Solution: Mocking library
class MockRegistry {
inline fun <reified T : Any> mock(): T = mock(T::class)
private var recording = false
private var bodyToStore: (() -> Any?)? = null
private var handlers = mutableMapOf<Record, () -> Any?>()
inline fun <reified T: Any> mock(): T = mock(T::class.java)
fun <T : Any> mock(classRef: KClass<T>): T {
val proxy = Any()
return Proxy.newProxyInstance(
classRef.java.classLoader,
arrayOf(classRef.java)
) { _, method, args ->
val record = Record(proxy, method, args?.toList())
if (recording) {
handlers[record] = checkNotNull(bodyToStore)
throw RecordedCorrectlyException()
} else {
checkNotNull(handlers[record]) { "No recording..." }
.invoke()
}
} as T
}
fun <T> setBody(functionCall: () -> T, body: () -> T) {
try {
recording = true
this.bodyToStore = body
functionCall()
} catch (e: RecordedCorrectlyException) {
// no-op
} finally {
recording = false
this.bodyToStore = null
}
}
fun <T> setReturnValue(functionCall: () -> T, value: T) {
setBody(functionCall) { value }
}
class RecordedCorrectlyException : RuntimeException()
data class Record(
val proxy: Any,
val method: Method,
val arguments: List<Any?>?,
)
}
Example solution in playground
import org.junit.Test
import java.lang.reflect.Method
import java.lang.reflect.Proxy
import kotlin.reflect.KClass
import java.lang.reflect.InvocationHandler
import kotlin.test.assertEquals
import kotlin.test.assertNotNull
class MockRegistry {
inline fun <reified T : Any> mock(): T = mock(T::class)
private var recording = false
private var bodyToStore: (() -> Any?)? = null
private var handlers = mutableMapOf<Record, () -> Any?>()
inline fun <reified T: Any> mock(): T = mock(T::class.java)
fun <T : Any> mock(classRef: KClass<T>): T {
val proxy = Any()
return Proxy.newProxyInstance(
classRef.java.classLoader,
arrayOf(classRef.java)
) { _, method, args ->
val record = Record(proxy, method, args?.toList())
if (recording) {
handlers[record] = checkNotNull(bodyToStore)
throw RecordedCorrectlyException()
} else {
checkNotNull(handlers[record]) { "No recording..." }
.invoke()
}
} as T
}
fun <T> setBody(functionCall: () -> T, body: () -> T) {
try {
recording = true
this.bodyToStore = body
functionCall()
} catch (e: RecordedCorrectlyException) {
// no-op
} finally {
recording = false
this.bodyToStore = null
}
}
fun <T> setReturnValue(functionCall: () -> T, value: T) {
setBody(functionCall) { value }
}
class RecordedCorrectlyException : RuntimeException()
data class Record(
val proxy: Any,
val method: Method,
val arguments: List<Any?>?,
)
}
fun main() {
val registry = MockRegistry()
val userRepository = registry.mock<UserRepository>()
val userService = registry.mock<UserService>()
registry.setReturnValue(
{ userRepository.getUser("alex") },
User("Alex Smith")
)
registry.setReturnValue(
{ userRepository.getUser("bell") },
User("Bell Rogers")
)
registry.setReturnValue(
{ userRepository.getUser("dan") },
null
)
registry.setBody({ userRepository.allUsers() }) {
listOf(User("James Bond"), User("Jane Doe"))
}
registry.setBody({ userService.getUser() }) {
User(userRepository.getUser("dan")?.name ?: "Unknown")
}
println(userRepository.getUser("alex"))
// User(name=Alex Smith)
println(userRepository.allUsers())
// [User(name=James Bond), User(name=Jane Doe)]
println(userRepository.getUser("bell"))
// User(name=Bell Rogers)
println(userService.getUser())
// User(name=Unknown)
registry.setReturnValue(
{ userRepository.getUser("dan") },
User("Dan Brown")
)
println(userService.getUser())
// User(name=Dan Brown)
}
class MockingTest {
interface GetValue {
