The Band - a Mockito kata

Overview

This document explains a little about mocking and the idea for the exercises.

Exercises is where the exercises are.

Spring Boot is a primer for Spring Boot and Flyway.

Background

Back in the 80's everyone asked the question. What would you use a personal computer for, except playing games? The answer was - keep a register of your record collection!

It's back for the benefit of practicing Mockito.

The domain

In this domain there are Albums, Artists, Bands and Song recordings.

A song recording appears on an album. An artist has contributed to zero or more song recordings. Artists has sometimes been members of bands that has produced albums.

How the domain model works in details are up to you. There is no right or wrong answer, we are here to explore mocking.

Architecture

It is a micro service, based on Spring Boot. Built with Maven and the pom includes some additions to what is included with Spring starter.

Flyway

Flyway maintains a database schema for you by reading the files in src/main/resources/db/migration. When you need to change something in the schema, add a file and standing with the naming convention of capital V, number and two underlines.

Postgresql

The database is assumed to be Postgresql, running on your localhost. Now, nobody wants to install a database on their development machine so use this docker command instead:

docker run --name pgdatabase -e POSTGRES_PASSWORD=mysecret -d -p 5432:5432 postgres

Docker can be found here: https://docs.docker.com/docker-for-mac/install/

Sonarqube

Test coverage with JaCoCo is also there. If you run SonarQube using docker, you can have your code analyzed.

docker run -d --name sonarqube -p 9000:9000 sonarqube

What to explore

Mocking is about isolating units so that you can test them in isolation. When you look at the code, you'll see that some code is just stubs. They may return an empty collection, no matter of what the input is. Still, we can test the clients that use that code by using mocks. A service may depend on another service that is not done yet. No problem, we just put in a mock and explore how the interaction between the services should be.

E.g. AlbumService depends on both the AlbumRepository and the SongService. But neither needs any implementation to test AlbumService. We just inject mocks.

Mockito when

The when is used to serve a canned answer when a method on a mock is called.

when(albumRepository.findById(isNull())).thenThrow(IllegalArgumentException.class);

It can also be used to throw an exception.

when(songService.artistsOnSong(SOME_SONG_ID)).thenReturn();

Returning a computed answer

You can return a computed answer, based on the input. For instance, if you wish to return what you got, as in the case of save an entity:

doAnswer(invocation -> invocation.getArgument(0)).when(albumRepository).save(isA(AlbumEntity.class));

This means: do answer with the first argument when the repository's save method is called with something that is an AlbumEntity.

If you don't specify anything, the default is to return null for objects and the default value for primitives.

Mockito verify

The verify is used when you wish to verify interactions. Instead of doing an assert on the result, you verify that some interaction has been taking place, like between a service and a repository.

verify(albumRepository).save(isA(AlbumEntity.class));

You can also say there should be no interactions at all.

verify(albumRepository, never()).save(isA(AlbumEntity.class));

Mockito capture

When you verify interaction, sometimes you not only want to verify that it did happen, you may also want to see that the correct parameters were passed. For this you will use an argument captor.

1. Get the captor: ArgumentCaptor<AlbumEntity> captor = ArgumentCaptor.forClass(AlbumEntity.class);

2. Verify and capture the interaction: verify(albumRepository).save(captor.capture());

3. Assert the correct value was passed: assertEquals(SOME_ALBUM_ID, captor.getValue().getId());

Read more

Mockito https://site.mockito.org/