Java SDK generation fails on discriminator value _ #17431

Java SDK generation fails on discriminator value _

When a discriminated union's discriminator value is a single underscore ("_"), the Java generator aborts with:

IllegalArgumentException: not a valid name: 

(empty identifier — note the trailing space, nothing after the colon)

Reproduction

openapi: 3.0.3
info:
  title: Underscore discriminator repro
  version: 1.0.0

paths:
  /filter:
    post:
      operationId: createFilter
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: "#/components/schemas/Filter"
      responses:
        "204":
          description: OK

components:
  schemas:
    Filter:
      oneOf:
        - $ref: "#/components/schemas/StringFilter"
        - $ref: "#/components/schemas/UnderscoreFilter"
      discriminator:
        propertyName: type
        mapping:
          string: "#/components/schemas/StringFilter"
          "_": "#/components/schemas/UnderscoreFilter"

    StringFilter:
      type: object
      properties:
        type:
          type: string
          enum: ["string"]
        value:
          type: string
      required: [type, value]

    UnderscoreFilter:
      type: object
      properties:
        type:
          type: string
          enum: ["_"]
        field:
          type: string
      required: [type, field]

Running fern ir confirms the IR correctly preserves "discriminantValue": "_".

Running fern generate --local against fernapi/fern-java-sdk:4.19.0 fails with:

Exception in thread "main" java.lang.RuntimeException:
java.lang.IllegalArgumentException: not a valid name: 
    at com.fern.java.AbstractGeneratorCli.run(AbstractGeneratorCli.java:412)
    at com.fern.java.client.Cli.main(Cli.java:122)
Caused by: java.lang.IllegalArgumentException: not a valid name: 
    at com.squareup.javapoet.MethodSpec.methodBuilder(MethodSpec.java:188)
    at com.fern.java.generators.UnionGenerator$ModelUnionSubTypes
        .getStaticFactory(UnionGenerator.java:440)
    at com.fern.java.generators.union.UnionTypeSpecGenerator
        .getStaticConstructors(UnionTypeSpecGenerator.java:162)

Root cause

This hits the same crash site as #17429 (UnionGenerator.java:440) but for a different reason. CasingConfiguration.computeName() builds the Java identifier by first calling toCamelCase(), which splits the input into words using this regex (splitWords):

[A-Z]+(?=[A-Z][a-z])|[A-Z]?[a-z]+|[A-Z]+|[0-9]+

That pattern doesn't match underscores at all, so splitWords("_") returns an empty list, and toCamelCase("_") returns "". The reserved-keyword check in sanitizeName() runs after this transformation, so there's no string left to compare against the keyword list by that point — computeName("_").camelSafe comes back as "", not "_" and not "__".

This is a distinct bug from #17429: that one was a missing entry in a keyword list; this one is the word-splitting step silently discarding a legal identifier before the keyword list is ever consulted. Just adding "_" to CasingConfiguration.JAVA_RESERVED_KEYWORDS on its own is a no-op — verified directly against the real class.

Worth noting: KeyWordUtils.java (a separate reserved-word list used for object property/class names elsewhere in the same generator) already treats _ as reserved and escapes it correctly there. It's specifically the CasingConfiguration word-splitting path (used for union discriminants, among other compressed/reconstructed names) that's affected.

Expected behavior

A valid, non-empty Java identifier — _ escaped to __, matching the existing convention used for other reserved words — while the serialized discriminator value stays "_".

Suggested fix

A complete fix needs both parts: (1) preserve all-underscore input during casing conversion instead of letting it collapse to "" (e.g. short-circuit before word-splitting when the input matches ^_+$), and (2) treat _ as a reserved Java identifier in JAVA_RESERVED_KEYWORDS so the preserved value gets escaped to __. I've implemented and verified this locally: unit tests against the real class confirm computeName("_").camelSafe == "__" (with "__"/"___" and ordinary names like "user_id" unaffected), and rebuilding the fernapi/fern-java-sdk image from source with the fix and re-running the reproduction above now generates successfully, with public static Filter __(UnderscoreFilter value) and @JsonTypeName("_") — a valid identifier, unchanged wire value. Happy to open a PR.

Note on parity: the canonical TypeScript CasingsGenerator has the same default behavior (_ also collapses to "" via lodash's camelCase() unless its preserveUnderscores option is set, which isn't currently invoked anywhere in the generator pipeline), and _ is likewise absent from reserved.ts's Java keyword set. So this isn't a case of Java being out of sync with existing TypeScript behavior — neither implementation handles it today. Bringing TypeScript to parity would be a separate, broader change.

Follow-up to #17429 / #17430, found while verifying whether that fix would also cover _ (it didn't, until now).