AVRO-4322: [Java] Only apply java-class annotations to SpecificData models

[RyanSkraba]

What is the purpose of the change

When the fastread function was set to be the default in Avro 1.12.1, the behaviour around create GenericData objects changed.

In the past, the GenericData returned standard and reliable "generic" data types that didn't involve casting or reflection, while SpecificData allowed the user to create "specific" java classes (including the SpecificData.CLASS_PROP and SpecificData.KEY_CLASS_PROP to control the representation of a STRING datum.

Currently, fastread changed this behaviour to apply those properties to the GenericData model, which causes tests to fail in Parquet. This restores the original behaviour.

Verifying this change

This change added tests and can be verified by running the TestFastReaderBuilderJavaClassTest.

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? no

[Copilot]

Pull request overview

This PR restores Avro’s pre-1.12.1 behavior by ensuring java-class / java-key-class schema annotations are applied only when using the SpecificData model, not GenericData, while adding/adjusting tests to lock in that behavior (including class-loading validation under ClassSecurityValidator).

Changes:

• Restricts FastReaderBuilder’s application of SpecificData.CLASS_PROP / SpecificData.KEY_CLASS_PROP to SpecificData model instances.
• Expands FastReaderBuilderJavaClassTest to validate both “Generic ignores / Specific applies” behavior for string fields and map keys.
• Updates the class-loading security test to use a SpecificData model and shared round-trip helper.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
lang/java/avro/src/main/java/org/apache/avro/io/FastReaderBuilder.java	Gates java-class / java-key-class transformations to SpecificData model usage and slightly refactors map key reader creation.
lang/java/avro/src/test/java/org/apache/avro/io/FastReaderBuilderJavaClassTest.java	Adds comprehensive regression tests asserting Generic vs Specific behavior for CLASS_PROP and KEY_CLASS_PROP.
lang/java/avro/src/test/java/org/apache/avro/io/TestFastReaderBuilderClassLoading.java	Aligns the security/class-loading test with the new “Specific-only” behavior and reuses the shared round-trip helper.

Suppressed comments (4)

lang/java/avro/src/test/java/org/apache/avro/io/FastReaderBuilderJavaClassTest.java:120

• This test relies on the global default for fast-reader enablement (system property + GenericData singleton). If fast-reader is disabled in the test JVM, this won’t exercise FastReaderBuilder and may not catch regressions. Prefer using a fresh model instance with fast-reader explicitly enabled.

    GenericRecord result = roundTrip(RECORD_WITH_CLASS_PROP, GenericData.get());

lang/java/avro/src/test/java/org/apache/avro/io/FastReaderBuilderJavaClassTest.java:152

• This test should explicitly enable fast-reader on a fresh SpecificData instance to ensure it validates the FastReaderBuilder path regardless of JVM/system-property configuration.

    GenericRecord result = roundTrip(RECORD_WITH_CLASS_PROP, SpecificData.get());

lang/java/avro/src/test/java/org/apache/avro/io/FastReaderBuilderJavaClassTest.java:169

• This test relies on the global default for fast-reader enablement (system property + GenericData singleton). If fast-reader is disabled in the test JVM, this won’t exercise FastReaderBuilder and may not catch regressions in the fast path. Prefer using a fresh model instance with fast-reader explicitly enabled.

    GenericRecord result = roundTrip(RECORD_WITH_MAP_KEY_CLASS_PROP, GenericData.get());

lang/java/avro/src/test/java/org/apache/avro/io/FastReaderBuilderJavaClassTest.java:192

• This test should explicitly enable fast-reader on a fresh SpecificData instance to ensure it validates the FastReaderBuilder path regardless of JVM/system-property configuration.

    GenericRecord result = roundTrip(RECORD_WITH_MAP_KEY_CLASS_PROP, SpecificData.get());

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