Pact Consumer-Driven Contract Testing (JVM)

A production-style Consumer-Driven Contract Testing (CDCT) demo using Pact JVM.

• The consumer defines the HTTP contract (Pact) by testing against a mock server.
• The provider verifies it against the real Spring Boot API.
• Contracts are automatically copied consumer → provider during the build.

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What this repo demonstrates

• Consumer-driven contracts: the contract is owned by the consumer test suite.
• Provider verification: the provider test suite verifies every interaction from the Pact file.
• Multiple scenarios in a single contract:
  • GET /api/users/1 → 200 OK + JSON body
  • GET /api/users/999 → 404 Not Found
• Build-time wiring:
  • running :provider:test implicitly runs :consumer:test first
  • generated pact files are copied into provider/build/pacts

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Modules

Module	Purpose
consumer	Defines contracts (Pact consumer tests) and contains the API client (UserClient)
provider	Spring Boot API + Pact provider verification tests

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Quick start

Run all tests (consumer + provider verification):

./gradlew test

Run only consumer contract tests (generates pact files in consumer/build/pacts):

./gradlew :consumer:test

Run only provider verification (will still generate/copy pacts automatically):

./gradlew :provider:test

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Architecture

flowchart LR
  subgraph Consumer
    CT[Consumer Pact tests]
    UC[UserClient WebClient]
  end

  subgraph Provider
    API[Spring Boot REST API]
    VT[Provider verification tests]
  end

  CT -->|starts mock server| MS[(Pact Mock Server)]
  UC -->|HTTP calls| MS
  CT -->|writes pact JSON| P[(consumer/build/pacts)]
  P -->|Gradle copy| PB[(provider/build/pacts)]
  VT -->|verifies| API

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Tech stack

• Language: Java 21
• Build: Gradle (Kotlin DSL)
• Framework: Spring Boot 3 (provider), Spring WebClient (consumer)
• Contract testing: Pact JVM (consumer + provider)
• Testing: JUnit 5, Spring Boot Test

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Notes

• Pact files are generated by the consumer tests and treated as build artifacts.
• Provider state handlers are implemented via @State(...) methods in the provider verification test.

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License

MIT. See LICENSE.