feat: Improve Jackson 2 compatibility for Swagger mappers

[ewaostrowska]

Pull Request

Restores selected Jackson 2 behavior after the Jackson 3 migration.

Each MapperBuilder now calls configureForJackson2() before Swagger Core applies its own configuration.

The compatibility configuration also restores behavior that configureForJackson2() does not cover:

• Accept lower-case and non-letter accessor names.
• Fail when ordered map keys cannot be compared.

Changes

• Apply Jackson 2 compatibility to all JSON, YAML, converter, and strict mappers.
• Apply Swagger Core overrides after the Jackson 2 configuration.
• Preserve enum output through Enum.toString().
• Read enum values through Enum.name().
• Preserve the strict mapper check for trailing tokens.
• Keep plain BigDecimal output limited to public output mappers.
• Remove separate manual configuration for Jackson 2 date and time behavior.

Known differences from Jackson 2

This PR does not restore all Jackson 2 behavior.

java.time.Month

Jackson 3 handles Month as a date and time type. Jackson 2 handled Month as a general enum.

This PR disables ONE_BASED_MONTHS. Therefore, numeric month values keep the Jackson 2 meaning:

• 0 is January.
• 11 is December.

The serialized type still changes:

• Jackson 2 writes Month.JANUARY as "JANUARY".
• The current Jackson 3 mapper writes Month.JANUARY as 0.

Text input such as "JANUARY" remains valid. The generated schema currently describes Month as a string.

Therefore, the serialized value and the generated schema use different types. A separate change is necessary to restore Jackson 2 string output.

Accessor acronyms

Jackson 3 removed the Jackson 2 legacy acronym rule. configureForJackson2() does not restore this rule.

The explicit accessor configuration accepts names such as getvalue() and get_value(). It does not restore legacy acronym conversion.

Examples:

Accessor	Jackson 2 property	Current property
getURL()	url	URL
getIPhone()	iphone	IPhone
isURL()	url	URL

This difference affects JSON, YAML, input names, and generated schema properties. A custom accessor strategy is necessary for full acronym compatibility.

Type of Change

• 🐛 Bug fix
• ✨ New feature
• ♻️ Refactor (non-breaking change)
• 🧪 Tests
• 📝 Documentation
• 🧹 Chore (build or tooling)

Checklist

• I have added/updated tests as needed
• I have added/updated documentation where applicable
• The PR title is descriptive
• The code builds and passes tests locally
• I have linked related issues (if any)

Screenshots / Additional Context

[Copilot]

Copilot review overview

🟢 Approval recommended

The configuration ordering matches the stated compatibility goals and is comprehensively covered by focused tests.

Review effort: Balanced
Findings: None

What changed in this PR

Restores Jackson 2 compatibility defaults across Swagger Core mappers while retaining Swagger-specific overrides.

Changes:

• Applies shared Jackson 2 compatibility and accessor behavior.
• Preserves enum, date/time, decimal, ordering, and strict parsing policies.
• Expands compatibility tests across public and strict mappers.

File	Description
ObjectMapperFactory.java	Centralizes compatibility configuration and mapper-specific overrides.
ObjectMapperFactoryTest.java	Adds broad compatibility and strict-mapper coverage.
OrderedMapKeyCompatibilityTest.java	Extends ordered-map validation to the strict mapper.

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[daniel-kmiecik]

Code review

Found 2 issues:

1. buildStrictGenericObjectMapper UTC date format silently changed from Z to +00:00

Previously this method did not call any compatibility setup, so WRITE_UTC_AS_OFFSET remained false (Jackson 3 default) and UTC timestamps serialized with a Z suffix. After this PR, configureJackson2Compatibility() is applied to the strict mapper, setting WRITE_UTC_AS_OFFSET=true via configureForJackson2(). The test that enforced the old behavior (assertFalse(mapper.isEnabled(DateTimeFeature.WRITE_UTC_AS_OFFSET), name)) was silently removed. MIGRATION.md documents the flag value globally but does not call out that the strict mapper specifically changes its UTC format — consumers relying on it to produce Z-suffix examples will see a silent behavioral change.

swagger-core/modules/swagger-core/src/main/java/io/swagger/v3/core/util/ObjectMapperFactory.java

Lines 433 to 437 in 0c8a90c

	public static ObjectMapper buildStrictGenericObjectMapper() {
	JsonMapper.Builder builder = JsonMapper.builder();
	configureJackson2Compatibility(builder);
	configureSwaggerPolicy(builder);
	builder.configure(DeserializationFeature.FAIL_ON_TRAILING_TOKENS, true);

2. buildStrictGenericObjectMapper placed in the publicMappers data provider

ObjectMapperFactoryTest maintains a strict publicMappers / strictMapper provider split — the strict mapper is explicitly kept separate because it intentionally omits MapperCustomizer registration. Adding it to publicMappers in OrderedMapKeyCompatibilityTest breaks that convention: any future test added to that provider with the expectation it covers only public-API mappers will silently also run against the internal strict mapper.

swagger-core/modules/swagger-core/src/test/java/io/swagger/v3/core/util/OrderedMapKeyCompatibilityTest.java

Lines 39 to 43 in 0c8a90c

	mapper("createYaml(false)", () -> ObjectMapperFactory.createYaml(false)),
	mapper("createYaml(true)", () -> ObjectMapperFactory.createYaml(true)),
	mapper("buildStrictGenericObjectMapper", ObjectMapperFactory::buildStrictGenericObjectMapper)
	};
	}

[ewaostrowska]

@daniel-kmiecik

1. Is intended behavior to align it with Jackson 2 behavior. I have double checked and it behaves the same on master.
2. Point 2 is fair - publicMappers should not contain the strict mapper