# OpenAPI Generator — Generate Client SDKs and Server Stubs from API Specs

> A code generation tool that produces client libraries, server stubs, API documentation, and configuration from OpenAPI 2.0/3.x specifications in over 50 languages.

## Install

Save in your project root:

# OpenAPI Generator — Generate Client SDKs and Server Stubs from API Specs

## Quick Use
```bash
npm install @openapitools/openapi-generator-cli -g
openapi-generator-cli generate -i petstore.yaml -g python -o ./out
```

## Introduction

OpenAPI Generator turns an OpenAPI specification file into production-ready client SDKs, server stubs, or documentation. It removes the tedious work of hand-writing HTTP wrappers and keeps your codebase synchronized with your API contract.

## What OpenAPI Generator Does

- Generates client libraries in 50+ languages including TypeScript, Python, Java, Go, and Rust
- Produces server stubs for frameworks like Spring Boot, Flask, Express, and Gin
- Outputs static HTML or Markdown documentation from spec files
- Supports OpenAPI 2.0 (Swagger), 3.0, and 3.1 specifications
- Allows deep customization through Mustache or Handlebars templates

## Architecture Overview

The tool parses an OpenAPI spec into an internal model of operations, schemas, and security definitions. It then passes this model through a selected generator module that applies language-specific templates to emit source files. A post-processing pipeline handles formatting, import resolution, and file placement in the output directory.

## Self-Hosting & Configuration

- Install via npm, Homebrew, Docker, or download the standalone JAR
- Requires Java 11+ runtime for the core engine (CLI wrappers hide this)
- Configure output with a `openapitools.json` or command-line flags for package names, namespaces, and date libraries
- Override individual templates by placing custom `.mustache` files in a templates directory
- Integrate into CI with the Maven or Gradle plugin for automatic regeneration on spec changes

## Key Features

- Broadest language coverage of any open-source API code generator
- Template-based architecture allows per-project customization without forking
- Generates model validation, serialization, and authentication handling automatically
- Supports discriminator-based polymorphism and oneOf/anyOf/allOf composition
- Active community with frequent releases and generator-specific configuration options

## Comparison with Similar Tools

- **Swagger Codegen** — the original fork; OpenAPI Generator has broader community governance and faster release cadence
- **oapi-codegen (Go)** — Go-only but produces idiomatic code; OpenAPI Generator covers 50+ languages
- **AutoRest (Microsoft)** — focused on Azure SDKs; OpenAPI Generator is vendor-neutral
- **Kiota (Microsoft)** — newer graph-based approach; OpenAPI Generator has more mature template ecosystem
- **openapi-typescript** — TypeScript-only type generation; OpenAPI Generator also produces runtime HTTP clients

## FAQ

**Q: Can I generate only models without the client wrapper?**
A: Yes. Use the `--global-property models` flag to emit only data model classes.

**Q: How do I handle breaking spec changes?**
A: Regenerate into a separate branch, diff the output, and review changes before merging.

**Q: Does it support async/await in generated clients?**
A: Many generators (TypeScript-fetch, Python-aiohttp, C#) produce async code by default or via configuration.

**Q: Can I add custom validation logic to generated models?**
A: Use partial class extensions or template overrides to inject custom validators without modifying generated files.

## Sources

- https://github.com/OpenAPITools/openapi-generator
- https://openapi-generator.tech/docs/installation

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Source: https://tokrepo.com/en/workflows/asset-a951911e
Author: AI Open Source