Mockito BDDMockito.willAnswer Method

The willAnswer method in the BDDMockito class is part of the Behavior-Driven Development (BDD) style of writing tests using Mockito. It allows you to specify a custom behavior for a method call on a mock object. 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. willAnswer Method Syntax
3. Examples
   • Basic Usage
   • Using willAnswer with Complex Logic
   • Using willAnswer with Multiple 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 willAnswer method is used to specify custom behavior for method calls on mock objects, making tests more readable and intuitive.

willAnswer Method Syntax

Specifying a Custom Answer

import org.mockito.BDDMockito;
import org.mockito.stubbing.Answer;

static <T> BDDMockito.BDDMyOngoingStubbing<T> willAnswer(Answer<?> answer)

Specifies custom behavior for a method call on a mock object.

Parameters:

• answer: The custom behavior to be executed when the method is called.

Returns:

• A BDDMyOngoingStubbing object that allows further stubbing.

Examples

Basic Usage

Use willAnswer to specify a basic custom behavior for a method call on a mock object.

import static org.mockito.BDDMockito.*;
import org.mockito.invocation.InvocationOnMock;
import org.mockito.stubbing.Answer;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

public class BasicWillAnswerTest {
    @Test
    void testWillAnswer() {
        UserService mockUserService = mock(UserService.class);

        // Set up the custom behavior
        willAnswer(new Answer<String>() {
            @Override
            public String answer(InvocationOnMock invocation) {
                String userId = invocation.getArgument(0);
                return "Mock user details for " + userId;
            }
        }).given(mockUserService).getUserDetails("user123");

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

        // Verify the result
        assertEquals("Mock user details for user123", result);
    }
}

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

Using willAnswer with Complex Logic

Use willAnswer to specify custom behavior with complex logic for a method call on a mock object.

import static org.mockito.BDDMockito.*;
import org.mockito.invocation.InvocationOnMock;
import org.mockito.stubbing.Answer;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

public class ComplexWillAnswerTest {
    @Test
    void testWillAnswerWithComplexLogic() {
        MathService mockMathService = mock(MathService.class);

        // Set up the custom behavior with complex logic
        willAnswer(new Answer<Integer>() {
            @Override
            public Integer answer(InvocationOnMock invocation) {
                int a = invocation.getArgument(0);
                int b = invocation.getArgument(1);
                return a * b;
            }
        }).given(mockMathService).multiply(anyInt(), anyInt());

        // Call the method
        int result = mockMathService.multiply(5, 3);

        // Verify the result
        assertEquals(15, result);
    }
}

interface MathService {
    int multiply(int a, int b);
}

Using willAnswer with Multiple Calls

Use willAnswer to specify different behavior for multiple calls to a method on a mock object.

import static org.mockito.BDDMockito.*;
import org.mockito.invocation.InvocationOnMock;
import org.mockito.stubbing.Answer;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

public class MultipleWillAnswerTest {
    @Test
    void testWillAnswerWithMultipleCalls() {
        UserService mockUserService = mock(UserService.class);

        // Set up the custom behavior for multiple calls
        willAnswer(new Answer<String>() {
            @Override
            public String answer(InvocationOnMock invocation) {
                String userId = invocation.getArgument(0);
                return "Mock user details for " + userId;
            }
        }).given(mockUserService).getUserDetails(anyString());

        // Call the method multiple times
        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;
    }
}

Real-World Use Case

Simplifying Tests for Services with Custom Behaviors

In a real-world scenario, you might need to test services with methods that have custom behaviors. Using willAnswer can simplify these tests by allowing you to specify the custom behaviors in a readable and intuitive way.

import static org.mockito.BDDMockito.*;
import org.mockito.invocation.InvocationOnMock;
import org.mockito.stubbing.Answer;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

interface NotificationService {
    void sendNotification(String recipient, String message);
}

class UserService {
    private final NotificationService notificationService;

    public UserService(NotificationService notificationService) {
        this.notificationService = notificationService;
    }

    public void notifyUser(String userId, String message) {
        notificationService.sendNotification(userId, message);
    }
}

public class UserServiceTest {
    @Test
    void testNotifyUserWithCustomBehavior() {
        NotificationService mockNotificationService = mock(NotificationService.class);
        UserService userService = new UserService(mockNotificationService);

        // Set up the custom behavior
        willAnswer(new Answer<Void>() {
            @Override
            public Void answer(InvocationOnMock invocation) {
                String recipient = invocation.getArgument(0);
                String message = invocation.getArgument(1);
                System.out.println("Sending notification to " + recipient + ": " + message);
                return null;
            }
        }).given(mockNotificationService).sendNotification(anyString(), anyString());

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

        // Verify the interaction
        then(mockNotificationService).should().sendNotification("user123", "Your account has been updated.");
    }
}

In this example, the UserServiceTest class uses Mockito’s BDDMockito.given method along with willAnswer to set up custom behavior for the sendNotification method. This simplifies the test by allowing you to specify the custom behavior and verify the interaction in a readable and intuitive way.

Conclusion

The BDDMockito.willAnswer method in Mockito is used for specifying custom behaviors for method calls on mock objects in a BDD style. By using willAnswer, 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.