fun getInt(): Int
fun getIntWithArg(arg: Int): Int
fun getString(): String
}
@Test
fun `should throw exception when no value set`() {
val registry = MockRegistry()
val mock = registry.mock<GetValue>()
val result = runCatching {
mock.getInt()
}
val exception = result.exceptionOrNull()
assertNotNull(exception)
assert(exception is IllegalStateException)
assertEquals("No handler for method getInt()", exception.message)
val result2 = runCatching {
mock.getIntWithArg(123)
}
val exception2 = result2.exceptionOrNull()
assertNotNull(exception2)
assert(exception2 is IllegalStateException)
assertEquals("No handler for method getIntWithArg(123)", exception2.message)
}
@Test
fun `should set int value`() {
val registry = MockRegistry()
val mock = registry.mock<GetValue>()
registry.setReturnValue({ mock.getInt() }, 123)
assertEquals(123, mock.getInt())
}
@Test
fun `should set string value`() {
val registry = MockRegistry()
val mock = registry.mock<GetValue>()
registry.setReturnValue({ mock.getString() }, "ABC")
assertEquals("ABC", mock.getString())
}
@Test
fun `should set int value with arg`() {
val registry = MockRegistry()
val mock = registry.mock<GetValue>()
registry.setReturnValue({ mock.getIntWithArg(123) }, 456)
registry.setReturnValue({ mock.getIntWithArg(456) }, 789)
assertEquals(456, mock.getIntWithArg(123))
assertEquals(789, mock.getIntWithArg(456))
}
@Test
fun `should set constant int body`() {
val registry = MockRegistry()
val mock = registry.mock<GetValue>()
registry.setBody({ mock.getInt() }) { 123 }
assertEquals(123, mock.getInt())
}
@Test
fun `should set int body`() {
val registry = MockRegistry()
val mock = registry.mock<GetValue>()
var counter = 0
registry.setBody({ mock.getInt() }) { counter++ }
assertEquals(0, mock.getInt())
assertEquals(1, mock.getInt())
assertEquals(2, mock.getInt())
assertEquals(3, mock.getInt())
assertEquals(4, mock.getInt())
assertEquals(5, mock.getInt())
}
@Test
fun `should set constant string body`() {
val registry = MockRegistry()
val mock = registry.mock<GetValue>()
registry.setBody({ mock.getString() }) { "ABC" }
assertEquals("ABC", mock.getString())
}
@Test
fun `should set string body`() {
val registry = MockRegistry()
val mock = registry.mock<GetValue>()
var counter = 0
registry.setBody({ mock.getString() }) { "ABC${counter++}" }
assertEquals("ABC0", mock.getString())
assertEquals("ABC1", mock.getString())
assertEquals("ABC2", mock.getString())
assertEquals("ABC3", mock.getString())
assertEquals("ABC4", mock.getString())
assertEquals("ABC5", mock.getString())
}
@Test
fun `should set body depending on another mock`() {
val registry = MockRegistry()
val userRepository = registry.mock<UserRepository>()
val userService = registry.mock<UserService>()
registry.setReturnValue(
{ userRepository.getUser("alex") },
User("Alex Smith")
)
registry.setReturnValue(
{ userRepository.getUser("bell") },
User("Bell Rogers")
)
registry.setReturnValue(
{ userRepository.getUser("dan") },
null
)
registry.setBody({ userRepository.allUsers() }) {
listOf(User("James Bond"), User("Jane Doe"))
}
registry.setBody({ userService.getUser() }) {
User(userRepository.getUser("dan")?.name ?: "Unknown")
}
assertEquals(User("Alex Smith"), userRepository.getUser("alex"))
assertEquals(listOf(User("James Bond"), User("Jane Doe")), userRepository.allUsers())
assertEquals(User("Bell Rogers"), userRepository.getUser("bell"))
assertEquals(User("Unknown"), userService.getUser())
registry.setReturnValue(
{ userRepository.getUser("dan") },
User("Dan Brown")
)
assertEquals(User("Dan Brown"), userService.getUser())
}
}
