Mockito BDDMockito willThrow Method

The willThrow method in the BDDMockito class is part of the Behavior-Driven Development (BDD) style of writing tests using Mockito. It is used to specify that a method on a mock object should throw an exception when 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. willThrow Method Syntax
3. Examples
   • Basic Usage
   • Using willThrow with Different Exceptions
   • Using willThrow with Checked Exceptions
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 willThrow method is used to specify that a method on a mock object should throw an exception when called, making tests more readable and aligning with the BDD approach.

willThrow Method Syntax

Specifying an Exception to be Thrown

import org.mockito.BDDMockito;

static BDDMockito.BDDStubber willThrow(Throwable throwable)

Specifies that a method should throw the given exception.

Parameters:

• throwable: The exception to be thrown.

Returns:

• A BDDStubber object that allows further stubbing.

Examples

Basic Usage

Use willThrow to specify that a method should throw an exception when called.

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

public class BasicWillThrowTest {
    @Test
    void testWillThrow() {
        UserService mockUserService = mock(UserService.class);

        // Set up the exception to be thrown
        willThrow(new RuntimeException("User not found")).given(mockUserService).getUserDetails("user123");

        // Call the method and verify the exception
        Exception exception = assertThrows(RuntimeException.class, () -> {
            mockUserService.getUserDetails("user123");
        });
        assertEquals("User not found", exception.getMessage());
    }
}

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

Using willThrow with Different Exceptions

Use willThrow to specify different exceptions for different method calls.

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

public class WillThrowDifferentExceptionsTest {
    @Test
    void testWillThrowDifferentExceptions() {
        UserService mockUserService = mock(UserService.class);

        // Set up the exceptions to be thrown
        willThrow(new RuntimeException("User not found")).given(mockUserService).getUserDetails("user123");
        willThrow(new IllegalArgumentException("Invalid user ID")).given(mockUserService).getUserDetails("user456");

        // Call the methods and verify the exceptions
        Exception exception1 = assertThrows(RuntimeException.class, () -> {
            mockUserService.getUserDetails("user123");
        });
        assertEquals("User not found", exception1.getMessage());

        Exception exception2 = assertThrows(IllegalArgumentException.class, () -> {
            mockUserService.getUserDetails("user456");
        });
        assertEquals("Invalid user ID", exception2.getMessage());
    }
}

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

Using willThrow with Checked Exceptions

Use willThrow to specify that a method should throw a checked exception.

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

public class WillThrowCheckedExceptionTest {
    @Test
    void testWillThrowCheckedException() throws Exception {
        UserService mockUserService = mock(UserService.class);

        // Set up the checked exception to be thrown
        willThrow(new Exception("Checked exception")).given(mockUserService).updateUserDetails("user123");

        // Call the method and verify the exception
        Exception exception = assertThrows(Exception.class, () -> {
            mockUserService.updateUserDetails("user123");
        });
        assertEquals("Checked exception", exception.getMessage());
    }
}

class UserService {
    public void updateUserDetails(String userId) throws Exception {
        // Update user details logic
    }
}

Real-World Use Case

Simplifying Tests for Services with Exception Handling

In a real-world scenario, you might need to test services with methods that handle exceptions. Using willThrow can simplify these tests by allowing you to specify the exceptions that should be thrown, making your tests more readable and intuitive.

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) throws EmailException;
}

class UserService {
    private final EmailService emailService;

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

    public void notifyUser(String userId, String message) throws EmailException {
        if (userId == null) {
            throw new IllegalArgumentException("User ID cannot be null");
        }
        emailService.sendEmail(userId, "Notification", message);
    }
}

class EmailException extends Exception {
    public EmailException(String message) {
        super(message);
    }
}

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

        // Set up the exception to be thrown
        willThrow(new EmailException("Email service is down")).given(mockEmailService).sendEmail("user123", "Notification", "Your account has been updated.");

        // Call the method and verify the exception
        EmailException exception = assertThrows(EmailException.class, () -> {
            userService.notifyUser("user123", "Your account has been updated.");
        });
        assertEquals("Email service is down", exception.getMessage());
    }
}

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

Conclusion

The BDDMockito.willThrow method in Mockito is used for specifying exceptions that should be thrown by methods on mock objects in a BDD style. By using willThrow, 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.