What is the purpose of the when method in Mockito?

Table of Contents

• Introduction
• Purpose of the when() Method in Mockito
  • 1. Stubbing Methods to Return Specific Values
    • Example: Returning a Value for a Mocked Method
  • 2. Mocking Void Methods and Actions
    • Example: Mocking Void Method with Action
  • 3. Throwing Exceptions Using when()
    • Example: Mocking an Exception
  • 4. Stubbing Multiple Calls with Different Return Values
    • Example: Mocking Multiple Returns
• Best Practices for Using when() in Mockito
• Conclusion

Introduction

In unit testing, particularly with Mockito, it’s essential to mock the behavior of dependencies or components that a class under test interacts with. This is where the when() method comes into play. The when() method in Mockito is used to define the behavior of mock objects. It allows you to specify what should happen when a mocked method is called, such as returning specific values, throwing exceptions, or executing particular actions.

In this guide, we will explore the purpose of the when() method in Mockito, how it works, and provide practical examples of its usage in unit tests.

Purpose of the when() Method in Mockito

The when() method is used to stub methods of mock objects, meaning it defines the return values or actions for specific method calls. This enables you to isolate the class under test by controlling the behavior of its dependencies. You can specify what should happen when a mock method is invoked, making your tests more focused and predictable.

1. Stubbing Methods to Return Specific Values

The most common use of the when() method is to specify a return value for a mocked method when it is called with certain arguments.

Example: Returning a Value for a Mocked Method

In this example:

• when(mockRepo.findById(1)) tells Mockito to return a User object with the name "John Doe" when findById(1) is called on the mocked UserRepository (mockRepo).
• The thenReturn() method specifies the return value for the mocked method.

2. Mocking Void Methods and Actions

In addition to returning values, the when() method can also be used to mock void methods. However, mocking void methods requires special handling, as they do not return values.

Example: Mocking Void Method with Action

In this case, doNothing().when(mockRepo).save(user) tells Mockito that the save() method does nothing when called. This is especially useful for mocking methods that perform actions, such as saving to a database.

3. Throwing Exceptions Using when()

You can also use when() to simulate exceptions being thrown when a method is called on a mock object. This is helpful when testing error scenarios.

Example: Mocking an Exception

In this example:

• when(mockRepo.findById(1)) tells Mockito to throw a RuntimeException with the message "User not found" when findById(1) is called.

4. Stubbing Multiple Calls with Different Return Values

Sometimes you might want to return different values on consecutive calls to the same method. The when() method can be chained with thenReturn() to specify different return values for different calls.

Example: Mocking Multiple Returns

In this case, the first time findById(1) is called, it returns User("John Doe"), and the second time it returns User("Jane Doe").

Best Practices for Using when() in Mockito

1. Always Use **thenReturn()** After **when()**: The when() method is used to define which method you are mocking, and thenReturn() specifies what should be returned when that method is called.
2. Mock Behavior Before Testing: Always set up the mock behavior before calling the method under test. This ensures the method behaves as expected during the test.
3. Use for Mocking External Dependencies: Use when() to simulate the behavior of external dependencies like databases, APIs, or third-party services that are not part of the unit under test.
4. Test Both Success and Failure Scenarios: Use when() to mock both successful outcomes (returning values) and failure scenarios (throwing exceptions) to cover all possible conditions in your tests.

Conclusion

The **when()** method in Mockito is an essential tool for defining the behavior of mock objects in unit tests. It allows you to specify return values, simulate exceptions, and mock void methods or actions, enabling controlled and predictable testing scenarios. By combining when() with other methods like thenReturn() or doNothing(), you can easily control the mock behavior and ensure that your tests focus on the logic of the class under test rather than external dependencies. This makes your unit tests more isolated, reliable, and easier to maintain.