Mockito BDDMockito willReturn Method

The willReturn method in the BDDMockito class is part of the Behavior-Driven Development (BDD) style of writing tests using Mockito. It is used to specify the value that should be returned when a method on a mock object is called. This method is particularly useful for making tests more readable and aligning with the BDD style, which focuses on describing the behavior of the application in a clear and human-readable format.

Table of Contents

1. Introduction
2. willReturn Method Syntax
3. Examples
   • Basic Usage
   • Using willReturn with Different Return Values
   • Using willReturn with Chained Calls
4. Real-World Use Case
5. Conclusion

Introduction

Behavior-Driven Development (BDD) is a software development approach that emphasizes collaboration between developers, QA, and non-technical or business participants in a software project. Mockito’s BDDMockito class provides methods that support the BDD style of writing tests. The willReturn method is used to specify the return value for a method call on a mock object in a more readable way.

willReturn Method Syntax

Specifying a Return Value

import org.mockito.BDDMockito;

static <T> BDDMockito.BDDMyOngoingStubbing<T> willReturn(T value)

Specifies the value to be returned by the mock object.

Parameters:

• value: The value to be returned by the mock object.

Returns:

• A BDDMyOngoingStubbing object that allows further stubbing.

Examples

Basic Usage

Use willReturn to specify a basic return value for a method call on a mock object.

import static org.mockito.BDDMockito.*;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

public class BasicWillReturnTest {
    @Test
    void testWillReturn() {
        UserService mockUserService = mock(UserService.class);

        // Set up the return value
        given(mockUserService.getUserDetails("user123")).willReturn("Mock user details");

        // Call the method
        String result = mockUserService.getUserDetails("user123");

        // Verify the result
        assertEquals("Mock user details", result);
    }
}

class UserService {
    public String getUserDetails(String userId) {
        return "Real user details for " + userId;
    }
}

Using willReturn with Different Return Values

Use willReturn to specify different return values for different method calls.

import static org.mockito.BDDMockito.*;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

public class WillReturnDifferentValuesTest {
    @Test
    void testWillReturnDifferentValues() {
        UserService mockUserService = mock(UserService.class);

        // Set up the return values
        given(mockUserService.getUserDetails("user123")).willReturn("Mock user details for user123");
        given(mockUserService.getUserDetails("user456")).willReturn("Mock user details for user456");

        // Call the methods
        String result1 = mockUserService.getUserDetails("user123");
        String result2 = mockUserService.getUserDetails("user456");

        // Verify the results
        assertEquals("Mock user details for user123", result1);
        assertEquals("Mock user details for user456", result2);
    }
}

class UserService {
    public String getUserDetails(String userId) {
        return "Real user details for " + userId;
    }
}

Using willReturn with Chained Calls

Use willReturn to specify a sequence of return values for successive calls to a method.

import static org.mockito.BDDMockito.*;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

public class WillReturnChainedCallsTest {
    @Test
    void testWillReturnChainedCalls() {
        UserService mockUserService = mock(UserService.class);

        // Set up a sequence of return values
        given(mockUserService.getUserDetails("user123")).willReturn("Mock user details 1", "Mock user details 2");

        // Call the method multiple times
        String result1 = mockUserService.getUserDetails("user123");
        String result2 = mockUserService.getUserDetails("user123");

        // Verify the results
        assertEquals("Mock user details 1", result1);
        assertEquals("Mock user details 2", result2);
    }
}

class UserService {
    public String getUserDetails(String userId) {
        return "Real user details for " + userId;
    }
}

Real-World Use Case

Simplifying Tests for Services with Complex Behaviors

In a real-world scenario, you might need to test services with methods that have complex behaviors. Using willReturn can simplify these tests by allowing you to set up return values in a readable and intuitive way.

import static org.mockito.BDDMockito.*;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

interface EmailService {
    void sendEmail(String recipient, String subject, String body);
}

class UserService {
    private final EmailService emailService;

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

    public String notifyUser(String userId, String message) {
        if (userId == null) {
            throw new IllegalArgumentException("User ID cannot be null");
        }
        emailService.sendEmail(userId, "Notification", message);
        return "Notification sent to " + userId;
    }
}

public class UserServiceTest {
    @Test
    void testNotifyUser() {
        EmailService mockEmailService = mock(EmailService.class);
        UserService userService = new UserService(mockEmailService);

        // Set up the return value
        willDoNothing().given(mockEmailService).sendEmail("user123", "Notification", "Your account has been updated.");

        // Call the method
        String result = userService.notifyUser("user123", "Your account has been updated.");

        // Verify the result and the interaction
        assertEquals("Notification sent to user123", result);
        then(mockEmailService).should().sendEmail("user123", "Notification", "Your account has been updated.");
    }
}

In this example, the UserServiceTest class uses Mockito’s BDDMockito.given method along with willReturn to set up the return value for the sendEmail method. This simplifies the test by allowing you to set up and verify interactions in a readable and intuitive way.

Conclusion

The BDDMockito.willReturn method in Mockito is used for specifying return values for method calls on mock objects in a BDD style. By using willReturn, you can make your tests more readable and align them with the BDD approach, focusing on the behavior of the application rather than the implementation details. This helps ensure that your tests are clear, comprehensive, and easy to understand.