Cursor-rules-java 131-java-testing-unit-testing

Use when you need to review, improve, or write Java unit tests — including migrating from JUnit 4 to JUnit 5, adopting AssertJ for fluent assertions, structuring tests with Given-When-Then, ensuring test independence, applying parameterized tests, mocking dependencies with Mockito, verifying boundary conditions (RIGHT-BICEP, CORRECT, A-TRIP), leveraging JSpecify null-safety annotations, or eliminating testing anti-patterns such as reflection-based tests or shared mutable state. Part of the skills-for-java project

install

source · Clone the upstream repo

git clone https://github.com/jabrena/cursor-rules-java

Claude Code · Install into ~/.claude/skills/

T=$(mktemp -d) && git clone --depth=1 https://github.com/jabrena/cursor-rules-java "$T" && mkdir -p ~/.claude/skills && cp -r "$T/skills/131-java-testing-unit-testing" ~/.claude/skills/jabrena-cursor-rules-java-131-java-testing-unit-testing && rm -rf "$T"

manifest: skills/131-java-testing-unit-testing/SKILL.md

Java Unit testing guidelines

Review and improve Java unit tests using modern JUnit 5, AssertJ, and Mockito best practices.

What is covered in this Skill?

JUnit 5 annotations:

@Test

@BeforeEach

@AfterEach

@DisplayName

@Nested

@ParameterizedTest

AssertJ fluent assertions:
```
assertThat
```
,
```
assertThatThrownBy
```
Given-When-Then test structure, descriptive test naming, single-responsibility tests
Test independence and isolated state
Parameterized tests:
```
@ValueSource
```
/
```
@CsvSource
```
/
```
@MethodSource
```
Mockito dependency mocking:
```
@Mock
```
,
```
@InjectMocks
```
,
```
MockitoExtension
```
Code coverage guidance (JaCoCo), package-private test visibility
Testing anti-patterns: reflection, shared state, hard-coded values, testing implementation details
Error handling:
```
assertThatThrownBy
```
, exception messages
JSpecify null-safety:
```
@NullMarked
```
,
```
@Nullable
```
RIGHT-BICEP coverage principles, A-TRIP test quality, CORRECT boundary condition verification

Scope: The reference is organized by examples (good/bad code patterns) for each core area. Apply recommendations based on applicable examples.

Constraints

Before applying any unit test changes, ensure the project compiles. If compilation fails, stop immediately — do not proceed until resolved. After applying improvements, run full verification.

MANDATORY: Run
```
./mvnw compile
```
or
```
mvn compile
```
before applying any change
SAFETY: If compilation fails, stop immediately and do not proceed — compilation failure is a blocking condition
VERIFY: Run
```
./mvnw clean verify
```
or
```
mvn clean verify
```
after applying improvements
BEFORE APPLYING: Read the reference for detailed examples, good/bad patterns, and constraints

When to use this skill

Review Java code for unit tests
Apply best practices for unit tests in Java code

Reference

For detailed guidance, examples, and constraints, see references/131-java-testing-unit-testing.md.