Spring Boot 4 Complete Guide · Lesson

Slice Testing & TestContainers

Utilize slice tests (`@WebMvcTest`, `@DataJpaTest`) for specific layers and integrate Testcontainers for external services.

Slice Testing & TestContainers is a free Spring Boot 4 Complete Guide lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Spring Boot 4 Complete Guide learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Slice Testing: Focused Tests

Welcome to Lesson 3! In previous lessons, we learned about unit and full integration tests. Now, let's explore slice tests.

Slice tests focus on testing a specific 'slice' or layer of your application, like the web layer or the data layer, in isolation. This makes them:

Faster: They load only necessary parts of the Spring context.
More targeted: You test specific component interactions without the overhead of the entire application.

Testing Web Layers with @WebMvcTest

When you want to test your Spring MVC controllers and their interactions with HTTP requests, @WebMvcTest is your go-to annotation.

It auto-configures Spring MVC components (like @Controller, @RestController, @ControllerAdvice) but excludes other parts of your application, such as services and repositories. This ensures your test focuses solely on the web layer.

It uses MockMvc to simulate HTTP requests and responses without starting a full HTTP server.

Your First @WebMvcTest

Here's an example of a simple controller and how to test it using @WebMvcTest and MockMvc. Notice how we define the controller class in the annotation.

This test simulates a GET request to /hello and asserts the HTTP status and response content. Remember, this code runs within a JUnit test environment, not as a standalone main method.

package com.coddykit.controller;

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class HelloController {
    @GetMapping("/hello")
    public String sayHello() {
        return "Hello CoddyKit!";
    }
}

// --- Test Class ---
package com.coddykit;

import com.coddykit.controller.HelloController;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.web.servlet.WebMvcTest;
import org.springframework.test.web.servlet.MockMvc;

import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.content;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;

@WebMvcTest(HelloController.class)
public class HelloControllerTest {

    @Autowired
    private MockMvc mockMvc;

    @Test
    void helloEndpointReturnsGreeting() throws Exception {
        mockMvc.perform(get("/hello"))
                .andExpect(status().isOk())
                .andExpect(content().string("Hello CoddyKit!"));
    }
}

Mocking Services in Web Tests

What if your controller depends on a service? Since @WebMvcTest doesn't load service beans, you'll need to provide a mock. This is where @MockBean comes in handy.

@MockBean replaces an existing bean in the application context with a Mockito mock. This lets you define the behavior of the service without needing its actual implementation, keeping your test isolated to the controller logic.

package com.coddykit.service;

import org.springframework.stereotype.Service;

@Service
public class GreetingService {
    public String getGreeting() {
        return "Hello from Service!";
    }
}

package com.coddykit.controller;

import com.coddykit.service.GreetingService;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class GreetingController {
    private final GreetingService greetingService;

    public GreetingController(GreetingService greetingService) {
        this.greetingService = greetingService;
    }

    @GetMapping("/greeting")
    public String getCustomGreeting() {
        return greetingService.getGreeting();
    }
}

// --- Test Class ---
package com.coddykit;

import com.coddykit.controller.GreetingController;
import com.coddykit.service.GreetingService;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.web.servlet.WebMvcTest;
import org.springframework.boot.test.mock.mockito.MockBean;
import org.springframework.test.web.servlet.MockMvc;

import static org.mockito.Mockito.when;
import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.content;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;

@WebMvcTest(GreetingController.class)
public class GreetingControllerTest {

    @Autowired
    private MockMvc mockMvc;

    @MockBean
    private GreetingService greetingService;

    @Test
    void greetingEndpointReturnsMockedGreeting() throws Exception {
        when(greetingService.getGreeting()).thenReturn("Mocked Greeting!");

        mockMvc.perform(get("/greeting"))
                .andExpect(status().isOk())
                .andExpect(content().string("Mocked Greeting!"));
    }
}

Testing Data Layers with @DataJpaTest

Moving to the data layer, @DataJpaTest is designed for testing JPA repositories. It's ideal for verifying that your entities are correctly mapped and your repository methods work as expected.

This annotation:

Configures an in-memory embedded database (like H2) by default.
Scans for @Entity classes and Spring Data JPA repositories.
Provides a TestEntityManager for interacting with the database directly in tests.

Your First @DataJpaTest

Let's create a simple Product entity and its repository, then write a @DataJpaTest to ensure data can be saved and retrieved. The TestEntityManager helps set up test data.

Each @DataJpaTest runs in a transaction and rolls back at the end, ensuring a clean state for every test.

package com.coddykit.entity;

import jakarta.persistence.Entity;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.GenerationType;
import jakarta.persistence.Id;

@Entity
public class Product {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
    private double price;

    public Product() {}
    public Product(String name, double price) {
        this.name = name;
        this.price = price;
    }

    public Long getId() { return id; }
    public void setId(Long id) { this.id = id; }
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    public double getPrice() { return price; }
    public void setPrice(double price) { this.price = price; }
}

package com.coddykit.repository;

import com.coddykit.entity.Product;
import org.springframework.data.jpa.repository.JpaRepository;

public interface ProductRepository extends JpaRepository<Product, Long> {
    Product findByName(String name);
}

// --- Test Class ---
package com.coddykit;

import com.coddykit.entity.Product;
import com.coddykit.repository.ProductRepository;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.orm.jpa.DataJpaTest;
import org.springframework.boot.test.autoconfigure.orm.jpa.TestEntityManager;

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

@DataJpaTest
public class ProductRepositoryTest {

    @Autowired
    private TestEntityManager entityManager;

    @Autowired
    private ProductRepository productRepository;

    @Test
    void productCanBeSavedAndFound() {
        Product newProduct = new Product("Laptop", 1200.00);
        entityManager.persist(newProduct);
        entityManager.flush();

        Product foundProduct = productRepository.findByName("Laptop");

        assertThat(foundProduct).isNotNull();
        assertThat(foundProduct.getName()).isEqualTo("Laptop");
        assertThat(foundProduct.getPrice()).isEqualTo(1200.00);
    }
}

Testcontainers: Real External Services

While in-memory databases are great for speed, they don't always mimic production environments perfectly. This is where Testcontainers shine!

