How to use partial mocks in real life

Partial mocks are an advanced feature of modern mocking libraries like mockito. Partial mocks retain the original code of a class only stubbing the methods you specify. If you build your system largely from scratch you most likely will not need to use them. Sometimes there is no easy way around them when working with dependencies not designed for testability. Let us look at an example:

 * Evil dependency we cannot change
public final class CarvedInStone {

    public CarvedInStone() {
        // may do unwanted things

    public int thisHasSideEffects(int i) {
        return 31337;

    // many more methods

public class ClassUnderTest {

    public Result computeSomethingInteresting() {
        // some interesting stuff
        int intermediateResult = new CarvedInStone().thisHasSideEffects(42);
        // more interesting code
        return new Result(intermediateResult * 1337);

We want to test the computeSomethingInteresting() method of our ClassUnderTest. Unfortunately we cannot replace CarvedInStone, because it is final and does not implement an interface containing the methods of interest. With a small refactoring and partial mocks we can still test almost the complete class:

public class ClassUnderTest {
    public int computeSomethingInteresting() {
        // some interesting stuff
        int intermediateResult = intermediateResultsFromCarvedInStone(42);
        // more interesting code
        return intermediateResult * 1337;

    protected int intermediateResultsFromCarvedInStone(int input) {
        return new CarvedInStone().thisHasSideEffects(input);

We refactored our dependency into a protected method we can use to stub out with our partial mocking to be tested like this:

public class ClassUnderTestTest {
    public void interestingComputation() throws Exception {
        ClassUnderTest cut = spy(new ClassUnderTest());
        assertEquals(1649858, cut.computeSomethingInteresting());

Caveat: Do not use the usual when-thenReturn-style:


with partial mocks because the real method will get called once!

So the only untested code is a simple delegation. Measures like that refactoring and partial mocking generally serve as a first step and not the destination.

Where to go from here

To go the whole way we would encapsulate all unmockable dependencies into wrapper objects providing the functionality we need here and inject them into our ClassUnderTest. Then we can replace our wrapper(s) easily using regular mocking.

Doing all this can be a lot of work and/or risk depending on the situation so the depicted process serves as an low risk intermediate step for getting as much important code under test as possible.

Note that the wrappers themselves stay largely untestable like our protected delegating method.

Spice up your unit testing

Writing unit tests shouldn’t be a chore. This article presents six tools (with alternatives) that help to improve your developer experience.

Writing unit tests is an activity every reasonable developer does frequently. While it certainly is a useful thing to do, it shouldn’t be a chore. To help you with the process of creating, running and evaluating unit tests, there are numerous tools and add-ons for every programming language around. This article focusses on improving the developer experience (the counterpart of “user experience”) for Java, JUnit and the Eclipse IDE. I will introduce you to the toolset we are using, which might not be the complete range of tools available.

Creating unit tests

  • MoreUnit – This plugin for Eclipse helps you to organize your unit test classes by maintaining a connection between the test and the production class. This way you’ll always see which classes and methods still lack a corresponding test. You can take shortcuts in the navigation by jumping directly into the test class and back. And if you move one file, MoreUnit will move the other one alongside. It’s a swiss army knife for unit test writers and highly recommended.
  • EqualsVerifier – If you ever wrote a custom implementation of the equals()/hashcode() method pair, you’ll know that it’s not a triviality. What’s even more intimidating is that you probably got it wrong or at least not fully correct. The effects of a flawed equals() method aren’t easily determinable, so this is a uncomfortable situation. Luckily, there is a specialized tool to help you with this task exactly. The EqualsVerifier library tests your custom implementation against all aspects of the art of writing an equals() method with just one line of code.
  • Mockito (and EasyMock) – When dealing with dependencies of classes under test, mock objects can come in handy. But writing them by hand is tedious, boring and error-prone. This is where mock frameworks can help by reducing the setup and verification of a mock object to just a few lines of code. EasyMock is the older of the two projects, but it manages to stay up-to-date by introducing new features and syntax with every release. Mockito has a very elegant and readable syntax and provides a rich feature set. There are other mock frameworks available, too.

Running unit tests

  • InfiniTest (and JUnit Max) – Normally, you have to run the unit tests in your IDE by manually clicking the “run” button or hitting some obscure keyboard shortcut. These two continuous testing tools will run your tests while you still type. This will shorten your test feedback loop to nearly milliseconds after each change. Your safety net was never closer. InfiniTest and JUnit Max are both Eclipse plugins, but the latter costs a small annual fee. It’s written by Kent Beck himself, though.

Evaluating unit tests

  • EclEmma (and Cobertura) – If you want to know about the scope or “coverage” of your tests, you should consult a code coverage tool. Cobertura produces really nice HTML reports for all your statistical needs. EclEmma is an Eclipse plugin that integrates the code coverage tool Emma with Eclipse in the finest way possible. Simply run “coverage as” instead of “run as” and you are done. All the hassle with instrumenting your classes and setting up the classpath in the right order (major hurdles when using cobertura) is dealt with behind the scenes.
  • Jester (and Jumble) – The question “who tests my tests?” is totally legit. And it has an answer: Every mutation testing tool around. For Java and JUnit, there are at least two that do their job properly: Jester works on the source code while Jumble uses the bytecode. Mutation testing injects little changes into your production code to test if your tests catch them. This is a different approach on test coverage that can detect code that is executed but not pinned down by an assertion. While Jester has a great success story to tell, Jumble tends to produce similar results as cobertura’s condition coverage report, at least in my experience.


As you can see, there is a wide range of tools available to improve your efforts to write well-tested software. This list is in no way comprehensive. If you know about a tool that should be mentioned, we would love to read your comment.