Mockito never()

Introduction

The never() method in Mockito is used to verify that a certain method on a mock object was never called. This is particularly useful for ensuring that no unwanted interactions have occurred. This tutorial will demonstrate how to use the never() method in Mockito to check that certain method invocations did not happen.

Maven Dependencies

To use Mockito with JUnit 5, add the following dependencies to your pom.xml file:

<dependency>
    <groupId>org.mockito</groupId>
    <artifactId>mockito-core</artifactId>
    <version>4.8.1</version>
    <scope>test</scope>
</dependency>
<dependency>
    <groupId>org.mockito</groupId>
    <artifactId>mockito-junit-jupiter</artifactId>
    <version>4.8.1</version>
    <scope>test</scope>
</dependency>
<dependency>
    <groupId>org.junit.jupiter</groupId>
    <artifactId>junit-jupiter-engine</artifactId>
    <version>5.9.2</version>
    <scope>test</scope>
</dependency>

Example Scenario

We will create a NotificationService class that has a dependency on a EmailService. Our goal is to test the NotificationService methods using the never() method in Mockito to ensure that no unwanted interactions with the EmailService occur.

NotificationService and EmailService Classes

First, create the Email class, the EmailService interface, and the NotificationService class.

public class Email {
    private String recipient;
    private String message;

    // Constructor, getters, and setters
    public Email(String recipient, String message) {
        this.recipient = recipient;
        this.message = message;
    }

    public String getRecipient() {
        return recipient;
    }

    public void setRecipient(String recipient) {
        this.recipient = recipient;
    }

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }
}

public interface EmailService {
    void sendEmail(Email email);
}

public class NotificationService {
    private final EmailService emailService;

    public NotificationService(EmailService emailService) {
        this.emailService = emailService;
    }

    public void sendNotification(String recipient, String message) {
        // Only send email if recipient and message are not empty
        if (recipient != null && !recipient.isEmpty() && message != null && !message.isEmpty()) {
            Email email = new Email(recipient, message);
            emailService.sendEmail(email);
        }
    }
}

JUnit 5 Test Class with Mockito

Create a test class for NotificationService using JUnit 5 and Mockito.

import static org.mockito.Mockito.*;
import static org.junit.jupiter.api.Assertions.*;
import org.junit.jupiter.api.Test;
import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.mockito.junit.jupiter.MockitoExtension;
import org.junit.jupiter.api.extension.ExtendWith;

@ExtendWith(MockitoExtension.class)
public class NotificationServiceTest {

    @Mock
    private EmailService emailService;

    @InjectMocks
    private NotificationService notificationService;

    @Test
    public void testSendNotification() {
        // Given
        String recipient = "test@example.com";
        String message = "Hello, World!";

        // When
        notificationService.sendNotification(recipient, message);

        // Then
        verify(emailService, times(1)).sendEmail(new Email(recipient, message));
    }

    @Test
    public void testSendNotificationNeverCalled() {
        // Given
        String recipient = "";
        String message = "Hello, World!";

        // When
        notificationService.sendNotification(recipient, message);

        // Then
        verify(emailService, never()).sendEmail(any(Email.class));
    }
    
    @Test
    public void testSendNotificationNullRecipient() {
        // Given
        String recipient = null;
        String message = "Hello, World!";

        // When
        notificationService.sendNotification(recipient, message);

        // Then
        verify(emailService, never()).sendEmail(any(Email.class));
    }
    
    @Test
    public void testSendNotificationNullMessage() {
        // Given
        String recipient = "test@example.com";
        String message = null;

        // When
        notificationService.sendNotification(recipient, message);

        // Then
        verify(emailService, never()).sendEmail(any(Email.class));
    }
}

Explanation

1. Creating Mocks with @Mock:

   • The @Mock annotation creates a mock instance of the EmailService interface. This mock instance can be used to simulate the behavior of the EmailService in a controlled way.

2. Injecting Mocks with @InjectMocks:

   • The @InjectMocks annotation injects the mock EmailService into the NotificationService instance to provide a controlled test environment. This allows the NotificationService methods to be tested in isolation from the actual EmailService implementation.

3. Verifying Interactions with never():

   • The verify(emailService, never()).sendEmail(any(Email.class)); method checks if the sendEmail method was never called on the EmailService mock object. This ensures that the sendNotification method of the NotificationService class does not interact with the EmailService when the recipient is empty, null, or when the message is null.

Additional Scenarios

Scenario: Verifying No Interactions

In this scenario, we will demonstrate how to verify that no interactions occurred with the mock object using the verifyNoInteractions() method.

@Test
public void testNoInteractionsWithEmailService() {
    // Given
    // No interactions expected

    // When
    // No method calls

    // Then
    verifyNoInteractions(emailService);
}

Scenario: Verifying Method Never Called with Specific Argument

In this scenario, we will demonstrate how to verify that a method was never called with a specific argument.

@Test
public void testSendNotificationNeverCalledWithSpecificArgument() {
    // Given
    String recipient = "test@example.com";
    String message = "Hello, World!";

    // When
    notificationService.sendNotification(recipient, "");

    // Then
    verify(emailService, never()).sendEmail(new Email(recipient, message));
}

Conclusion

The never() method in Mockito simplifies the verification that certain method calls on mock objects did not occur. By using never(), you can ensure that your code does not interact with dependencies in unwanted ways. This step-by-step guide demonstrated how to effectively use the never() method in your unit tests, covering different scenarios to ensure comprehensive testing of the NotificationService class.

Related Mockito Methods

Mockito mock()
Mockito spy()
Mockito when()
Mockito thenThrow()
Mockito verify()
Mockito times()
Mockito never()
Mockito any()
Mockito eq()
Mockito inOrder()
Mockito doReturn()
Mockito doThrow()
Mockito doAnswer()
Mockito timeout()
Mockito ArgumentMatchers