Mocking

Objectives

• Understand what is a mock

• Practice mocking on a code example

Connection - 10'

In groups of 3 answer to this question : in which cases do you use a Mock ?

Concepts - 5'

What is a Mock ?

• Mock objects allow you to simulate the behavior of classes and interfaces

  • Letting the code in the test interact with them as if they were real

• Mocks are used to test behavior of other objects, reals, but link to another object not available in test context or not implemented yet

In which cases do we use them ?

Be able to execute tests in isolation

• Replaces dependency from other object

• Set expectations on calls to the dependent object

• Set the exact return values it should give you to perform the test you want

When using mocks you need to always ask yourselves what are your test boundaries.

Examples of situation

• The system needs a DB access

• The system call services that are not available

• The system call APIs developed by other teams which are not implemented yet

How does it work ?

Concrete Practice - 45'

We would like to test a system which convert an amount in a specific currency to another one. The converter method call a service to retrieve the change rate between the two currencies.

Some rules are checked during treatment :

- The amount must be equal or greater than zero
- Service checks if currencies exist
- The service could throw other types of exception following the context
- The return rate must be equal or greater than zero
- The result must be between 0 and the maximum value

In our Unit Tests methods the service is not accessible -> we need to “mock” it

How to ?

• Open the source code here

• Demonstrate how to add Mockito to our tests

pom.xml

<dependency>
    <groupId>org.mockito</groupId>
    <artifactId>mockito-core</artifactId>
    <version>3.3.3</version>
    <scope>test</scope>
</dependency>
<dependency>
    <groupId>org.mockito</groupId>
    <artifactId>mockito-junit-jupiter</artifactId>
    <version>3.3.3</version>
    <scope>test</scope>
</dependency>

• This annotation allows us to use the @Mock annotation (inject Mocks)

  • Learn more about it here

@ExtendWith(MockitoExtension.class)
@Mock private ChangeRateService changeRateService;

Implement tests - 35'

• The production code is implemented as expected

• Please add the tests to check it

When working with existing code and adding test on it : use your Code Coverage

Debrief the exercise - 10'

• Group sharing

• Demo the solution in the dedicated branch

Conclusion - 10'

• Think about what we did today.

  • If you had to explain the main idea of mocking to someone else, what would you say?

• Write your explanation in a sentence or two on a post-it