Java Testing

Core testing patterns for plain Java using JUnit 5, Mockito, AssertJ, and Testcontainers.

Note: This skill covers plain Java (25+) without Spring Boot. For Spring Boot projects, use springboot-tdd which covers MockMvc, @SpringBootTest, and the full Spring test slice annotations.

When to Activate

• Writing tests for plain Java, Jakarta EE, or Quarkus projects
• Setting up JUnit 5 + Mockito + AssertJ test stack
• Writing integration tests with Testcontainers
• Configuring JaCoCo coverage enforcement
• Applying TDD in Java without Spring context
• Diagnosing flaky or order-dependent tests caused by shared mutable state
• Adding parameterized tests to cover multiple input variants without duplicating test methods
• Enforcing 80% line coverage via JaCoCo as a build gate in CI

Dependencies

<!-- pom.xml -->
<dependencies>
  <!-- JUnit 5 -->
  <dependency>
    <groupId>org.junit.jupiter</groupId>
    <artifactId>junit-jupiter</artifactId>
    <version>5.11.0</version>
    <scope>test</scope>
  </dependency>
  <!-- Mockito -->
  <dependency>
    <groupId>org.mockito</groupId>
    <artifactId>mockito-core</artifactId>
    <version>5.12.0</version>
    <scope>test</scope>
  </dependency>
  <!-- AssertJ -->
  <dependency>
    <groupId>org.assertj</groupId>
    <artifactId>assertj-core</artifactId>
    <version>3.26.3</version>
    <scope>test</scope>
  </dependency>
  <!-- Testcontainers -->
  <dependency>
    <groupId>org.testcontainers</groupId>
    <artifactId>junit-jupiter</artifactId>
    <version>1.20.1</version>
    <scope>test</scope>
  </dependency>
</dependencies>

<build>
  <plugins>
    <!-- JaCoCo coverage -->
    <plugin>
      <groupId>org.jacoco</groupId>
      <artifactId>jacoco-maven-plugin</artifactId>
      <version>0.8.12</version>
      <executions>
        <execution>
          <goals><goal>prepare-agent</goal></goals>
        </execution>
        <execution>
          <id>check</id>
          <goals><goal>check</goal></goals>
          <configuration>
            <rules>
              <rule>
                <limits>
                  <limit>
                    <counter>LINE</counter>
                    <value>COVEREDRATIO</value>
                    <minimum>0.80</minimum>
                  </limit>
                </limits>
              </rule>
            </rules>
          </configuration>
        </execution>
      </executions>
    </plugin>
  </plugins>
</build>

JUnit 5 Patterns

import org.junit.jupiter.api.*;
import static org.assertj.core.api.Assertions.*;

class PriceFormatterTest {

    private PriceFormatter formatter;

    @BeforeEach
    void setUp() {
        formatter = new PriceFormatter();
    }

    @Test
    void formatsUsdCorrectly() {
        assertThat(formatter.format(1000, "USD")).isEqualTo("$10.00");
    }

    @Test
    void throwsOnNegativeAmount() {
        assertThatThrownBy(() -> formatter.format(-1, "USD"))
            .isInstanceOf(IllegalArgumentException.class)
            .hasMessageContaining("negative");
    }

    @ParameterizedTest
    @CsvSource({
        "100, USD, $1.00",
        "100, EUR, €1.00",
        "100, GBP, £1.00",
    })
    void formatsMultipleCurrencies(int cents, String currency, String expected) {
        assertThat(formatter.format(cents, currency)).isEqualTo(expected);
    }

    @Nested
    class WhenCurrencyIsUnknown {
        @Test
        void throwsUnsupportedCurrencyException() {
            assertThatThrownBy(() -> formatter.format(100, "XYZ"))
                .isInstanceOf(UnsupportedCurrencyException.class);
        }
    }
}

Mockito Patterns

import org.mockito.*;
import static org.mockito.Mockito.*;

@ExtendWith(MockitoExtension.class)
class OrderServiceTest {

    @Mock
    private OrderRepository repository;

    @Mock
    private EmailService emailService;

    @InjectMocks
    private OrderService orderService;

    @Test
    void createsOrderAndSendsConfirmation() {
        // Arrange
        var request = new CreateOrderRequest("user-123", List.of("item-1"));
        var savedOrder = new Order("order-456", "user-123");
        when(repository.save(any())).thenReturn(savedOrder);

        // Act
        var result = orderService.createOrder(request);

        // Assert
        assertThat(result.id()).isEqualTo("order-456");
        verify(emailService).sendConfirmation("user-123", "order-456");
        verify(repository, times(1)).save(any(Order.class));
    }

    @Test
    void throwsWhenUserNotFound() {
        when(repository.save(any())).thenThrow(new UserNotFoundException("user-123"));

        assertThatThrownBy(() -> orderService.createOrder(new CreateOrderRequest("user-123", List.of())))
            .isInstanceOf(UserNotFoundException.class);

        verifyNoInteractions(emailService);
    }
}

AssertJ Best Practices

// Prefer fluent AssertJ over JUnit assertions
// BAD:
assertEquals("expected", actual);
assertTrue(list.contains("item"));

// GOOD:
assertThat(actual).isEqualTo("expected");
assertThat(list).contains("item");

