Wokrskshop: API Integration testing for Spring Boot apps, with Testcontainers and WireMock

Testcontainers have become one of the most popular tools for software integration testing. If you can put your system under test into a container, Docker compose cluster, or pod, you’re likely to use this. If your target isn’t ready, not containerizable, or just too heavy for testing as is, you can always use mock testing frameworks, e.g., WireMock or MockServer in the JVM ecosystem. Can you combine both approaches? Sure!

In this talk, we will classify integration testing of Spring projects and discuss application areas for WireMock and Testcontainers, and discuss which tool to choose for each use-case. And why not both? We will discuss putting WireMock into Testcontainers, and getting the best from their features thanks to the new WireMock modules for Testcontainers. Last but not least, we will see how to put WireMock in front of the Testcontainers or your real system to do deeper tests or even chaos engineering.

Bonus part from Oleg Šelajev: Let's add some AI? Using Spring AI and SLMs for test purposes

P.S: We will focus on the Java/Kotlin examples and Spring, but the same approaches can be used everywhere

Transcript

1. WORKSHOP: API Integration testing for Spring apps, with Testcontainers and WireMock Oleg Nenashev, Gradle Oleg Šelajev, Docker
2. Async Q&A: Testcontainers and WireMock Slack
3. Demo & Workshop repository • github.com/oleg-nenashev/ spring_io_2025-Testcontainers-workshop • Target app: Tracking the TODO list and listing some news for your morning coffee! NOTE: Trademarked names, project and company logos belong to their respective owners 3
5. We will use Gradle Build Tool for the demo this time, Oleg TL;DR: Two Olegs 5
6. Oleg #1 Dr. Nenashev / Mr. Jenkins Community Builder, Gradle Developer Tools Hacker @oleg_nenashev oleg-nenashev #RussiansAgainstPutin #StandWithUkraine
7. Oleg #2 • • • • Surprisingly, a Java Champion 18+ years software development experience 11 years Developer Advocate Loves to stare at the code of Open Source projects Quarkus Spring Boot LangChain4j Microcks Testcontainers (sometimes contributes bugs too!) linkedin.com/in/shelajev/ youtube.com/user/shelajev/shorts
8. What we talk about today? 1. Spring Dev Services 2. Testcontainers 3. WireMock 4. Everything together 5. Local models for modeling / testing AI 6. Integrations 8
9. About You 9
10. You Have a LOT to Integrate 10
11. When IT Goes wrong 11
14. Do you `make it`? IT - Integration Tests 14
15. Today’s lab The workshop! On-demands: • API Mocking 101 • Integration Testing 101 • Testcontainers 101 • WireMock 101 • Testing AI applications 15
16. Before We Start • Level of experience with tech we use • Do you use Testcontainers? • Do you use WireMock or Microcks? • Do you already use Spring Boot Dev Services? 16
17. Want to follow along? • A Laptop (?) • Recommended: MacOS / Windows laptops • Your Favorite IDE • Docker Desktop 17
18. Before we start • docker model pull ai/gemma3-qat:1B-Q4_K_M 18
19. 101s Integration & API Testing 19
20. Spring Boot and Dev Services
21. See: Presentation by Moritz Halbritter https://www.youtube.com/watch?v=Yqss7tYP890 21
22. Options • Manual Docker Compose & Co • Dev Services with Docker Compose • Dev Service with Testcontainers (!) ◦ Test app with ./gradlew bootTestRun https://docs.spring.io/spring-boot/reference/fe atures/dev-services.html 22
23. Integration Testing 101
25. Integration Testing • Multiple technology stacks and toolchains • Many teams involved • Sacred knowledge • [Almost] always too late 25
28. APIs are the Key for Developer Productivity Client App request request response response HTTP/2 API API Server Storage
29. API specs API tech
31. We will develop a custom framework for integration tests Just use xUnit Just use xUnit 31
32. Gaining confidence in you software Build Tests “Shift Left” with modern tools Integration tests Build Staging & QA Tests 32
33. https://martinfowler.com/articles/ mocksArentStubs.html 33
34. Ways to do API integration testing 1. Testing against Production/Staging servers Accurate Slow Nope Fast 2. Testing against a simplified/containerized instance (e.g. Testcontainers) 3. Modeling (aka Mocking) at the API provider level (e.g. WireMock) 4. Mocking at the code level * For Testcontainers, see the slides
35. Ways to do API integration testing 1. Testing against Production/Staging servers Accurate Slow Nope Fast 2. Testing against a simplified/containerized instance (e.g. Testcontainers) 3. Modeling (aka Mocking) at the API provider level (e.g. WireMock or Microcks) 4. Mocking at the code level * For Testcontainers, see the slides
36. Testcontainers 101 36
38. Example - Java 38
39. https://testcontainers.com/modules/
40. LOCAL STACK https://testcontainers.com/modules/ 40
41. SDKs testcontainers.org … and 41
