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java-junit

github/awesome-copilot

JUnit 5 best practices and data-driven testing patterns for Java unit tests.

What is java-junit?

This skill provides guidance on writing effective unit tests with JUnit 5, including standard test structure, parameterized/data-driven tests, assertions, mocking, and test organization. Use it when setting up or improving test suites in Java projects.

Write standard unit tests following Arrange-Act-Assert pattern with descriptive naming
Create parameterized tests using @ParameterizedTest with @ValueSource, @MethodSource, @CsvSource, and @CsvFileSource
Organize and isolate tests using @BeforeEach, @AfterEach, @BeforeAll, @AfterAll lifecycle annotations
Mock dependencies with Mockito using @Mock and @InjectMocks for test isolation
Group and categorize tests with @Tag, @Nested, and @TestMethodOrder for better organization
Use fluent assertions with AssertJ and grouped assertions with assertAll for clarity

How to install java-junit

npx skills add https://github.com/github/awesome-copilot --skill java-junit

Prerequisites

Maven or Gradle project with standard src/test/java structure
JUnit 5 dependencies: junit-jupiter-api, junit-jupiter-engine, junit-jupiter-params
Optionally: Mockito for mocking, AssertJ for fluent assertions

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How to use java-junit

1.Add JUnit 5 and parameterized test dependencies to your pom.xml or build.gradle
2.Create test classes with Test suffix in src/test/java following package structure
3.Write test methods with @Test annotation using Arrange-Act-Assert pattern and descriptive names
4.For data-driven tests, use @ParameterizedTest with appropriate source annotation (@ValueSource, @CsvSource, @MethodSource, etc.)
5.Use @BeforeEach/@AfterEach for per-test setup/teardown and @BeforeAll/@AfterAll for class-level setup
6.Apply @Mock and @InjectMocks for dependency injection and Mockito mocking
7.Organize tests with @Tag for categorization and @Nested for grouping related test classes
8.Run tests with mvn test or gradle test

Use cases

Good for

Writing parameterized tests that validate a function across multiple input/output combinations from CSV or method sources
Setting up test fixtures and teardown logic for database or resource-heavy tests
Organizing large test suites into nested classes by feature or component
Mocking external service dependencies to test business logic in isolation
Creating tagged test groups (e.g., 'fast', 'integration') for selective test execution

Who it's for

Java developers writing unit tests
QA engineers automating test coverage
Teams adopting or upgrading to JUnit 5
Projects using Maven or Gradle build tools

java-junit FAQ

What is the difference between @BeforeEach and @BeforeAll?

@BeforeEach runs before each test method (can use instance fields), while @BeforeAll runs once before all tests in the class (must be static). Use @BeforeEach for per-test setup like creating fresh objects, and @BeforeAll for expensive one-time setup like database initialization.

How do I test that an exception is thrown?

Use assertThrows(ExceptionType.class, () -> { code that should throw }). For testing that no exception is thrown, use assertDoesNotThrow.

What is the best way to provide test data for parameterized tests?

Use @ValueSource for simple literals, @CsvSource for inline comma-separated values, @MethodSource for complex objects from a factory method, and @CsvFileSource to load data from a CSV file on the classpath.

Should I use AssertJ or standard JUnit assertions?

Both work; AssertJ provides more fluent, readable syntax (assertThat(...).is...) and better error messages. Standard Assertions are sufficient for simple tests. Choose based on team preference and readability needs.

How do I skip a test temporarily?

Use @Disabled on the test method or class with a reason string, e.g., @Disabled("Waiting for bug fix"). This prevents the test from running while documenting why.

Full instructions (SKILL.md)

Source of truth, from github/awesome-copilot.

name: java-junit description: 'Get best practices for JUnit 5 unit testing, including data-driven tests'

JUnit 5+ Best Practices

Your goal is to help me write effective unit tests with JUnit 5, covering both standard and data-driven testing approaches.

Project Setup

Use a standard Maven or Gradle project structure.
Place test source code in src/test/java.
Include dependencies for junit-jupiter-api, junit-jupiter-engine, and junit-jupiter-params for parameterized tests.
Use build tool commands to run tests: mvn test or gradle test.

Test Structure

Test classes should have a Test suffix, e.g., CalculatorTest for a Calculator class.
Use @Test for test methods.
Follow the Arrange-Act-Assert (AAA) pattern.
Name tests using a descriptive convention, like methodName_should_expectedBehavior_when_scenario.
Use @BeforeEach and @AfterEach for per-test setup and teardown.
Use @BeforeAll and @AfterAll for per-class setup and teardown (must be static methods).
Use @DisplayName to provide a human-readable name for test classes and methods.

Standard Tests

Keep tests focused on a single behavior.
Avoid testing multiple conditions in one test method.
Make tests independent and idempotent (can run in any order).
Avoid test interdependencies.

Data-Driven (Parameterized) Tests

Use @ParameterizedTest to mark a method as a parameterized test.
Use @ValueSource for simple literal values (strings, ints, etc.).
Use @MethodSource to refer to a factory method that provides test arguments as a Stream, Collection, etc.
Use @CsvSource for inline comma-separated values.
Use @CsvFileSource to use a CSV file from the classpath.
Use @EnumSource to use enum constants.

Assertions

Use the static methods from org.junit.jupiter.api.Assertions (e.g., assertEquals, assertTrue, assertNotNull).
For more fluent and readable assertions, consider using a library like AssertJ (assertThat(...).is...).
Use assertThrows or assertDoesNotThrow to test for exceptions.
Group related assertions with assertAll to ensure all assertions are checked before the test fails.
Use descriptive messages in assertions to provide clarity on failure.

Mocking and Isolation

Use a mocking framework like Mockito to create mock objects for dependencies.
Use @Mock and @InjectMocks annotations from Mockito to simplify mock creation and injection.
Use interfaces to facilitate mocking.

Test Organization

Group tests by feature or component using packages.
Use @Tag to categorize tests (e.g., @Tag("fast"), @Tag("integration")).
Use @TestMethodOrder(MethodOrderer.OrderAnnotation.class) and @Order to control test execution order when strictly necessary.
Use @Disabled to temporarily skip a test method or class, providing a reason.
Use @Nested to group tests in a nested inner class for better organization and structure.

Surgical code refactoring to improve maintainability without changing behavior.

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