Testing is an essential part of any software development process. Mockito, a powerful mocking framework, can help you test different parts of your application, including void methods. Void methods can be challenging to test, as the focus is on the program flow rather than the output. In this blog post, we will explore how to effectively test void methods using Mockito in a Spring Boot application.
Getting Started: Setting Up Dependencies
To begin, ensure that you have the necessary dependencies in your project. Add the following dependencies to your pom.xml
or build.gradle
file:
For Maven:
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-test</artifactId>
<scope>test</scope>
</dependency>
For Gradle:
testImplementation 'org.springframework.boot:spring-boot-starter-test'
These dependencies will bring in Mockito and other necessary testing libraries. For a complete project setup, feel free to use the prepared Spring initializr
Testing Void Methods
Let's consider a simple example: a Spring Boot application with a service that sends email notifications. The EmailService
class has a sendEmail
method, which is void and sends an email to a given recipient.
@Service
public class EmailService {
private final EmailClient emailClient;
public EmailService(EmailClient emailClient) {
this.emailClient = emailClient;
}
public void sendEmail(String recipient, String subject, String content) {
emailClient.send(recipient, subject, content);
}
}
public interface EmailClient {
void send(String recipient, String subject, String content);
}
To test the sendEmail
method, we will use Mockito to verify the program flow of the method execution.
1. Create a test class
First, create a test class named EmailServiceTest
:
@ExtendWith(MockitoExtension.class)
public class EmailServiceTest {
private EmailService emailService;
// Test cases will be added here
}
2. Mock the dependencies
Next, we need to mock any dependencies or external systems the EmailService
interacts with, in this case, an EmailClient
:
Mock the EmailClient
in the test class:
@ExtendWith(MockitoExtension.class)
class EmailServiceTest {
@Mock
private EmailClient emailClient;
@InjectMocks
private EmailService emailService;
}
3. Test the void method
Now, we can create a test case for the sendEmail
method. We will use Mockito's verify()
method to ensure that the send()
method of the EmailClient
is called with the correct parameters.
@ExtendWith(MockitoExtension.class)
class EmailServiceTest {
@Mock
private EmailClient emailClient;
@InjectMocks
private EmailService emailService;
@Test
void sendEmail() {
// given
String recipient = "test@example.com";
String subject = "Test Subject";
String content = "Test Content";
// when
emailService.sendEmail(recipient, subject, content);
// then
verify(emailClient, times(1)).send(recipient, subject, content);
}
}
Testing Exceptions with assertThrows()
In some cases, (void) methods may throw exceptions when specific conditions are met. It is essential to test these scenarios to ensure the application's resilience and robustness. In this section, we will cover how to test (void) methods that throw exceptions using JUnit 5's assertThrows()
or AssertJ's assertThatThrownBy()
functions.
Continuing with our EmailService
example, let's assume that the sendEmail
method throws an InvalidEmailException
when the recipient's email address is invalid.
@Service
public class EmailService {
public void sendEmail(String recipient, String subject, String content) throws InvalidEmailException {
// Implementation for sending an email
}
}
To test the scenario where the method throws an InvalidEmailException
, we will follow these steps:
1. Update the EmailClient
interface to declare the exception:
public interface EmailClient {
void send(String recipient, String subject, String content) throws InvalidEmailException;
}
2. Configure the mock to throw the exception:
In the test class, use Mockito's doThrow()
method to configure the emailClient
mock to throw an InvalidEmailException
when the send()
method is called with an invalid email address.
@Test
public void testSendEmailThrowsInvalidEmailException() {
String invalidRecipient = "invalid-email";
String subject = "Test Subject";
String content = "Test Content";
doThrow(InvalidEmailException.class).when(emailClient).send(eq(invalidRecipient), anyString(), anyString());
// Use JUnit's assertThrows() to test the exception
assertThrows(InvalidEmailException.class, () -> emailService.sendEmail(invalidRecipient, subject, content));
}
In this test case, we use JUnit 5's assertThrows()
method to test that the InvalidEmailException
is thrown when calling the sendEmail
method with the invalid email address.
If you prefer to test with AssertJ, here is an example that makes use of AssertJ's assertThatThrownBy()
function to essentially test the same thing as we did above:
@Test
public void testSendEmailThrowsInvalidEmailException() {
String invalidRecipient = "invalid-email";
String subject = "Test Subject";
String content = "Test Content";
doThrow(InvalidEmailException.class).when(emailClient).send(eq(invalidRecipient), anyString(), anyString());
// Use AssertJ's assertThatThrownBy() to test the exception
assertThatThrownBy(() -> emailService.sendEmail(invalidRecipient, subject, content))
.isInstanceOf(InvalidEmailException.class);
}
Back in JUnit 4, there was another approach to testing exception throwing of methods, namely
@Test(expected = InvalidEmailException.class)
public void testSendEmailThrowsInvalidEmailException() throws InvalidEmailException {
// Some test that will throw the exception
}
Both assertThrows()
from JUnit 5 and assertThatThrownBy()
from AssertJ are more modern and straightforward compared to the expected
attribute of the @Test
annotation from JUnit 4 for the following reasons:
Explicitness: Both
assertThrows()
andassertThatThrownBy()
make it clear which part of the code is expected to throw the exception. In contrast, theexpected
attribute of the @Test annotation only specifies that the test method is expected to throw an exception, but it doesn't pinpoint the exact line of code where the exception is expected.Exception capturing: Both
assertThrows()
andassertThatThrownBy()
allow you to capture the thrown exception, enabling you to perform additional assertions on the exception itself, such as checking the exception message, cause, or other properties. In comparison, the expected attribute doesn't provide a way to capture the exception for further assertions.Readability: Both
assertThrows()
andassertThatThrownBy()
improve the readability of the test code by making the exception testing more explicit and more focused on the specific code block that is expected to throw an exception. This enhances the test's clarity and makes it easier for developers to understand the test's purpose.
Conclusion
Testing void methods' general behavior and testing methods that throw exceptions is crucial for ensuring the reliability and robustness of an application. By using Mockito to configure mocks and AssertJ's or JUnit5's assert methods to verify exceptions, we can create comprehensive test cases that cover various scenarios, including those involving exceptions. As a result, we can develop more reliable and resilient code.
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