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Testing Strategy

Comprehensive testing examples covering unit, integration, and architectural testing.

Test Pyramid

        ╱  E2E  ╲          Few — Selenium, Playwright (see guide below)
       ╱─────────╲
      ╱ Integration╲       Some — TestContainers, REST Assured (*IT.java)
     ╱───────────────╲
    ╱   Unit Tests     ╲   Many — JUnit 5, Mockito, AssertJ (*Test.java)
   ╱─────────────────────╲
  ╱  Architecture Tests    ╲  Always — ArchUnit (runs with unit tests)
 ╱───────────────────────────╲

Module Contents

File Framework Type
JUnit5FeaturesTest JUnit 5 + AssertJ Unit
MockitoFeaturesTest Mockito Unit
ArchitectureRulesTest ArchUnit Unit
PostgresContainerIT TestContainers Integration
RestAssuredIT REST Assured + TestContainers Integration

How to Run

# Unit tests only (fast, no Docker needed)
mvn test -pl examples/testing

# Integration tests (requires Docker)
mvn verify -pl examples/testing -P integration-tests

# Run specific test class
mvn test -pl examples/testing -Dtest=JUnit5FeaturesTest

# Run tagged tests
mvn test -pl examples/testing -Dgroups=slow

# With coverage report (target/site/jacoco/index.html)
mvn verify -pl examples/testing

Frameworks

JUnit 5

The standard Java testing framework. Key features demonstrated:

  • @DisplayName — readable test names in reports
  • @Nested — group related tests in inner classes
  • @ParameterizedTest — run same test with different inputs (@ValueSource, @CsvSource, @MethodSource)
  • @Tag — filter tests by category (mvn test -Dgroups=slow)
  • @EnabledOnOs — conditional execution
  • assertAll — grouped assertions that report all failures
  • assertTimeout — fail if operation exceeds duration

AssertJ

Fluent assertion library (preferred over JUnit's built-in assertions):

assertThat(user.name()).isEqualTo("Alice");
assertThat(list).hasSize(3).contains("a", "b");
assertThatThrownBy(() -> riskyCall())
    .isInstanceOf(RuntimeException.class)
    .hasMessageContaining("failed");

Mockito

Mock dependencies to isolate the unit under test:

  • @Mock — create mock instances
  • @InjectMocks — auto-inject mocks into constructor
  • @Captor — capture arguments for inspection
  • when(...).thenReturn(...) — stub return values
  • verify(...) — assert interactions occurred
  • BDD style — given(...).willReturn(...) / then(...).should()
  • spy(...) — partial mock wrapping a real object

TestContainers

Spin up real infrastructure (databases, message brokers, etc.) in Docker:

@Testcontainers
class MyIT {
    @Container
    static PostgreSQLContainer<?> pg = new PostgreSQLContainer<>("postgres:16-alpine");

    @Test
    void test() {
        String url = pg.getJdbcUrl(); // real JDBC URL
    }
}

Available modules: postgresql, mysql, mongodb, kafka, redis, elasticsearch, localstack, and many more.

REST Assured

Fluent HTTP API testing:

given()
    .contentType(ContentType.JSON)
    .body("""{"name":"Alice"}""")
.when()
    .post("/api/users")
.then()
    .statusCode(201)
    .body("name", equalTo("Alice"));

ArchUnit

Enforce architecture rules as tests — no special tooling needed:

@ArchTest
static final ArchRule rule = noClasses()
    .that().resideInAPackage("..repository..")
    .should().dependOnClassesThat().resideInAPackage("..service..");

Selenium (E2E Overview)

Selenium is not included as a dependency (it requires a browser), but here's the pattern:

// pom.xml: org.seleniumhq.selenium:selenium-java:4.18.1
//          io.github.bonigarcia:webdrivermanager:5.7.0

@Test
void loginFlow() {
    WebDriverManager.chromedriver().setup();
    var driver = new ChromeDriver();
    try {
        driver.get("http://localhost:8080/login");
        driver.findElement(By.id("username")).sendKeys("admin");
        driver.findElement(By.id("password")).sendKeys("secret");
        driver.findElement(By.id("submit")).click();

        var welcome = driver.findElement(By.id("welcome"));
        assertThat(welcome.getText()).contains("Welcome");
    } finally {
        driver.quit();
    }
}

For modern projects, consider Playwright (com.microsoft.playwright:playwright:1.42.0) as a faster alternative with auto-wait and better API.

Naming Conventions

Pattern Plugin When
*Test.java Surefire mvn test
*IT.java Failsafe mvn verify -P integration-tests

Best Practices

  1. One assertion concept per test — test one behavior, use descriptive names
  2. Arrange-Act-Assert (or Given-When-Then) — consistent structure
  3. Don't mock what you don't own — wrap third-party APIs, mock the wrapper
  4. Use TestContainers over H2 — test against real databases
  5. Keep unit tests fast — mock I/O, no network, no disk
  6. Integration tests in CI — run with Docker in pipeline, skip locally if needed
  7. Coverage as a guide, not a goal — aim for meaningful coverage, not 100%
  8. Test behavior, not implementation — tests should survive refactoring

Related Documentation