/test-quality

name: test-quality
description: Write high-quality JUnit 5 tests with AssertJ assertions. Use when user says "add tests", "write tests", "improve test coverage", or when reviewing/creating test classes for Java code.
license: MIT

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)
    .hasNoCause(); // or verify cause chain

// 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() {