AVRO-4225: FastReaderBuilder throws ClassCastException for schemas with java-class attribute

[kevinburkesegment]

What is the purpose of the change

When using GenericDatumReader with schemas containing "java-class" attributes on string fields (e.g., java.math.BigDecimal), FastReaderBuilder throws:

ClassCastException: Utf8 cannot be cast to String

The bug is in getTransformingStringReader() which casts the result of stringReader.read() directly to String, but GenericData returns Utf8.

This commit adds a failing test to reproduce the issue.

Verifying this change

Run the test suite.

Documentation

• Does this pull request introduce a new feature? No

[kevinburkesegment]

I'd be happy to try and propose a fix as well but I'm not great at Java

[m8719-github]

Just run into what looks to be a related issue with decimal logical type:

java.lang.ClassCastException: class java.math.BigDecimal cannot be cast to class java.nio.ByteBuffer

This is because the schema has a bytes field annotated with the decimal logical type, but the POJO is generated via maven avro plugin with enableDecimalLogicalType = false (the default) and so the receiving instance field type java.nio.ByteBuffer is miss-matched.

This logic is responsible for attaching conversions based on schema logical type annotations; a possible fix would involve interrogating the receiving instance via reflection for receiving field type and matching this with what's returned from Conversion.getConvertedType() or simply an informational/documentation fix stating that schema logical type annotations have to be accompanied by enableDecimalLogicalType = true setting in the avro compiler.