Mockito Mocking Void Methods

1. Overview

In Mockito, mocking void methods can be a unique challenge. Unlike methods that return a value, void methods don't allow us to use a return statement. But there are times when we need these void methods to throw exceptions or execute specific tasks in a testing context. This tutorial will guide you through how to mock void methods using Mockito effectively.

2. Development Steps

1. Set up a new Maven project.

2. Add the necessary Mockito and JUnit 5 dependencies.

3. Create a simple class named 'Notifier' to demonstrate the mocking of void methods.

4. Craft a test class to show how to mock the behavior of the void method.

5. Execute the tests and understand the results.

6. Conclude on the importance of mocking void methods.

3. Dependencies (Mockito and JUnit 5)

<!-- JUnit 5 -->
<dependency>
    <groupId>org.junit.jupiter</groupId>
    <artifactId>junit-jupiter-engine</artifactId>
    <version>5.10.0</version>
    <scope>test</scope>
</dependency>
<!-- Mockito -->
<dependency>
    <groupId>org.mockito</groupId>
    <artifactId>mockito-core</artifactId>
    <version>5.6.0</version>
    <scope>test</scope>
</dependency>

4. Code Program

// Step 3: Define the Notifier class
class Notifier {
    public void notifyUser(String message) {
        System.out.println("Notification sent: " + message);
    }
}
// Step 4: Create the test class for Notifier
import org.junit.jupiter.api.Test;
import static org.mockito.Mockito.*;
public class NotifierTest {
    @Test
    public void testMockingVoidMethod() {
        // Mocking the Notifier
        Notifier mockNotifier = mock(Notifier.class);
        // Use Mockito to mock behavior of void method
        doThrow(new IllegalArgumentException()).when(mockNotifier).notifyUser("Bad Message");
        // Test with a valid message
        mockNotifier.notifyUser("Good Message");
        // Test with a bad message, expecting an exception
        try {
            mockNotifier.notifyUser("Bad Message");
        } catch (IllegalArgumentException e) {
            System.out.println("Caught expected exception: " + e.getMessage());
        }
        // Verify the interactions
        verify(mockNotifier, times(1)).notifyUser("Good Message");
        verify(mockNotifier, times(1)).notifyUser("Bad Message");
    }
}

Output:

Notification sent: Good Message
Caught expected exception: null

Code Explanation:

1. The Notifier class contains a simple void method notifyUser which sends a notification (in this context, prints a message).

2. In the test class, we've mocked Notifier.

3. Using Mockito's doThrow() method, we set our mock to throw an IllegalArgumentException when notifyUser is called with the argument "Bad Message".

4. Then, we call notifyUser with a "Good Message", and no exception is thrown.

5. We subsequently call notifyUser with a "Bad Message" and capture the expected exception.

6. The verify() methods ensure our mock methods were called the expected number of times with the expected arguments.

5. Conclusion

Mocking void methods in Mockito might seem a bit unorthodox at first, especially since we're used to the when().thenReturn() pattern. But with tools like doThrow(), Mockito provides a smooth experience. Being able to mock void methods is crucial, as they often encapsulate crucial behaviors in our applications, such as sending notifications, logging, or interacting with external systems. 

By understanding how to test these methods effectively, we ensure that our applications behave as expected in various scenarios.