fix(java): emit guarded undiscriminated-union members before all-optional ones in deserializer

[iamnamananand996]

Description

Linear ticket: Closes

Fix incorrect deserialization of Java undiscriminated unions where one variant has all-optional fields. Because Jackson's @JsonIgnoreProperties(ignoreUnknown = true) silently accepts any JSON object when every field is optional, an all-optional variant could greedily match a payload intended for a more specific sibling that requires certain keys.

The generated deserializer in UndiscriminatedUnionGenerator now stably partitions non-primitive members so that variants with at least one required wire key (a "key guard") are attempted first, and unguarded (all-optional) variants are tried last. This preserves prior ordering within each partition while ensuring the more specific match wins.

Concrete failure mode this fixes: a union containing Check (requires achHolder) and PayMethodCloud (all optional) previously deserialized {"achHolder": "..."} as PayMethodCloud instead of Check.

Changes Made

• generators/java/generator-utils/src/main/java/com/fern/java/generators/UndiscriminatedUnionGenerator.java: filter out primitives/boxed primitives, then List#sort non-primitive members so those with required wire keys (getRequiredWireKeys(...) non-empty) come before those without, before emitting their try { ... } catch blocks.
• generators/java/sdk/changes/unreleased/fix-undiscriminated-union-all-optional-ordering.yml: add a fix changelog entry describing the deserialization fix.
• Updated README.md generator (if applicable)

Testing

• Unit tests added/updated
• Manual testing completed

Link to Devin session: https://app.devin.ai/sessions/2e4e587dd160400390839880a5434932
Requested by: @iamnamananand996

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✅ Devin Review: No Issues Found

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SDK Generation Benchmark Results

Comparing PR branch against median of 5 nightly run(s) on main (latest: 2026-05-12T05:12:12Z).

Full benchmark table (click to expand)

Generator	Spec	main (generator)	main (E2E)	PR (generator)	Delta
java-sdk	square	212s (n=5)	271s (n=5)	196s	-16s (-7.5%)

main (generator): generator-only time via --skip-scripts (includes Docker image build, container startup, IR parsing, and code generation — this is the same Docker-based flow customers use via fern generate). main (E2E): full customer-observable time including build/test scripts (nightly baseline, informational). Delta is computed against generator-only baseline.
⚠️ = generation exited with a non-zero exit code (timing may not reflect a successful run).
Baseline from nightly runs on main (latest: 2026-05-12T05:12:12Z). Trigger benchmark-baseline to refresh.
Last updated: 2026-05-12 20:50 UTC