OpenAPI-Generated Clients

About

OpenAPI-generated clients are HTTP clients automatically created from an OpenAPI (Swagger) specification file. Instead of manually configuring and managing REST clients like RestTemplate, we generate strongly-typed client classes that map directly to our service contracts defined in the OpenAPI spec.

These clients abstract the complexity of request creation, serialization, deserialization, error handling, and endpoint management. They help reduce boilerplate code and enforce consistency between service and client.

How It Works ?

1. Define the API Contract: An OpenAPI (YAML or JSON) file defines endpoints, parameters, responses, request bodies, and schemas.

2. Generate Code: Tools like OpenAPI Generator, Swagger Codegen, or Gradle/Maven plugins generate client-side code in our language of choice.

3. Use Generated Client: The generated classes offer type-safe methods for interacting with remote services, eliminating manual endpoint string construction and request mapping.

OpenAPI-Generated Clients vs Manual RestTemplate Configuration

Aspect	OpenAPI-Generated Client	Manual RestTemplate
Setup	Requires OpenAPI file and code generation setup	Manual setup of RestTemplate bean
Endpoint Definition	Auto-generated methods per endpoint	Manual string-based URL management
Data Models	Generated POJOs matching API schemas	Manually created DTOs
Serialization / Deserialization	Handled internally	Manually configured message converters
Error Handling	Can be auto-handled or customized	Must handle explicitly via ResponseErrorHandler
Maintainability	Easier to maintain with contract updates	High effort in updating method signatures and URLs
Type Safety	High; compile-time guarantees	Lower; string-based, requires defensive checks
Interceptors / Config	Can be added via HTTP client	Custom interceptors added to RestTemplate
Testing	Can mock client interfaces easily	Needs manual mocking or wrapper layers
Asynchronous Support	Supported via HTTP client options	Manual integration with CompletableFuture, etc.

Typical Project Usage

Many enterprise projects define OpenAPI specs as the source of truth. CI pipelines automatically generate and integrate the clients into consumer services.

Teams can:

• Version API clients

• Share clients across teams as artifacts

• Generate clients in different languages (Java for backend, Typescript for frontend)

Drawbacks and Considerations

While OpenAPI-generated clients provide many advantages, there are several trade-offs and challenges that teams should carefully evaluate before adopting them:

1. Added Build Complexity

Integrating OpenAPI generation into our build process (via Maven, Gradle, or CI pipelines) introduces extra steps. This includes:

• Managing the OpenAPI spec version.

• Configuring the generator tool.

• Dealing with compatibility between generator versions and codebase.

If not managed well, it can slow down build cycles and increase maintenance overhead.

2. Limited Flexibility in Customization

Generated clients follow the structure and behavior dictated by the OpenAPI spec and the generator templates. Custom scenarios such as:

• Dynamic request creation

• Runtime manipulation of headers, paths, or query params

• Specialized authentication logic are often harder to implement cleanly without overriding the generated code or writing adapters.

3. Tight Coupling to the OpenAPI Spec

Our application logic becomes tightly bound to the spec version. If the upstream service changes frequently or the spec is not well maintained, we will be forced to:

• Regenerate clients frequently

• Rework integration logic regularly

• Deal with breaking changes more often

This can slow development, especially in fast-moving teams where API-first discipline isn’t strictly followed.

4. Generated Code Volume

Generated clients often produce a large number of classes:

• Model classes for each schema

• API classes for each resource group

• Utility/configuration classes

This increases the footprint of our codebase, making debugging and onboarding slightly more complex. IDEs may also slow down with large generated packages.

5. Tooling Gaps and Inconsistencies

The behavior and quality of generated code vary depending on:

• The language used

• The generator version

• The specific generator configuration

Not all generated clients are equally optimized or idiomatic. We may need to tweak templates or even fix bugs in generated code depending on the tool.

6. Loss of Control over HTTP Layer

With manually configured clients like RestTemplate or WebClient, we have precise control over:

• Connection pooling

• Retries

• Error mapping

• Logging and tracing

Generated clients abstract this layer, which may make it harder to plug in our enterprise-wide cross-cutting concerns like observability, correlation IDs, or fine-grained retry strategies.

7. Version Management and Compatibility

When working in large teams or consuming multiple services:

• We may end up managing multiple versions of generated clients.

• If the server team makes incompatible spec changes, consumers need to upgrade their clients quickly to stay in sync.

This requires coordination and robust versioning practices, or We will face integration failures.

8. Learning Curve and Onboarding

Developers unfamiliar with OpenAPI tools may struggle initially with:

• Generator configuration

• Template customization

• Spec debugging

• Navigating the generated code

While the code itself is meant to abstract HTTP concerns, understanding what it does under the hood is still important, especially during debugging or error handling.