// For collections
assertThat(users)
    .hasSize(3)
    .extracting(User::name)
    .containsExactlyInAnyOrder("Alice", "Bob", "Charlie");

// For exceptions
assertThatThrownBy(() -> service.findUser("missing"))
    .isInstanceOf(UserNotFoundException.class)
    .hasMessageContaining("missing");

// For optionals
assertThat(optional).isPresent().hasValue("expected");

Integration Tests with Testcontainers

import org.testcontainers.containers.PostgreSQLContainer;
import org.testcontainers.junit.jupiter.Container;
import org.testcontainers.junit.jupiter.Testcontainers;

@Testcontainers
class UserRepositoryIT {

    @Container
    static final PostgreSQLContainer<?> postgres =
        new PostgreSQLContainer<>("postgres:16")
            .withDatabaseName("testdb")
            .withUsername("test")
            .withPassword("test");

    private UserRepository repository;

    @BeforeEach
    void setUp() {
        var dataSource = createDataSource(postgres.getJdbcUrl());
        repository = new JdbcUserRepository(dataSource);
    }

    @Test
    void savesAndRetrievesUser() {
        var user = new User(null, "Alice", "alice@example.com");
        var saved = repository.save(user);

        assertThat(saved.id()).isNotNull();
        var found = repository.findById(saved.id());
        assertThat(found).isPresent().hasValue(saved);
    }
}

Coverage with JaCoCo

# Run tests with coverage
mvn test jacoco:report

# Enforce 80% minimum (fails build if below)
mvn verify

# View report
open target/site/jacoco/index.html

TDD Cycle

1. RED: Write @Test method with assertThat(...) — it fails because class/method doesn't exist yet
2. GREEN: Implement the minimum code (often just return a hardcoded value) to make the test pass
3. REFACTOR: Replace hardcoding with real logic, add edge cases, keep tests green
4. VERIFY: mvn verify — JaCoCo enforces 80%+ coverage

Common Pitfalls

• Don't use @Mock without @ExtendWith(MockitoExtension.class) — mocks won't be injected
• Avoid new in test bodies for slow dependencies — use Mockito or Testcontainers
• Don't assert on toString() — use isEqualTo() with proper .equals()/records
• Use @Nested for grouping — keeps related test cases together without long method names
• verify() after assertThat() — verify side effects after asserting the primary result

Anti-Patterns

Using JUnit assertEquals Instead of AssertJ

Wrong:

import static org.junit.jupiter.api.Assertions.*;

assertEquals("expected", actual);           // swapped arg order is easy to get wrong
assertTrue(users.contains(alice));          // unhelpful failure message
assertNull(result);

Correct:

import static org.assertj.core.api.Assertions.*;

assertThat(actual).isEqualTo("expected");
assertThat(users).contains(alice);
assertThat(result).isNull();

Why: AssertJ produces human-readable failure messages and its fluent API prevents argument-order mistakes that silently reverse expected/actual.

---

Mocking Without @ExtendWith(MockitoExtension.class)

Wrong:

class OrderServiceTest {
    @Mock
    private OrderRepository repository;  // never initialized — always null

    @Test
    void test() {
        orderService.createOrder(...);  // NullPointerException
    }
}

Correct:

@ExtendWith(MockitoExtension.class)
class OrderServiceTest {
    @Mock
    private OrderRepository repository;

    @InjectMocks
    private OrderService orderService;
    // ...
}

Why: Without the extension, @Mock fields are never injected by Mockito and remain null, causing NPEs instead of meaningful test failures.

---

Verifying Interactions Before Asserting the Primary Result

Wrong:

@Test
void createsOrder() {
    var result = orderService.createOrder(request);
    verify(emailService).sendConfirmation(any());  // side effect checked first
    assertThat(result.id()).isNotNull();            // primary result buried after
}

Correct:

@Test
void createsOrder() {
    var result = orderService.createOrder(request);
    assertThat(result.id()).isNotNull();            // primary result first
    verify(emailService).sendConfirmation(any());  // side effects after
}

Why: Asserting the primary result first makes test intent clear; if the main assertion fails, the verify is irrelevant and the failure message is more meaningful.

---

Using @BeforeAll with Instance Fields

Wrong:

class ReportServiceTest {
    private ReportService service;

    @BeforeAll                              // static required, but field is not static
    void setUpAll() {
        service = new ReportService();      // compile error or NPE
    }
}

Correct:

class ReportServiceTest {
    private ReportService service;

    @BeforeEach
    void setUp() {
        service = new ReportService();      // fresh instance per test
    }
}

Why: @BeforeAll requires a static method (or @TestInstance(Lifecycle.PER_CLASS)); use @BeforeEach for instance setup to guarantee test isolation.

---

Asserting on toString() Output

Wrong:

assertThat(user.toString()).contains("Alice");  // breaks on any toString() change

Correct:

assertThat(user.name()).isEqualTo("Alice");     // assert the actual field
// or, for records with proper equals:
assertThat(user).isEqualTo(new User("alice-1", "Alice", "alice@example.com"));

Why: toString() is a debugging aid, not a contract; asserting on it couples tests to formatting details rather than behaviour.