Test Quality Skill (JUnit 5 + AssertJ)

Write high-quality, maintainable tests for Java projects using modern best practices.

When to Use

• Writing new test classes
• Reviewing/improving existing tests
• User asks to "add tests" / "improve test coverage"
• Code review mentions missing tests

Framework Preferences

JUnit 5 (Jupiter)

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Nested;
import static org.assertj.core.api.Assertions.*;

AssertJ over standard assertions

✅ Use AssertJ:

assertThat(plugin.getState())
    .as("Plugin should be started after initialization")
    .isEqualTo(PluginState.STARTED);

assertThat(plugins)
    .hasSize(3)
    .extracting(Plugin::getId)
    .containsExactly("plugin1", "plugin2", "plugin3");

❌ Avoid JUnit assertions:

assertEquals(PluginState.STARTED, plugin.getState()); // Less readable
assertTrue(plugins.size() == 3); // Less descriptive failures

Test Structure (AAA Pattern)

Always use Arrange-Act-Assert pattern:

@Test
@DisplayName("Should load plugin from valid directory")
void shouldLoadPluginFromValidDirectory() {
    // Arrange - Setup test data and dependencies
    Path pluginDir = Paths.get("test-plugins/valid-plugin");
    PluginLoader loader = new DefaultPluginLoader();
    
    // Act - Execute the behavior being tested
    Plugin plugin = loader.load(pluginDir);
    
    // Assert - Verify results
    assertThat(plugin)
        .isNotNull()
        .extracting(Plugin::getId, Plugin::getVersion)
        .containsExactly("test-plugin", "1.0.0");
}

Naming Conventions

Test class names

// Class under test: PluginManager
PluginManagerTest           // ✅ Simple, standard
PluginManagerShould         // ✅ BDD style (if team prefers)
TestPluginManager           // ❌ Avoid

Test method names

Option 1: should_expectedBehavior_when_condition (descriptive)

@Test
void should_throwException_when_pluginDirectoryNotFound() { }

@Test  
void should_returnEmptyList_when_noPluginsAvailable() { }

@Test
void should_loadPluginsInDependencyOrder_when_multipleDependencies() { }

Option 2: Natural language with @DisplayName (cleaner code)

@Test
@DisplayName("Should load all plugins from directory")
void loadAllPlugins() { }

@Test
@DisplayName("Should throw exception when plugin descriptor is invalid")
void invalidPluginDescriptor() { }

AssertJ Power Features

Collection assertions

// Basic collection checks
assertThat(plugins)
    .isNotEmpty()
    .hasSize(2)
    .doesNotContainNull();

// Advanced filtering and extraction
assertThat(plugins)
    .filteredOn(p -> p.getState() == PluginState.STARTED)
    .extracting(Plugin::getId)
    .containsExactlyInAnyOrder("plugin-a", "plugin-b");

// All elements match condition
assertThat(plugins)
    .allMatch(p -> p.getVersion() != null, "All plugins have version");

Exception assertions

// Basic exception check
assertThatThrownBy(() -> loader.load(invalidPath))
    .isInstanceOf(PluginException.class)
    .hasMessageContaining("Invalid plugin descriptor");

// Detailed exception verification
assertThatThrownBy(() -> manager.startPlugin("missing-plugin"))
    .isInstanceOf(PluginException.class)
    .hasMessageContaining("Plugin not found")
    .hasCauseInstanceOf(IllegalArgumentException.class);

// With assertThatExceptionOfType (more readable)
assertThatExceptionOfType(PluginException.class)
    .isThrownBy(() -> loader.load(invalidPath))
    .withMessageContaining("Invalid")
    .withMessageMatching("Invalid .* descriptor");

Object assertions

// Extract and verify multiple properties
assertThat(plugin)
    .isNotNull()
    .extracting("id", "version", "state")
    .containsExactly("my-plugin", "1.0", PluginState.STARTED);

// Using method references (type-safe)
assertThat(plugin)
    .extracting(Plugin::getId, Plugin::getVersion, Plugin::getState)
    .containsExactly("my-plugin", "1.0", PluginState.STARTED);

// Field by field comparison
assertThat(actualPlugin)
    .usingRecursiveComparison()
    .isEqualTo(expectedPlugin);

Soft assertions (multiple checks)

@Test
void shouldHaveValidPluginDescriptor() {
    SoftAssertions softly = new SoftAssertions();
    
    softly.assertThat(descriptor.getId())
        .as("Plugin ID")
        .isNotBlank()
        .matches("[a-z0-9-]+");
    
    softly.assertThat(descriptor.getVersion())
        .as("Plugin version")
        .matches("\\d+\\.\\d+\\.\\d+");
    
    softly.assertThat(descriptor.getDependencies())
        .as("Dependencies")
        .isNotNull()
        .doesNotContainNull();
    
    softly.assertAll(); // All assertions evaluated, even if some fail
}

String assertions

assertThat(errorMessage)
    .startsWith("Error:")
    .contains("plugin", "failed")
    .doesNotContain("success")
    .matches("Error: .* failed")
    .hasLineCount(3);

