JUnit 5 @Test Annotation

Table of Contents

1. Overview
2. Maven Configuration
3. Method Under Test
4. Testing the Method
5. Conclusion

1. Overview

In this article, we’ll make a quick review of JUnit’s @Test annotation. This annotation provides a powerful tool for performing unit and regression testing.

2. Maven Configuration

To use the latest version of JUnit 5, we’ll need to add the following Maven dependency:

<dependency>
    <groupId>org.junit.jupiter</groupId>
    <artifactId>junit-jupiter-engine</artifactId>
    <version>5.1.0</version>
    <scope>test</scope>
</dependency>

We use the test scope because we don’t want Maven to include this dependency in our final build.

Since the surefire plugin doesn’t still natively fully support JUnit 5, we’ll also need to add a provider, which tells Maven where to find our tests:

<plugin>
    <artifactId>maven-surefire-plugin</artifactId>
    <version>2.19.1</version>
    <dependencies>
        <dependency>
            <groupId>org.junit.platform</groupId>
            <artifactId>junit-platform-surefire-provider</artifactId>
            <version>1.0.2</version>
        </dependency>
    </dependencies>
</plugin>

In our configuration, we’ll use surefire 2.19.1 because, at the time of writing, version 2.20.x is not compatible with the junit-platform-surefire-provider.

3. Method Under Test

First of all, let’s build a simple method that we’ll use in our test scenarios to showcase the @Test annotation’s capabilities:

public boolean isNumberEven(Integer number) {
    return number % 2 == 0;
}

This method should return true if the argument passed is an even number and false otherwise. Now, let’s check out if it works the way it’s supposed to.

4. Testing the Method

For our example, we want to specifically check two scenarios:

when given an even number, the method should return true
when given an odd number, the method should return false

This means that the implementation code will call our isNumberEven method with different parameters and check that the result is what we expect.

In order for the tests to be recognized as such, we’ll add the @Test annotation. We can have as many of these as we want in a class, but it’s a good practice to put together only the related ones. Notice also that a test must not be private, nor may it return a value —otherwise it’ll just be ignored.

Given these considerations, let’s write our test methods:

@Test
void givenEvenNumber_whenCheckingIsNumberEven_thenTrue() {
    boolean result = bean.isNumberEven(8);

    Assertions.assertTrue(result);
}

@Test
void givenOddNumber_whenCheckingIsNumberEven_thenFalse() {
    boolean result = bean.isNumberEven(3);

    Assertions.assertFalse(result);
}

If we now run a Maven build, the surefire plugin will go through all the annotated methods in the classes placed under src/test/java and execute them, causing the build to fail if any test failures occur.

If you come from JUnit 4, be aware that in this version the annotation doesn’t accept any parameters. To check for a timeout or an exception thrown we would use assertions instead:

@Test
void givenLowerThanTenNumber_whenCheckingIsNumberEven_thenResultUnderTenMillis() {
    Assertions.assertTimeout(Duration.ofMillis(10), () -> bean.isNumberEven(3));
}
	
@Test
void givenNull_whenCheckingIsNumberEven_thenNullPointerException() {
    Assertions.assertThrows(NullPointerException.class, () -> bean.isNumberEven(null));
}