131-java-testing-unit-testing

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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

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SKILL.md Content

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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

Reference

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