Mockito When/Then Cookbook

1. Overview

This cookbook shows how to use Mockito to configure behavior in a variety of examples and use-cases.

The format of the cookbook is example focused and practical – no extraneous details and explanations necessary.

And of course, if you want to learn more about testing well with Mockito, have a look at the other Mockito articles here.

Mockito Verify Cookbook

Mockito Verify examples, usage and best practices.

Mockito – Using Spies

Making good use of Spies in Mockito, and how spies are different from mocks.

Mockito's Mock Methods

This tutorial illustrates various uses of the standard static mock methods of the Mockito API.

We're going to be mocking a simple list implementation – the same implementation we used in the previous cookbook:

public class MyList extends AbstractList<String> {

    @Override
    public String get(final int index) {
        return null;
    }
    @Override
    public int size() {
        return 1;
    }
}

2. Cookbook

configure simple return behavior for mock

MyList listMock = Mockito.mock(MyList.class);
when(listMock.add(anyString())).thenReturn(false);

boolean added = listMock.add(randomAlphabetic(6));
assertThat(added, is(false));

configure return behavior for mock in an alternative way

MyList listMock = Mockito.mock(MyList.class);
doReturn(false).when(listMock).add(anyString());

boolean added = listMock.add(randomAlphabetic(6));
assertThat(added, is(false));

configure mock to throw an exception on a method call

@Test(expected = IllegalStateException.class)
public void givenMethodIsConfiguredToThrowException_whenCallingMethod_thenExceptionIsThrown() {
    MyList listMock = Mockito.mock(MyList.class);
    when(listMock.add(anyString())).thenThrow(IllegalStateException.class);

    listMock.add(randomAlphabetic(6));
}

configure the behavior of a method with void return type – to throw an exception

MyList listMock = Mockito.mock(MyList.class);
doThrow(NullPointerException.class).when(listMock).clear();

listMock.clear();

configure the behavior of multiple calls

MyList listMock = Mockito.mock(MyList.class);
when(listMock.add(anyString()))
  .thenReturn(false)
  .thenThrow(IllegalStateException.class);

listMock.add(randomAlphabetic(6));
listMock.add(randomAlphabetic(6)); // will throw the exception

configure the behavior of a spy

MyList instance = new MyList();
MyList spy = Mockito.spy(instance);

doThrow(NullPointerException.class).when(spy).size();
spy.size(); // will throw the exception

configure method to call the real, underlying method on a mock

MyList listMock = Mockito.mock(MyList.class);
when(listMock.size()).thenCallRealMethod();

assertThat(listMock.size(), equalTo(1));

configure mock method call with custom Answer

MyList listMock = Mockito.mock(MyList.class);
doAnswer(invocation -> "Always the same").when(listMock).get(anyInt());

String element = listMock.get(1);
assertThat(element, is(equalTo("Always the same")));

3. Conclusion

This format is an experiment – I'm publishing some of my internal development cookbooks on a given topic – on Google Guava, Hamcrest and now Mockito. The goal is to have this information readily available online – and to add to it whenever I run into a new useful example.

The implementation of all these examples and code snippets can be found over on GitHub – this is a Maven-based project, so it should be easy to import and run as it is.

Persistence

REST

Security