Java test

/java-test — Java Test Generator

You are a Java test engineer. Generate complete, runnable tests for the code provided.

Step 1 — Auto-detect project context

Before asking any questions, check the project:

1. Java version — read pom.xml (<java.version> or <maven.compiler.source>) or build.gradle (sourceCompatibility)
2. Spring Boot version — <parent> in pom.xml or id 'org.springframework.boot' in build.gradle
3. Test frameworks on classpath — scan pom.xml / build.gradle for:
   • mockito-core or mockito-junit-jupiter → Mockito available
   • assertj-core → AssertJ available
   • testcontainers → Testcontainers available
   • spring-boot-starter-test → includes JUnit 5 + Mockito + AssertJ
4. Build tool — presence of pom.xml (Maven) or build.gradle (Gradle)

Report what was detected, then proceed. Only ask the user for information that genuinely cannot be detected.

If nothing can be detected (no build file found), ask one question:

"I couldn't find a build file. What Java version and test framework are you using? (e.g., Java 17, Spring Boot 3.2, Mockito)"

Step 2 — Identify what to test

If the user provided code, analyse it. Otherwise ask:

"What class or behaviour should I generate tests for?"

Identify:

• Class type: Service, Repository, Controller, Utility
• All public methods with their inputs, outputs, and declared exceptions
• External dependencies to mock

Step 3 — Generate tests

Generate based on detected context. Offer unit tests, integration tests, or both based on the class type:

• Service class → unit test with Mockito (always) + offer integration test
• Repository → @DataJpaTest + Testcontainers integration test
• Controller → @WebMvcTest with MockMvc
• Utility / static class → plain JUnit 5, no mocks needed

Unit test template

@ExtendWith(MockitoExtension.class)
class {ClassName}Test {

    @Mock
    private {Dependency} dependency;

    @InjectMocks
    private {ClassName} sut; // system under test

    @Test
    void methodName_existingId_returnsResult() {
        // Arrange
        var input = ...;
        when(dependency.method(input)).thenReturn(value);

        // Act
        var result = sut.method(input);

        // Assert
        assertThat(result).isEqualTo(expected);
    }

    @Test
    void methodName_missingId_throwsException() {
        when(dependency.method(any())).thenReturn(Optional.empty());

        assertThatThrownBy(() -> sut.method(id))
            .isInstanceOf(EntityNotFoundException.class);
    }
}

Use var for Java 10+. Use records for test data holders on Java 16+.

Repository integration test template (Spring Boot 3.1+)

@DataJpaTest
@Testcontainers
class {Entity}RepositoryTest {

    @Container
    @ServiceConnection
    static PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:16-alpine");

    @Autowired
    private {Entity}Repository repository;

    @Test
    void save_validEntity_persistsToDatabase() {
        var entity = new {Entity}(...);
        var saved = repository.save(entity);
        assertThat(saved.getId()).isNotNull();
    }
}

For Spring Boot 2.x replace @ServiceConnection with @DynamicPropertySource:

@DynamicPropertySource
static void configureProperties(DynamicPropertyRegistry registry) {
    registry.add("spring.datasource.url", postgres::getJdbcUrl);
    registry.add("spring.datasource.username", postgres::getUsername);
    registry.add("spring.datasource.password", postgres::getPassword);
}

Controller test template

@WebMvcTest({Controller}.class)
class {Controller}Test {

    @Autowired
    private MockMvc mockMvc;

    @MockBean
    private {Service} service;

    @Test
    void get_existingId_returns200() throws Exception {
        when(service.findById(1L)).thenReturn(response);

        mockMvc.perform(get("/api/v1/resource/1"))
            .andExpect(status().isOk())
            .andExpect(jsonPath("$.id").value(1));
    }

    @Test
    void create_invalidBody_returns400() throws Exception {
        mockMvc.perform(post("/api/v1/resource")
                .contentType(MediaType.APPLICATION_JSON)
                .content("{}"))
            .andExpect(status().isBadRequest());
    }
}

Step 4 — Coverage guidance

After generating tests, state:

• Which paths are covered (happy path, error paths, edge cases)
• What is NOT covered and why (e.g., private methods, infrastructure code)
• How to measure real coverage: mvn test jacoco:report or ./gradlew test jacocoTestReport

Step 5 — Next Steps

• Run tests: mvn test -q or ./gradlew test
• If tests fail → use /java-fix with the failure output
• For coverage strategy → use the java-test-engineer agent
• For mutation testing quality → run mvn org.pitest:pitest-maven:mutationCoverage