junit

Q: How do I install junit?

Run npx skills add https://github.com/claude-dev-suite/claude-dev-suite --skill junit in your project. The skills CLI detects the agents you use and copies the skill into the right folder. For Claude Code you can also place the skill folder at ~/.claude/skills/junit/ (personal) or .claude/skills/junit/ (project).

by claude-dev-suiteTesting & QA

JUnit 5 testing framework with Mockito integration. Covers unit tests, mocking, and Spring Boot test integration. USE WHEN: user mentions "junit", "java test", "mockito", asks about "@Test", "@Mock", "@InjectMocks", "unit test java", "spring boot test" DO NOT USE FOR: Integration tests with containers - use testcontainers; REST API tests - use rest-assured; E2E tests - use Selenium; JavaScript/TypeScript - use vitest or jest

Installs: 52
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Security: Not audited

Install

Works with every agent the skills CLI supports.

npx skills add https://github.com/claude-dev-suite/claude-dev-suite --skill junit

01
Install
Run the command in your project. The CLI asks which agents to install for.
02
Describe the task
Ask your agent for something this skill covers. It loads junit automatically when relevant.
03
Review the result
The agent follows the SKILL.md steps, scripts and references bundled with the skill.

SKILL.md

The instructions your agent reads when the skill is triggered.

JUnit 5 - Quick Reference

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: junit for comprehensive documentation.

When NOT to Use This Skill

Integration Tests with Containers - Use testcontainers for Docker-based tests
REST API Testing - Use rest-assured for HTTP/REST testing
E2E Web Testing - Use Selenium or Playwright
JavaScript/TypeScript - Use vitest or jest for JS/TS
Database Integration Tests - Combine with spring-boot-integration skill

Essential Patterns

Basic Test

@Test
void shouldAddNumbers() {
    assertEquals(5, calculator.add(2, 3));
    assertThrows(ArithmeticException.class, () -> calculator.divide(10, 0));
}

Mockito + Service Test

@ExtendWith(MockitoExtension.class)
class UserServiceTest {

    @Mock
    private UserRepository userRepository;

    @InjectMocks
    private UserServiceImpl userService;

    @Test
    void shouldCreateUser() {
        when(userRepository.save(any())).thenReturn(user);

        UserResponse result = userService.create(request);

        assertNotNull(result);
        verify(userRepository, times(1)).save(any());
    }
}

Spring Boot Test

@SpringBootTest
@ActiveProfiles("test")
class UserServiceIntegrationTest {

    @Autowired
    private UserService userService;

    @MockBean
    private UserRepository userRepository;

    @Test
    void contextLoads() {
        assertNotNull(userService);
    }
}

Controller Test

@WebMvcTest(UserController.class)
class UserControllerTest {

    @Autowired
    private MockMvc mockMvc;

    @MockBean
    private UserService userService;

    @Test
    void shouldReturnUsers() throws Exception {
        when(userService.findAll()).thenReturn(users);

        mockMvc.perform(get("/api/v1/users"))
            .andExpect(status().isOk())
            .andExpect(jsonPath("$[0].name").value("John"));
    }
}

Repository Test

@DataJpaTest
class UserRepositoryTest {

    @Autowired
    private TestEntityManager entityManager;

    @Autowired
    private UserRepository userRepository;

    @Test
    void shouldFindByEmail() {
        entityManager.persist(user);
        Optional<User> found = userRepository.findByEmail("john@email.com");
        assertTrue(found.isPresent());
    }
}

Common Annotations

Annotation	Usage
`@Test`	Test method
`@BeforeEach`	Setup before each test
`@Mock`	Creates mock
`@InjectMocks`	Injects mocks
`@SpringBootTest`	Integration test
`@WebMvcTest`	Controller test
`@DataJpaTest`	Repository test

Anti-Patterns

Anti-Pattern	Why It's Bad	Solution
Using @SpringBootTest for unit tests	Extremely slow	Use @ExtendWith(MockitoExtension.class)
Testing private methods	Coupled to implementation	Test through public API
No mock cleanup	Tests affect each other	Use @BeforeEach, Mockito.reset()
Hardcoded test data	Hard to maintain	Use test data builders or factories
Not verifying mock interactions	Silent failures	Use verify() to ensure methods called
Too many assertions per test	Hard to debug	One logical assertion per test
Ignoring @Disabled tests	Technical debt accumulates	Fix or remove disabled tests

Quick Troubleshooting

Problem	Likely Cause	Solution
"NullPointerException in test"	Mock not injected	Check @Mock and @InjectMocks annotations
"Wanted but not invoked"	Method not called or wrong args	Verify method call, check argument matchers
Test takes too long	Using @SpringBootTest unnecessarily	Use Mockito for unit tests
"UnnecessaryStubbingException"	Mock setup but not used	Remove unused when() statements
Flaky test	Shared state or timing	Isolate setup, avoid Thread.sleep
"No tests found"	Wrong naming convention	Use test* prefix or @Test annotation

Reference Documentation

FAQ

What does the junit skill do?

How do I install junit?

Run npx skills add https://github.com/claude-dev-suite/claude-dev-suite --skill junit in your project. The skills CLI detects the agents you use and copies the skill into the right folder. For Claude Code you can also place the skill folder at ~/.claude/skills/junit/ (personal) or .claude/skills/junit/ (project).

Which AI agents can use junit?

Any agent that supports the open Agent Skills format (a folder with a SKILL.md file), including Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, Windsurf, Cline, OpenCode and more.

Is junit safe to install?

No partner security audit is available yet. Skills can run commands and scripts, so read the SKILL.md and any bundled files before installing.

Who maintains junit?

It is published by claude-dev-suite in claude-dev-suite/claude-dev-suite. It has 52 recorded installs.