Testcontainers is a Java library that provides lightweight, throwaway instances of databases, message brokers, web browsers, or anything else that can run in a Docker container. You get a real service for your tests, ensuring higher fidelity to your production setup.

No more mock databases that behave differently.
Consistent test environments across machines.
Supports a wide range of services.

Setting Up Your First Container

To use Testcontainers, you need Docker running on your machine. Here's a basic example of how you might start a Redis container within a JUnit test using GenericContainer. This demonstrates the lifecycle of a container for testing.

In a real test, you'd integrate a client (e.g., Spring Data Redis) to interact with this running Redis instance.

package com.coddykit;

import org.junit.jupiter.api.Test;
import org.testcontainers.containers.GenericContainer;
import org.testcontainers.utility.DockerImageName;

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

public class RedisContainerTest {

    private static GenericContainer<?> redis = new GenericContainer<>(DockerImageName.parse("redis:5.0.3-alpine"))
            .withExposedPorts(6379);

    @Test
    void redisContainerStartsAndIsAccessible() {
        redis.start();
        assertThat(redis.isRunning()).isTrue();
        System.out.println("Redis container is running on port: " + redis.getFirstMappedPort());
        redis.stop();
    }
}

DataJpaTest with Testcontainers

Combining @DataJpaTest with Testcontainers allows you to test your repositories against a real database, like PostgreSQL, instead of an in-memory one. This provides the best of both worlds: focused data layer testing with a realistic database environment.

We use @Testcontainers to enable Testcontainers integration and @Container to define our database container. @DynamicPropertySource dynamically sets Spring's datasource properties to connect to the running container.

package com.coddykit;

import com.coddykit.entity.Product;
import com.coddykit.repository.ProductRepository;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.jdbc.AutoConfigureTestDatabase;
import org.springframework.boot.test.autoconfigure.orm.jpa.DataJpaTest;
import org.springframework.boot.test.autoconfigure.orm.jpa.TestEntityManager;
import org.springframework.test.context.DynamicPropertyRegistry;
import org.springframework.test.context.DynamicPropertySource;
import org.testcontainers.containers.PostgreSQLContainer;
import org.testcontainers.junit.jupiter.Container;
import org.testcontainers.junit.jupiter.Testcontainers;

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

@DataJpaTest
@Testcontainers
@AutoConfigureTestDatabase(replace = AutoConfigureTestDatabase.Replace.NONE)
public class ProductRepositoryPostgresTest {

    @Container
    static PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:13");

    @DynamicPropertySource
    static void configureProperties(DynamicPropertyRegistry registry) {
        registry.add("spring.datasource.url", postgres::getJdbcUrl);
        registry.add("spring.datasource.username", postgres::getUsername);
        registry.add("spring.datasource.password", postgres::getPassword);
    }

    @Autowired
    private TestEntityManager entityManager;

    @Autowired
    private ProductRepository productRepository;

    @Test
    void productCanBeSavedAndFoundInRealDb() {
        Product newProduct = new Product("Monitor", 300.00);
        entityManager.persist(newProduct);
        entityManager.flush();

        Product foundProduct = productRepository.findByName("Monitor");

        assertThat(foundProduct).isNotNull();
        assertThat(foundProduct.getName()).isEqualTo("Monitor");
    }
}

Check Your Understanding

Time to test what you've learned about focused testing strategies.

Slice Testing & Testcontainers Recap

You've successfully explored advanced testing techniques! Here's a quick recap:

Slice tests (like @WebMvcTest and @DataJpaTest) allow you to test specific layers of your application in isolation, making tests faster and more focused.
@WebMvcTest is for the web layer, using MockMvc to simulate requests.
@DataJpaTest is for the data layer, often with an in-memory database.
Testcontainers provide a powerful way to use real external services (like PostgreSQL or Redis) in Docker containers for your tests, ensuring high fidelity to production environments.

These tools are invaluable for building robust and reliable Spring Boot applications!

Frequently asked questions

Is the “Slice Testing & TestContainers” lesson free?

Yes — the full text of “Slice Testing & TestContainers” is free to read here on the web, and the Spring Boot 4 Complete Guide course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Spring Boot 4 Complete Guide course, upgrade to CoddyKit PRO.

What will I learn in “Slice Testing & TestContainers”?

Utilize slice tests (`@WebMvcTest`, `@DataJpaTest`) for specific layers and integrate Testcontainers for external services. You practise Spring Boot 4 Complete Guide with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Spring Boot 4 Complete Guide?

No prior experience is required. Spring Boot 4 Complete Guide on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Slice Testing & TestContainers” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Spring Boot 4 Complete Guide lesson?

Yes. Every Spring Boot 4 Complete Guide lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

← Back to Spring Boot 4 Complete Guide