junit - SKILL.md Agent Skill

name: junit description: JUnit Java testing framework. Use for Java testing.

JUnit 5

JUnit is the programmer-friendly testing framework for Java and the JVM. JUnit 5 is the current major version, composed of the JUnit Platform, JUnit Jupiter, and JUnit Vintage.

When to Use

Java Projects: The standard. Spring Boot starts with spring-boot-starter-test which includes JUnit 5.
Unit & Integration: From testing a single method to spinning up a database context (with Testcontainers).

Quick Start (JUnit 5 / Jupiter)

import static org.junit.jupiter.api.Assertions.assertEquals;
import org.junit.jupiter.api.Test;

class CalculatorTest {

    @Test
    void addition() {
        assertEquals(2, 1 + 1, "Optional failure message");
    }
}

Core Concepts

Lifecycle Annotations

@BeforeEach / @AfterEach: Run before/after every test method.
@BeforeAll / @AfterAll: Run once per class (must be static).

Parameterized Tests

Running the same test with different inputs.

@ParameterizedTest
@ValueSource(strings = { "racecar", "radar", "able was I ere I saw elba" })
void palindromes(String candidate) {
    assertTrue(StringUtils.isPalindrome(candidate));
}

Assertions

assertThrows, assertTimeout, assertAll (Grouped assertions).

Best Practices (2025)

Do:

Use @DisplayName: Give tests readable sentences as names ("Should return 404 when user not found").
Use AssertJ: JUnit assertions are basic. AssertJ (assertThat(actual).isEqualTo(expected)) is fluent and better.
Testcontainers: For integration tests, use Testcontainers with JUnit 5 annotations to spin up real Docker DBs.

Don't:

Don't use System.out.println: Use a logger or assertions.
Don't use JUnit 4: @Test (org.junit.Test) is old. Use @Test (org.junit.jupiter.api.Test).

References

JUnit 5 User Guide