JUNIT 5: ArgumentCaptor with Mockito

Unit testing isn’t just about confirming your code works; it’s about ensuring it excels. Mockito, a trusted mock framework, introduces the ArgumentCaptor, a potent tool for honing your unit tests. This feature empowers you to capture and assert method arguments, leading to highly targeted tests.

In this article, we’ll delve into the realm of ArgumentCaptor within the JUnit 5 framework. And provide some code examples of a Java Class with it’s Java unit test Class

Putting ArgumentCaptor to Work: A Real-world Scenario

Imagine you’re testing a search service that interacts with a search engine API. Your goal is to ensure the service accurately constructs and sends search queries to the API. This is where ArgumentCaptor proves invaluable. It lets you capture the search terms and other parameters, enabling you to assert their correctness with surgical precision.

Let’s dive into a hands-on example to demonstrate how ArgumentCaptor shines in a practical context. For our illustration, we’ll focus on a SearchService that interacts with a search engine to fetch relevant results based on user input.

SearchService.java

1
2
3
4
5
6
7
8
9
10
11

public class SearchService {
private SearchEngine searchEngine;

public SearchService(SearchEngine searchEngine) {
this.searchEngine = searchEngine;
}

public List<String> search(String searchTerm) {
return searchEngine.performSearch(searchTerm);
}
}

Our mission: to write a JUnit 5 test that verifies whether the search method constructs the search query correctly and calls the search engine API with the expected search term. Enter ArgumentCaptor.

SearchServiceTest.java

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28

import org.junit.jupiter.api.Test;
import org.mockito.ArgumentCaptor;

import java.util.List;

import static org.mockito.Mockito.*;

public class SearchServiceTest {

@Test
void testSearchService() {
SearchEngine searchEngine = mock(SearchEngine.class);
SearchService searchService = new SearchService(searchEngine);

ArgumentCaptor<String> searchTermCaptor = ArgumentCaptor.forClass(String.class);

String searchTerm = "AEM JUNIT5 test examples";

when(searchEngine.performSearch(searchTermCaptor.capture())).thenReturn(List.of("result1", "result2"));

List<String> searchResults = searchService.search(searchTerm);

verify(searchEngine).performSearch(searchTermCaptor.capture());

assertEquals(searchTerm, searchTermCaptor.getValue());
assertEquals(2, searchResults.size());
}
}

In this example, we utilize ArgumentCaptor to capture the search term passed to the performSearch method of the SearchEngine. This allows us to assert that the correct search term is used when interacting with the API.

Benefits and Best Practices

Focused Testing: ArgumentCaptor lets you zoom in on specific method arguments, enhancing the precision of your tests.
Readable Assertions: Your test assertions become highly readable and directly tied to real-world scenarios, improving code comprehension.
Use Cases: While particularly useful for testing interactions with external systems like APIs, remember to employ ArgumentCaptor judiciously, focusing on crucial scenarios.

Wrapping Up

Incorporating the ArgumentCaptor into your testing toolbox elevates your unit tests from good to exceptional. By capturing and asserting method arguments, you’re crafting tests that pinpoint potential issues. As you integrate this feature into your testing routine, you’ll discover the power of precision-driven tests that ensure your code operates flawlessly, especially in scenarios involving external APIs and intricate parameters. Embrace ArgumentCaptor, and watch your unit tests shine with accuracy and insight.

JUNIT 5: ArgumentCaptor with Mockito