Test Organization

Nested tests for clarity

@DisplayName("PluginManager")
class PluginManagerTest {
    
    private PluginManager manager;
    
    @BeforeEach
    void setUp() {
        manager = new DefaultPluginManager();
    }
    
    @Nested
    @DisplayName("when starting plugins")
    class WhenStartingPlugins {
        
        @Test
        @DisplayName("should start all plugins in dependency order")
        void shouldStartInDependencyOrder() {
            // Test implementation
        }
        
        @Test
        @DisplayName("should skip disabled plugins")
        void shouldSkipDisabledPlugins() {
            // Test implementation
        }
        
        @Test
        @DisplayName("should fail if circular dependency detected")
        void shouldFailOnCircularDependency() {
            // Test implementation
        }
    }
    
    @Nested
    @DisplayName("when stopping plugins")  
    class WhenStoppingPlugins {
        
        @Test
        @DisplayName("should stop plugins in reverse dependency order")
        void shouldStopInReverseOrder() {
            // Test implementation
        }
    }
}

Parameterized tests

@ParameterizedTest
@ValueSource(strings = {"1.0.0", "2.1.3", "10.0.0-SNAPSHOT"})
@DisplayName("Should accept valid semantic versions")
void shouldAcceptValidVersions(String version) {
    assertThat(VersionParser.parse(version))
        .isNotNull()
        .hasFieldOrPropertyWithValue("valid", true);
}

@ParameterizedTest
@CsvSource({
    "plugin-a, 1.0, STARTED",
    "plugin-b, 2.0, STOPPED",
    "plugin-c, 1.5, DISABLED"
})
@DisplayName("Should load plugin with expected state")
void shouldLoadPluginWithState(String id, String version, PluginState expectedState) {
    Plugin plugin = createPlugin(id, version);
    
    assertThat(plugin.getState()).isEqualTo(expectedState);
}

@ParameterizedTest
@MethodSource("invalidPluginDescriptors")
@DisplayName("Should reject invalid plugin descriptors")
void shouldRejectInvalidDescriptors(PluginDescriptor descriptor, String expectedError) {
    assertThatThrownBy(() -> validator.validate(descriptor))
        .hasMessageContaining(expectedError);
}

static Stream<Arguments> invalidPluginDescriptors() {
    return Stream.of(
        Arguments.of(descriptorWithoutId(), "Missing plugin ID"),
        Arguments.of(descriptorWithInvalidVersion(), "Invalid version format"),
        Arguments.of(descriptorWithEmptyId(), "Plugin ID cannot be empty")
    );
}

Common Patterns

Testing with mocks (Mockito)

@ExtendWith(MockitoExtension.class)
class PluginManagerTest {
    
    @Mock
    private PluginRepository repository;
    
    @Mock
    private PluginValidator validator;
    
    @InjectMocks
    private DefaultPluginManager manager;
    
    @Test
    @DisplayName("Should load plugins from repository")
    void shouldLoadPluginsFromRepository() {
        // Given
        List<PluginDescriptor> descriptors = List.of(
            createDescriptor("plugin1"),
            createDescriptor("plugin2")
        );
        when(repository.findAll()).thenReturn(descriptors);
        
        // When
        List<Plugin> plugins = manager.loadAll();
        
        // Then
        assertThat(plugins).hasSize(2);
        verify(validator, times(2)).validate(any(PluginDescriptor.class));
    }
}

Test fixtures with @BeforeEach

@BeforeEach
void setUp() throws IOException {
    // Create temporary directory for test plugins
    pluginDir = Files.createTempDirectory("test-plugins");
    
    // Initialize plugin manager with test config
    PluginConfig config = PluginConfig.builder()
        .pluginDirectory(pluginDir)
        .enableValidation(true)
        .build();
    
    pluginManager = new DefaultPluginManager(config);
}

@AfterEach
void tearDown() throws IOException {
    // Clean up test resources
    if (pluginManager != null) {
        pluginManager.stopAll();
    }
    if (pluginDir != null) {
        FileUtils.deleteDirectory(pluginDir.toFile());
    }
}

Testing async operations

@Test
@DisplayName("Should complete async plugin loading")
void shouldCompleteAsyncLoading() {
    CompletableFuture<Plugin> future = manager.loadAsync(pluginPath);
    
    assertThat(future)
        .succeedsWithin(Duration.ofSeconds(5))
        .satisfies(plugin -> {
            assertThat(plugin.getState()).isEqualTo(PluginState.STARTED);
            assertThat(plugin.getId()).isNotBlank();
        });
}

Token Optimization

When writing tests:

1. Generate test skeleton first

// Phase 1: List test cases as comments
// @Test void shouldLoadPlugin() { }
// @Test void shouldThrowExceptionForInvalidPlugin() { }
// @Test void shouldHandleMissingDependencies() { }

2. Implement incrementally

• One test at a time
• Verify compilation after each
• Run tests to validate
• Refactor if needed

3. Reuse patterns

// Extract common setup to helper methods
private Plugin createTestPlugin(String id, String version) {
    return Plugin.builder()
        .id(id)
        .version(version)
        .build();
}

Code Coverage Guidelines

• Aim for: 80%+ line coverage on core logic
• Focus on: Business logic, complex algorithms, edge cases
• Skip: Trivial getters/setters, POJOs, generated code
• Test: Happy paths + error conditions + boundary cases

What to test

✅ High priority:

• Public APIs
• Complex business logic
• Error handling
• Edge cases and boundaries
• Integration points

❌ Low priority:

// Simple getters/setters
public String getId() { return id; }
public void setId(String id) { this.id = id; }

// Simple POJOs with no logic
public class PluginInfo {
    private String id;
    private String version;
    // ... only getters/setters
}

Anti-patterns

❌ Avoid:

// 1. Generic test names
@Test void test1() { }
@Test void testPlugin() { }

// 2. Testing implementation details
assertThat(plugin.internalState.flag).isTrue(); // Couples to internals

// 3. Brittle assertions with timestamps
assertThat(message).isEqualTo("Error at 2024-01-26 10:30:15");

// 4. Multiple unrelated assertions
@Test void testEverything() {
    // 50 unrelated assertions
    assertThat(plugin.getId()).isNotNull();
    assertThat(manager.getCount()).isEqualTo(5);
    assertThat(config.isEnabled()).isTrue();
    // ... mixing multiple concerns
}

// 5. Ignoring exceptions
@Test void shouldFail() {
    try {
        loader.load(invalidPath);
        fail("Should have thrown exception");
    } catch (Exception e) {
        // Swallowing exception details
    }
}

✅ Prefer:

@Test
@DisplayName("Should reject plugin with missing dependencies")
void shouldRejectPluginWithMissingDependencies() {
    PluginDescriptor descriptor = PluginDescriptor.builder()
        .id("test-plugin")
        .dependencies(List.of("missing-dep"))
        .build();
    
    assertThatThrownBy(() -> manager.load(descriptor))
        .isInstanceOf(PluginException.class)
        .hasMessageContaining("Missing dependencies: missing-dep");
}

Integration with Coverage Tools

Maven configuration

<plugin>
    <groupId>org.jacoco</groupId>
    <artifactId>jacoco-maven-plugin</artifactId>
    <version>0.8.14</version> <!-- 0.8.14+ needed for Java 25 bytecode -->
    <executions>
        <execution>
            <goals>
                <goal>prepare-agent</goal>
            </goals>
        </execution>
        <execution>
            <id>report</id>
            <phase>test</phase>
            <goals>
                <goal>report</goal>
            </goals>
        </execution>
    </executions>
</plugin>

After test generation, suggest:

# Run tests with coverage
mvn clean test jacoco:report

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

# Check coverage threshold
mvn verify # Fails if below configured threshold

Quick Reference

// ===== Basic Assertions =====
assertThat(value).isEqualTo(expected);
assertThat(value).isNotNull();
assertThat(value).isInstanceOf(String.class);
assertThat(number).isPositive().isGreaterThan(5);

// ===== Collections =====
assertThat(list).hasSize(3);
assertThat(list).contains(item);
assertThat(list).containsExactly(item1, item2, item3);
assertThat(list).containsExactlyInAnyOrder(item2, item1, item3);
assertThat(list).doesNotContain(item);
assertThat(list).allMatch(predicate);

// ===== Strings =====
assertThat(str).isNotBlank();
assertThat(str).startsWith("prefix");
assertThat(str).endsWith("suffix");
assertThat(str).contains("substring");
assertThat(str).matches("regex\\d+");

// ===== Exceptions =====
assertThatThrownBy(() -> code())
    .isInstanceOf(PluginException.class)
    .hasMessageContaining("error");

assertThatNoException().isThrownBy(() -> code());

// ===== Custom Descriptions =====
assertThat(userId)
    .as("User ID should be positive")
    .isPositive();

// ===== Object Comparison =====
assertThat(actual)
    .usingRecursiveComparison()
    .ignoringFields("timestamp", "id")
    .isEqualTo(expected);

Best Practices Summary

1. Use AssertJ for all assertions
2. Follow AAA pattern (Arrange-Act-Assert)
3. Descriptive names with @DisplayName
4. One concept per test
5. Test behavior, not implementation
6. Extract helpers for common setup
7. Use @Nested for logical grouping
8. Parameterize similar tests
9. Soft assertions for multiple checks
10. Coverage on business logic, not boilerplate

References

• AssertJ Documentation
• JUnit 5 User Guide