bodduv opened a new pull request, #1137:
URL: https://github.com/apache/arrow-java/pull/1137
### Background
`ComplexCopier.copy` does a deep-copy using a `FieldReader` from the source
and a `FieldWriter` to write to the destination. On the read side in the
`EXTENSIONTYPE` branch, it uses `reader.getField().getType()` to get the
`ArrowType` it needs to forward to `writer.writeExtension(value, type)`.
Vector-backed extension readers (UuidReaderImpl, VariantReaderImpl) override
getField(), so this works. Holder readers (NullableUuidHolderReaderImpl,
NullableVariantHolderReaderImpl) on the other hand do not.
`AbstractFieldReader.getField` throws via `fail("getField")`, so any
ComplexCopier.copy call with a holder reader as input fails.
As #1109 points out, overriding getField() on a holder reader is a simple
fix but a holder does not have a notion of `Field`, the same way primitive (as
opposed to extension) holder readers also have no notion of `getField()` and
the method is not overriden.
## What's Changed
`ComplexCopier` only needs the `ArrowType` here, not the whole Field. So
this PR adds a narrower accessor for that:
```java
default ArrowType getExtensionType() {
return getField().getType();
}
```
on `ExtensionReader`. The default delegates to `getField()`, so every
existing vector-backed extension reader keeps working unchanged. