Mockito BDDMockito willThrow()

1. Overview

The willThrow() method, as provided by BDDMockito, serves as a cornerstone for Behavior-Driven Development (BDD) style testing using Mockito, especially when dealing with exceptions. By simulating exception scenarios, this method proves crucial in verifying how a system reacts to unexpected situations. This tutorial dives deep into the use of the willThrow() method, guiding you on how to harness its capabilities for your BDD tests.

2. Development Steps

1. Initialize a new Maven project.

2. Incorporate the required Mockito and JUnit 5 dependencies.

3. Design a simple service class named UserService for our demonstration.

4. Develop a BDD-style test for UserService, employing BDDMockito's willThrow().

5. Run the test and analyze the results.

6. Discuss the relevance and utility of the willThrow() method in BDD testing.

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 Core -->
<dependency>
    <groupId>org.mockito</groupId>
    <artifactId>mockito-core</artifactId>
    <version>5.6.0</version>
    <scope>test</scope>
</dependency>

4. Code Program

// A rudimentary UserService class for this tutorial
class UserService {
    void deleteUser(int userId) {
        // In an actual scenario, this could remove a user from a database
    }
}
// BDD-style test class for UserService using BDDMockito's willThrow()
import org.junit.jupiter.api.Test;
import static org.mockito.BDDMockito.*;
public class UserServiceBDDTest {
    @Test
    public void testDeleteUserExceptionScenario() {
        // Given: Establish the situation
        UserService mockUserService = mock(UserService.class);
        int invalidUserId = -1;
        willThrow(new IllegalArgumentException("Invalid user ID")).given(mockUserService).deleteUser(invalidUserId);
        try {
            // When: Perform the action under scrutiny
            mockUserService.deleteUser(invalidUserId);
        } catch (Exception e) {
            // Then: Confirm the result
            System.out.println("Exception caught: " + e.getMessage());
        }
    }
}

Output:

Exception caught: Invalid user ID

Code Explanation:

1. We've created a straightforward UserService class possessing a method named deleteUser().

2. The UserServiceBDDTest class is designed to implement a BDD test for UserService using BDDMockito.

3. In the test, BDDMockito's willThrow() function is deployed to set up an expected exception scenario. In this case, an IllegalArgumentException is expected to be thrown when trying to delete a user with an invalid ID.

4. The "When" section enacts the behavior we aim to test. Here, we call the deleteUser() method, anticipating it to throw the exception we've mocked.

5. The catch block captures and prints the exception, affirming the expected behavior.

5. Conclusion

With BDDMockito's willThrow() method at our disposal, we can simulate exception scenarios in a BDD style, ensuring our system can gracefully handle unexpected situations. This method not only facilitates better test clarity but also aligns our tests closely with natural language requirements, a hallmark of BDD. Through effective exception-handling tests, developers can ensure that their systems remain robust and resilient, even in the face of unforeseen challenges.

Related Mockito BDDMockito Class Methods (Behavior-Driven Development Style)

Mockito BDDMockito
Mockito BDDMockito given()
Mockito BDDMockito willThrow()
Mockito BDDMockito willAnswer()
Mockito BDDMockito willReturn()
Mockito BDDMockito willDoNothing()
Mockito BDDMockito willCallRealMethod()
Mockito BDDMockito then()
Mockito BDDMockito.any()
Mockito BDDMockito.times()