Java Testing & Quality

Master Java testing — JUnit 5, Mockito, integration testing, and building a comprehensive test strategy for Java applications.

50 min•By Priygop Team•Last updated: Feb 2026

JUnit 5 & Mockito

JUnit 5: @Test, @BeforeEach, @AfterEach — test lifecycle. @DisplayName for readable test names. @Nested for grouped test classes. @ParameterizedTest for data-driven tests
Assertions: assertEquals, assertTrue, assertThrows, assertAll — verify behavior. assertAll groups multiple assertions so all are checked even if one fails
Mockito: @Mock + @InjectMocks — create mock dependencies for unit testing. when(mock.method()).thenReturn(value). verify(mock).method() confirms calls were made
Argument Matchers: any(), eq(), argThat(...) — flexible mock matching. ArgumentCaptor captures passed arguments for detailed verification
Test Slices: @WebMvcTest (controller only), @DataJpaTest (repository only), @SpringBootTest (full context) — test specific layers without loading the entire application
TestContainers: @Container PostgreSQLContainer — spin up real databases in Docker for integration tests. Realistic testing without shared test databases

Try It Yourself: Design Patterns

Try It Yourself: Design PatternsJava

Java Editor

✓ ValidTab = 2 spaces

// Observer Pattern - Event System
import java.util.ArrayList;
import java.util.List;

interface EventListener {
    void onEvent(String event, String data);
}

class EventBus {
    private List<EventListener> listeners = new ArrayList<>();

    public void subscribe(EventListener listener) {
        listeners.add(listener);
    }

    public void publish(String event, String data) {
        System.out.println("Publishing: " + event);
        for (EventListener l : listeners) {
            l.onEvent(event, data);
        }
    }
}

class Logger implements EventListener {
    public void onEvent(String event, String data) {
        System.out.println("[LOG] " + event + ": " + data);
    }
}

class Notifier implements EventListener {
    public void onEvent(String event, String data) {
        System.out.println("[NOTIFY] Alert for " + event);
    }
}

public class Main {
    public static void main(String[] args) {
        EventBus bus = new EventBus();
        bus.subscribe(new Logger());
        bus.subscribe(new Notifier());

        bus.publish("USER_LOGIN", "alice@example.com");
        bus.publish("ORDER_PLACED", "Order #1234");
    }
}

// Observer Pattern - Event System
import java.util.ArrayList;
import java.util.List;

interface EventListener {
    void onEvent(String event, String data);
}

class EventBus {
    private List<EventListener> listeners = new ArrayList<>();

public void subscribe(EventListener listener) {
        listeners.add(listener);
    }

public void publish(String event, String data) {
        System.out.println("Publishing: " + event);
        for (EventListener l : listeners) {
            l.onEvent(event, data);
        }
    }
}

class Logger implements EventListener {
    public void onEvent(String event, String data) {
        System.out.println("[LOG] " + event + ": " + data);
    }
}

class Notifier implements EventListener {
    public void onEvent(String event, String data) {
        System.out.println("[NOTIFY] Alert for " + event);
    }
}

public class Main {
    public static void main(String[] args) {
        EventBus bus = new EventBus();
        bus.subscribe(new Logger());
        bus.subscribe(new Notifier());

bus.publish("USER_LOGIN", "alice@example.com");
        bus.publish("ORDER_PLACED", "Order #1234");
    }
}

Output

Click ▶ Run to see the result

Edit the code on the left, then click Run

Java|45 lines|1159 chars|✓ Valid syntax

UTF-8

Java Testing & Quality

JUnit 5 & Mockito

Try It Yourself: Design Patterns

Quick Quiz — Spring & Modern Java