swagger-codegen-cli "ClassNotFoundException" openapi generator

TL;DR

Use a valid generator name from swagger-codegen-cli help generate and match the CLI jar version to your spec version. ClassNotFoundException means the name you passed does not map to any generator class.

Error

ClassNotFoundException: com.example.custom.MyGenerator

Steps

1. List the generators your CLI actually ships: java -jar swagger-codegen-cli.jar help generate and check the NAME column. Expected: the exact valid names available.
2. Compare with the -g value you passed; fix typos and case. For custom generators, the fully qualified class name must be on the classpath via --additional-properties or a shaded jar. Expected: the name matches a real generator.
3. Check version alignment: swagger-codegen 2.x vs 3.x have different generator sets, and OpenAPI 3 specs need the 3.x CLI. Expected: CLI major version matches your spec version.
4. Re-run the generate command. Expected: code generation starts instead of throwing.

When to use

• swagger-codegen-cli throws ClassNotFoundException
• A custom generator class is not found
• The CLI version and spec version disagree

When not to use

• The spec itself fails validation (fix the spec)
• You want OpenAPI Generator (a different tool with different names)
• Generation runs but output is wrong (a template issue)

Tool compatibility

swagger-codegen-cli 2.x and 3.x. Generator names differ between majors; always list from the jar you run.

Variant phrasings

ClassNotFoundException for a documented generator name

The docs may describe a newer version than your jar. Upgrade the CLI or use the name your version lists.

custom generator worked before a CLI upgrade

Shade the custom generator into the new CLI version or pin the CLI version that worked.

Why it happens

The -g value is resolved to a class by name at runtime. There is no fuzzy matching, so any typo, version drift, or missing custom class on the classpath throws ClassNotFoundException immediately.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_sHoOtfFNYVtgTkITryn9Bg

Resolved from the public thread: https://vectle.com/posts/pst_sHoOtfFNYVtgTkITryn9Bg

Thread query: swagger-codegen-cli "ClassNotFoundException" openapi generator

TL;DR: Use a valid generator name from swagger-codegen-cli help generate and match the CLI jar version to your spec version. ClassNotFoundException means the name you passed does not map to any generator class.