Stubbed Mock


Object Mocking
MIT License
Oct 3, 2019
See also
Android Mock


Mar 11, 2019
Oct 2, 2019
Blacklane GmbH (blacklane)
Milos Marinkovic (milosmns)
Source code


Stubbed Mock

Stubbed Mock is an open-source library written in Kotlin for JVM that allows users to stub data models in a Mockito-like fashion during unit tests.

Stubbed Mock supports many language features like plain Java and Kotlin classes, constructor and field injection of stubbed values, lazy, Kotlin's backing field properties and more.

Important to note: This library isn't meant to replace Mockito and doesn't mock class (or function) behavior. Its primary use is for data model classes.

Table of Contents

Getting started

To get an idea of how Stubbed Mock works, let's look at an example. Imagine this is a data model that you use in many places throughout your project:

// Kotlin
data class Person(
  val name: String,
  val nickname: String? = null,
  val age: Int = 18
// Java
public final class Person {

  @NotNull private final String name;
  @Nullable private final String nickname;
  private final int age;

  @NotNull public final String getName() { return name; }
  @Nullable public final String getNickname() { return nickname; }
  public final int getAge() { return age; }

  public Person(@NotNull String name, @Nullable String nickname, int age) { = name;
    this.nickname = nickname;
    this.age = age;

   // More Java boilerplate

It'd be repetitive to fill in these three data values for every test you have to write. And, in reality, your data models will actually have 10 or 20 fields - all of which need to be filled with data before you can use the instance in your tests.

Here are some of the most representative examples of what this library does. The examples are in Kotlin but you can use the same approach from Java too. For other examples, take a look at the demo directory.

Stubbing a data model

@StubbedMock lateinit var person: Person

This produces the instance: Person(name="name", nickname="nickname", age=0). All class member variables are set to their type's default value. String properties are set to the property name (even for nullable strings).

Stubbing a data model with custom values*

  Stub("name", /* stringValue = */ "Marcus"),
  Stub("nickname", /* stringValue = */ "Mark"),
  Stub("age", intValue = 50)
lateinit var person: Person

This produces the instance: Person(name="Marcus", nickname="Mark", age=50). Notice that we manually set values for each of the properties using the Stub child annotation. @Stub supports all basic JVM types and you need to reference the property name you want to inject the value to.

Initializing the mocks

As you might have suspected, there needs to be an entry point. Similar to Mockito, you need to invoke our stubbing function before you use the properties annotated with @StubbedMock. You can do this in your setup function:

@Before fun setup() {

Or even shorter if this is your only line in setup:

@Before fun setup() = initStubbedMocks(this)

How stubbing works

Our stubber is a recursive instance generator. This means that it also supports more complex data models, like this one:

data class Family(
  val members: List<Person>,
  val head: Person,
  val name: String

In this case, the stubber will go through all constructor arguments and try to stub in default values for them. If it encounters a complex data type (such as Person), it recursively does the same stubbing process for this data type.

For lists, it only stubs in an empty list of the proposed data type - but we're working on improving this in the future.

Filtering properties

If you don't want to stub special properties or pre-filled data types that class will modify later, you can use the advanced stubbing feature called Filtering. It allows you to skip that property and keep its value as-is.

A common use case is the lazy property. To add the lazy filter to your initializer function, you can pass it in as an argument:

@Before fun setup() {
  val filters = listOf(LazyFilter())
  initStubbedMocks(this, filters)

When your property will be filled by the class (or even externally) later on, you might not want to touch it until that moment. In this case, you can filter it out. For example, if we wanted to skip the list stubbing in our Family model from earlier, we can do that like so:

@Before fun setup() {
  val filters = listOf(ListFilter())
  initStubbedMocks(this, filters)

Because list filtering isn't supported out of the box, you need to provide a custom filter for this data type. You can do this by replacing the List with another type or you can use a more sophisticated check if necessary. Here's one way to do it:

class ListFilter : StubFilter {
  override fun isFiltered(field: Field) = field.type.simpleName == "List"

Including Stubbed Mock in your projects

To use stubbed-mock in your tests, the easiest way is to include it using Gradle. Put the following line in your module dependencies:

testImplementation 'com.blacklane.jvm:stubbedmock-lib:<latest_version>'

The latest version could be found here:

Tech used

Reporting issues and how to contribute

Our contributing guide outlines how you can report issues and get involved with Stubbed Mock. There's also information on the project structure and other guidelines for developing new features, fixing bugs or other changes related to the codebase.


MIT License