42. github.com/testcontainers/ testcontainers-go 42
43. WireMock 101 43
44. No Implementation? Fake it until you “$ make it” 44
45. Ways to do API integration testing 1. Testing against Production/Staging servers Accurate Slow Nope Fast 2. Testing against a simplified/containerized instance (e.g. Testcontainers) 3. Modeling (aka Mocking) at the API provider level (e.g. WireMock or Microcks) 4. Mocking at the code level * For Testcontainers, see the slides
46. API Mocking for Java. Examples Open Source Proprietary ` en.wikipedia.org/wiki/Comparison_of_API_simulation_tools
47. API Mocking Tools Main implementation in Java Sandbox CNCF Project Native lib, Docker images and Testcontainers modules Golang / Microservice arch Support for GraalVM Docker image, Testcontainers module, All-in K8s Charts Rich Ecosystem Less Rich Ecosystem
49. API Mocking 101 WireMock Config JSON: IF (request_url) THEN (response) * Client library request response HTTP/2 Client App Mock API Server * it gets MUCH more complex wiremock.org/docs/stubbing
50. WireMock Ecosystem & Features wiremock.org/docs
51. WireMock Ecosystem Gradle Build Tool wiremock.org/docs
52. WireMock in Java wiremock.org/docs/stubbing
53. WireMock Ecosystem / Java • Java • Kotlin • Scala • Groovy • Clojure • (?)Android • • • • • • • • JUnit 4/5 Testcontainers Sprint Boot Spring Cloud Contract Quarkus Maven / Gradle Build Tool Spock Pact 53
54. Example - JUnit 5 54
55. WireMock Plugin for Gradle Build Tool William-Hill-Online/wiremock-gradle-plugin Gradle Build Tool 55
56. WireMock for Spring Boot • There is a library for it! • https://wiremock.org/docs/solutions/ spring-boot-integration/ 56
57. WireMock has an official Testcontainers module! testcontainers.com/modules/wiremock 57
58. Demos • https://github.com/wiremock/wiremock-docker • https://github.com/oleg-nenashev/mocks-as-code-demo • https://github.com/wiremock/wiremock-grpc-demos • https://github.com/wiremock/wiremock-spring-boot-oauth2-demo 58
59. WireMock or Testcontainers?
60. Choosing a right tool * • Need database or complex dynamic environment? ◦ YES => Testcontainers • Do you have a ready implementation? ◦ NO => WireMock/Microcks • Is it containerized and lightweight? ◦ NO => WireMock/Microcks • Can you easily trigger the desired scenarios? ◦ NO => WireMock/Microcks • Else: => Testcontainers * Likely, you want to use both * Your mileage may vary ** More later *** I just like footnotes 60
62. WireMock has an official Testcontainers module! testcontainers.com/modules/wiremock 62
63. Proxy can be made transparent in your test! Tests 63
64. WireMock Proxy? Why? • Record & Playback for Request sequences • Reverse engineering of legacy undocumented APIs • NEW: OpenAPI Standard Compliance Verification (in 3.0 Beta) • Emulating complex cases ◦ Overriding particular responses ◦ Emulating response sequences / Stateful behavior ◦ Failure Injection 64
65. More Demos • Simple WireMock and Testcontainers demo: github.com/testcontainers-community/testcontainers-wiremock-demo • WireMock and Docker integration tests: https://github.com/wiremock/wiremock-docker • Quarkus Dev Service for WireMock and Testcontainers: https://github.com/oleg-nenashev/gradle-quarkus-plugin-demo • Spring Boot + Testcontainers and WireMock + LocalStack: https://github.com/oleg-nenashev/mocks-as-code-demo
66. Docker Model Runner 66
67. Docker Model Runner ➔ Run models locally with full GPU support ➔ Distribute models through container registries ➔ Easily integrate using any OpenAI-compatible SDKs and libraries compose.yaml services: llm: provider: type: model options: model: ai/gemma3:4B-F16 app:
68. Resources • Docker Model Runner docs: https://docs.docker.com/model-runner/ • Using Docker Model in Docker Compose https://docs.docker.com/compose/how-tos/model-runner/#provider-services • Using Docker Model Runner for AI assistant setup: https://dev.to/olegshelajev/easy-private-ai-assistant-with-goose-and-docker-model-runner-4b41
69. Lessons Learned 69
70. Takeaways • Shift left the integration testing • API Mocking / abstraction are great… when used right • There is no silver bullet • WireMock and Testcontainers address their own use-cases • They work well together! 70
71. Choosing a right tool * • Need database or complex dynamic environment? ◦ YES => Testcontainers • Do you have a ready implementation? ◦ NO => WireMock/Microcks • Is it containerized and lightweight? ◦ NO => WireMock/Microcks • Can you easily trigger the desired scenarios? ◦ NO => WireMock/Microcks • Else: => Testcontainers * Likely, you want to use both * Your mileage may vary ** More later *** I just like footnotes 71
72. Keep Learning Tools * AKA: Burning Out 101 72
74. It’s [always] a great time to contribute! testcontainers.org/contributing wiremock.org/participate 74
75. Questions? Testcontainers Slack WireMock Slack 75