Improve OneOf handling with new normalizer REPLACE_ONE_OF_BY_DISCRIMINATOR_MAPPING

[jpfinne]

fixes #19261, #912, #23527, #22013, #23276, #14769, #16391, #19194, #19261, #23577 (and probably others) when using the new REPLACE_ONE_OF_BY_DISCRIMINATOR_MAPPING normalizer option.

The goal is to use inheritance when composition or useOneOfInterfaces don't work.

The normalizer converts oneOf with discriminators.
It ensures that:

• discriminator mappings are created (using x-discriminator-value if present or single enum value).
• It removes the oneOf.
• It ensures that the referenced children correctly has an allOf to the parent.
• it removes the single enum value in the referenced mappings if already present in the parent.

For the java generators, it means that inheritance with a base class class is generated instead of an interface.

Warning: the normalizer does not support inline oneOf (as seen in #23276 and in #15 using composed-oneof.yaml). A warning is outputted Inline oneOf schema not supported by REPLACE_ONE_OF_BY_DISCRIMINATOR_MAPPING normalization. No transformation is done.
It might be nice to refactor the inline oneOfs in the OpenAPINormalizer, similar to REFACTOR_ALLOF_INLINE_SCHEMAS done in the InlineModelResolver.
ps: it can't be done in InlineModelResolver because it runs after the normalizer.

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---

Summary by cubic

Adds a normalizer that converts oneOf into discriminator.mapping, removes oneOf, and sets up allOf inheritance so Java and spring generators emit proper class hierarchies. Also hardens parent detection to avoid missed inheritance in nested allOf graphs.

• New Features

  • REPLACE_ONE_OF_BY_DISCRIMINATOR_MAPPING=true: builds discriminator.mapping from oneOf $refs (keeps existing mappings), removes oneOf, adds an allOf parent $ref to children and moves child properties under that allOf; removes the discriminator property from children when the base defines it; skips inline oneOf with a warning.
  • Discriminator values prefer x-discriminator-value, else a single enum on the discriminator property, else the schema name. Docs updated with a spring CLI example; new samples/tests (issues 912, 14769, 19261, 22013, 23276, 23527, 23577) validate JsonTypeInfo/JsonSubTypes annotations and children extending the base.

• Bug Fixes

  • Fixes the hasParent recursion guard to prevent early termination or infinite loops and reliably detect existing parents without duplicating allOf links.

^{Written for commit c1da8d4. Summary will update on new commits.}

[winklerm]

Thanks a lot for this quick fix!

I have verified it resolves the issue for our use-case.

[jpfinne]

Thanks a lot for this quick fix!

I have verified it resolves the issue for our use-case.

This bug has bother me for months. I've initially tried to fix it in DefaultCodeGen, but the handling of composition vs inheritance is very complex.

I'll work on the PR. In the meantime you can create a custom normalizer:

NORMALIZER_CLASS: Set to full classname of a class extending the default org.openapitools.codegen.OpenAPINormalizer. It allows customization of the default normalizer

[cubic-dev-ai]

5 issues found across 18 files

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="docs/customization.md">

<violation number="1" location="docs/customization.md:654">
P3: Malformed Markdown adds a stray backtick to the documented normalizer name, making the docs show/copy the option incorrectly.</violation>
</file>

<file name="pom.xml">

<violation number="1" location="pom.xml:27">
P1: Root-level module declarations duplicate modules already listed in the default-active openapi-generator profile, which can make Maven fail with duplicated reactor projects.</violation>
</file>

<file name="modules/openapi-generator/src/main/java/org/openapitools/codegen/OpenAPINormalizer.java">

<violation number="1" location="modules/openapi-generator/src/main/java/org/openapitools/codegen/OpenAPINormalizer.java:1608">
P2: `schema.getOneOf()` is iterated without a null guard after `processSimplifyOneOf()` can clear `oneOf`, causing a potential NPE in discriminator replacement.</violation>

<violation number="2" location="modules/openapi-generator/src/main/java/org/openapitools/codegen/OpenAPINormalizer.java:1619">
P1: Unconditionally removing `oneOf` can drop subtype metadata when the discriminator mapping is partial or not rebuilt.</violation>

<violation number="3" location="modules/openapi-generator/src/main/java/org/openapitools/codegen/OpenAPINormalizer.java:1630">
P2: `hasParent()` returns false immediately when it encounters a visited `allOf` branch, so later siblings are never checked and an existing parent ref can be added again.</violation>
</file>

_{Reply with feedback, questions, or to request a fix. Tag @cubic-dev-ai to re-run a review.}

[cubic-dev-ai]

P1: Root-level module declarations duplicate modules already listed in the default-active openapi-generator profile, which can make Maven fail with duplicated reactor projects.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At pom.xml, line 27:

<comment>Root-level module declarations duplicate modules already listed in the default-active openapi-generator profile, which can make Maven fail with duplicated reactor projects.</comment>

<file context>
@@ -23,6 +23,15 @@
         <url>https://github.com/openapitools/openapi-generator</url>
     </scm>
+    <modules>
+        <module>modules/openapi-generator-core</module>
+        <module>modules/openapi-generator</module>
+        <module>modules/openapi-generator-cli</module>
</file context>

[cubic-dev-ai]

1 issue found across 1 file (changes from recent commits).

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="modules/openapi-generator/src/main/java/org/openapitools/codegen/OpenAPINormalizer.java">

<violation number="1" location="modules/openapi-generator/src/main/java/org/openapitools/codegen/OpenAPINormalizer.java:1780">
P1: Regression in `hasParent()`: the new visited check short-circuits recursive traversal, so existing parent refs nested under `allOf` are missed and duplicate parent inheritance refs can be added.</violation>
</file>

_{Reply with feedback, questions, or to request a fix. Tag @cubic-dev-ai to re-run a review.}