktor-client-mock-js:$ktor_version" api "io.ktor:ktor-client-mock-native:$ktor_version" }
dependencies { testCompile("io.ktor:ktor-client-mock:$ktor_version,io.ktor:ktor-client-mock-jvm:$ktor_version,io.ktor:ktor-client-mock-js:$ktor_version,io.ktor:ktor-client-mock-native:$ktor_version") }
<project> ... <dependencies> <dependency> <groupId>io.ktor</groupId> <artifactId>ktor-client-mock</artifactId> <version>${ktor.version}</version> <scope>test</scope> </dependency> <dependency> <groupId>io.ktor</groupId> <artifactId>ktor-client-mock</artifactId> <version>${ktor.version}</version> <scope>test</scope> </dependency> <dependency> <groupId>io.ktor</groupId> <artifactId>ktor-client-mock</artifactId> <version>${ktor.version}</version> <scope>test</scope> </dependency> <dependency> <groupId>io.ktor</groupId> <artifactId>ktor-client-mock</artifactId> <version>${ktor.version}</version> <scope>test</scope> </dependency> </dependencies> </project>

Usage

The usage is straightforward: the MockEngine class has a method addHandler in MockEngineConfig, that receives a block/callback that will handle the request. This callback receives an HttpRequest as a parameter, and must return a HttpResponseData. There are many helper methods to construct the response.

Full API description and list of helper methods could be found here.

A sample illustrating this:

val client = HttpClient(MockEngine) {
    engine {
        addHandler { request ->
            when (request.url.fullUrl) {
                "https://example.org/" -> {
                    val responseHeaders = headersOf("Content-Type" to listOf(ContentType.Text.Plain.toString()))
                    respond("Hello, world", headers = responseHeaders)
                }
                else -> error("Unhandled ${request.url.fullUrl}")
            }
        }
    }
}

private val Url.hostWithPortIfRequired: String get() = if (port == protocol.defaultPort) host else hostWithPort
private val Url.fullUrl: String get() = "${protocol.name}://$hostWithPortIfRequired$fullPath"