# Generating Clients and Server Stubs — OpenAPI / Swagger

Source: https://www.skillbyai.com/en/openapi/c-generators

> Generate SDKs and server interfaces with OpenAPI Generator and understand the trade-offs.

## Code from the contract

**OpenAPI Generator** (a community fork of Swagger Codegen) produces **client SDKs** in dozens of languages (TypeScript, Java, Kotlin, Python, Go, C#, Swift and more) and **server stubs** or interfaces for frameworks such as Spring, ASP.NET Core and others. Generated clients give consumers typed models and methods named after `operationId`s, and keep many client languages consistent with one contract. Generated **server interfaces** (for example Spring `*Api` interfaces with `interfaceOnly=true`) let you implement controllers against the contract, so compile errors reveal drift. Trade-offs: generated code reflects the quality of your document (vague schemas produce `Object` types), different generators vary in maturity, and customisation is done through configuration options or templates rather than editing output. Treat generated code as a **build artefact**: regenerate it in the build, do not hand-edit it, and pin the generator version. Lighter alternatives exist too, such as **openapi-typescript** for TypeScript types only, and several commercial SDK generators.

## One contract, many SDKs

A generator turns the contract into typed clients for several languages and server interfaces.

![A document icon feeding a gear, which emits several small code-file icons with different coloured tabs.](assets/figures/openapi/section-7-map.svg) — Figure 7.1 — Generating clients and server interfaces from one document.

## Generating a TypeScript client and Spring server interfaces

Run as part of the build so generated code never drifts.

```bash
# TypeScript client using fetch
npx @openapitools/openapi-generator-cli generate \
  -i api/openapi.yaml \
  -g typescript-fetch \
  -o clients/ts \
  --additional-properties=supportsES6=true,npmName=@shop/orders-client

# Spring Boot server interfaces only (you implement them)
npx @openapitools/openapi-generator-cli generate \
  -i api/openapi.yaml \
  -g spring \
  -o build/generated/orders-api \
  --additional-properties=interfaceOnly=true,useSpringBoot3=true,useTags=true

# TypeScript types only (lightweight alternative)
npx openapi-typescript api/openapi.yaml -o src/api/schema.d.ts
```

## Generated code is a mirror of your schemas

If generated clients are full of `any` or `Object`, the document is underspecified. Fix schemas, required lists and formats in the contract rather than patching generated output.

**Quiz:** Why should generated code not be edited by hand?

- [x] Edits are lost the next time code is regenerated from the contract
- [ ] It is read-only on disk
- [ ] Editing breaks the OpenAPI document
- [ ] Generators encrypt the code

*Answer:* Edits are lost the next time code is regenerated from the contract. Generated code should be reproducible from the contract; customise via configuration or templates.