InjectMocks doesn't work with Kotlin constructor arguments with default values

Question

EDIT: I've created a ticket with mockito-kotlin here

I have a class defined like so:

package me.jpalacios.poc

class MyClass(
    private val myDependency: MyDependency = MyDependency()
) {
    fun run() {
        myDependency.doSomething()
    }
}

package me.jpalacios.poc

class MyDependency {

    fun doSomething() {
        println("I did something")
    }
}

package me.jpalacios.poc

import org.junit.jupiter.api.Test
import org.junit.jupiter.api.extension.ExtendWith
import org.mockito.InjectMocks
import org.mockito.Mock
import org.mockito.junit.jupiter.MockitoExtension
import org.mockito.kotlin.verify

@ExtendWith(MockitoExtension::class)
class MyClassTest {

    @Mock
    private lateinit var myDependency: MyDependency
    @InjectMocks
    private lateinit var myClass: MyClass

    @Test
    fun `Test InjectMocks`() {
        myClass.run()

        verify(myDependency).doSomething()
    }
}

Looks like a test defined like so does not work because the mocks are not injected:

@ExtendWith(MockitoExtension::class)
class MyClassTest {
    @Mock
    private lateinit var dependency: MyDependency
    @InjectMocks
    private lateinit var underTest: MyClass
}

plugins {
    kotlin("jvm") version "1.5.20"
}

group = "me.jpalacios"
version = "1.0-SNAPSHOT"

repositories {
    mavenCentral()
}

dependencies {
    implementation(kotlin("stdlib"))

    testImplementation("org.junit.jupiter:junit-jupiter:5.7.2")
    testImplementation("org.junit.jupiter:junit-jupiter-params:5.7.2")
    testImplementation("org.assertj:assertj-core:3.20.2")
    testImplementation("org.mockito.kotlin:mockito-kotlin:3.2.0")
    testImplementation("org.mockito:mockito-junit-jupiter:3.11.2")
}

tasks{
    jar {
        duplicatesStrategy = DuplicatesStrategy.EXCLUDE

        configurations["compileClasspath"].forEach { file: File ->
            from(zipTree(file.absoluteFile))
        }
    }
    compileKotlin {
        kotlinOptions {
            jvmTarget = "${JavaVersion.VERSION_11}"
        }
    }
    test {
        useJUnitPlatform()
    }
}

Any thoughts as to why?

The output is:

I did something

Wanted but not invoked: myDependency.doSomething(); -> at me.jpalacios.poc.MyDependency.doSomething(MyDependency.kt:6) Actually, there were zero interactions with this mock.

Wanted but not invoked: myDependency.doSomething(); -> at me.jpalacios.poc.MyDependency.doSomething(MyDependency.kt:6) Actually, there were zero interactions with this mock.

at me.jpalacios.poc.MyDependency.doSomething(MyDependency.kt:6) at me.jpalacios.poc.MyClassTest.Test InjectMocks(MyClassTest.kt:22) at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ...

Answer 1

I really am no expert in Mockito, but adding default values for constructor arguments literally says that you don't need to be passed a dependency, so I wouldn't be surprised if Mockito doesn't try to inject mocks in this case. It really sees a no-arg constructor available, so wouldn't you want it to use it?

In any case, I'm not even sure you need this at all. Why not just instantiate the class under test yourself, and pass it the mock where you need it?

@ExtendWith(MockitoExtension::class)
class MyClassTest {

    @Mock
    private lateinit var myDependency: MyDependency

    @Test
    fun `Test InjectMocks`() {
        MyClass(myDependency).run()

        verify(myDependency).doSomething()
    }
}

If you need to share such initialization among multiple tests, you can use a lazy property, or a @BeforeTest method to create it.