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Héctor Valls

Sr. Software Engineer

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Ensuring REST API contract compliance

In software development, RESTful APIs have become a standard for communication between different components of a system. These APIs define a contract or agreement between the backend API provider and the frontend or third-party consumers. The contract outlines the expected behavior, data formats, and rules for exchanging information.

Why respecting API contracts is important?

Testing REST API contracts is highly significant for the following reasons:

  • Contractual compliance: Just like in any business transaction, adherence to a contract is crucial. By testing your API against the specified contract, you ensure that your implementation aligns with the agreed-upon terms. It ensures that your API behaves as expected, follows the correct data structures, and honors the commitments made to API consumers.

  • Integration confidence: Integrating different systems can be a really challenging task. Testing REST API contracts helps mitigate integration risks and instills confidence in the process. By validating your API against the contract, you verify that it plays well with other components and services. By testing REST API contracts, you lay the foundation for effective communication and collaboration between API providers and consumers.

In this article I will compare two approaches for testing REST API contracts: generating classes from the API specification and manually writing tests.

Code generation from API spec

Code generation from the API specification is a process that automatically generates code artifacts based on the defined API contract or specification. In the context of REST APIs, this approach uses tools or libraries to parse the API specification, typically written in formats such as OpenAPI, and generates code in a programming language of choice. Note that this generated code has nothing to do with the business logic. Generated classes and structures represents the resources the API will consume and return.

To ensure ongoing compliance with the API spec, code generation can be integrated into the development workflow: whenever the API specification changes or updates, the code generation process can be triggered again to regenerate the code artifacts, keeping them in sync with the latest contract.

Advantages of code generation

  • Contract adherence assurance: Automated code generation approach minimizes the risk of overlooking contract details and helps maintain compliance throughout the development process.

  • Automated payload validation: When data is exchanged with the API, it can be automatically validated against the expected format and structure. This helps catch inconsistencies and discrepancies, ensuring that both the API provider and consumer communicate seamlessly based on the contract.

  • Reduced development effort: It saves developers significant effort and time. Manually creating classes, interfaces, and data structures for each endpoint can be tedious and error-prone. With code generation, developers can focus on implementing business logic rather than worrying about the details of the API contract.

Challenges of code generation

  • Handling specification changes: When the API specification changes, regenerating the code becomes necessary to reflect the updated contract. Managing these changes and ensuring synchronization between the specification and the generated code can be a challenge. Developers must be diligent in managing these changes and keeping the generated code up to date, especially when dealing with large APIs or frequent specification updates.

  • Flexibility and extensibility: Code generation can sometimes limit the flexibility and extensibility of the codebase. Generated code might not always accommodate customizations or specific requirements beyond the contract. Developers may need to find workarounds or manually modify the generated code to meet certain edge cases or unique business needs.

Manually writting tests

Manually writing tests to validate contract adherence involves the creation of custom test cases and assertions that ensure the API implementation complies with the specified contract. Instead of relying on code generation, this approach focuses on designing and executing test scenarios to verify that the API behaves as expected.

When manually writing tests to validate REST API contracts, using libraries and tools for JSON schema validation can greatly simplify the process. Two popular options in Java/JVM ecosystem are JSON Schema Validator and RestAssured. With these tools, developers can easily validate API responses against a specified JSON schema, ensuring contract adherence.

Advantages of manually writting tests

  • Flexibility and customization: Manual testing allows developers to have complete control over the testing process. They can design test cases tailored to specific scenarios, including edge cases and complex business logic.

  • Greater understanding of the API: By manually writing tests, developers gain a deeper understanding of the API's functionality, endpoints, and expected behavior. This hands-on approach fosters a comprehensive understanding of the API contract and enhances the developer's knowledge of how different components interact with each other.

Challenges of manually writting tests

  • Increased development effort: Manual testing requires additional development effort compared to automated code generation. Developers need to design and write test cases, define assertions, and maintain the test suite over time. This can lead to higher development costs and increased time investment, especially for complex APIs with extensive test coverage requirements.

  • Potential for omitted test scenarios: As manual testing relies on human effort, there is a possibility of missing certain test scenarios or edge cases. Developers need to be diligent in designing comprehensive test cases that cover a wide range of scenarios to ensure effective contract validation.

  • Maintenance overhead: Test cases and assertions may need updates to reflect changes in the contract, which adds an additional overhead in terms of effort and time.

Which approach should I choose?

As always, it depends:

  • Project complexity: For simple APIs with straightforward contracts, automated code generation can provide efficient contract validation. On the other hand, for complex APIs with intricate business logic and edge cases, manual testing allows for greater customization and flexibility to cover diverse scenarios.

  • Team preferences and expertise: If your team is experienced in working with code generation tools and libraries, leveraging automated code generation may streamline the testing process. However, if your team prefers a more hands-on approach or has specific expertise in writing test cases and assertions, manual testing can be a favorable choice.

  • Integration and collaboration: If there is a strong need for close collaboration and communication, manual testing can provide better flexibility to address specific requirements and ensure smooth integration. Code generation, on the other hand, may offer a more standardized approach when working with multiple teams or third-party consumers.

  • Tooling and ecosystem: Evaluate the level of community support, documentation, and integration capabilities with your existing development ecosystem. A well-established tooling ecosystem can significantly impact the ease and efficiency of adopting either approach.


To sum up, testing REST API contracts is essential for ensuring reliable and compliant API implementations. Whether you choose to leverage code generation or manually write tests, the objective remains the same: to validate that your API adheres to the specified contract. By selecting the right approach, you can build robust and trustworthy systems that foster seamless communication and integration.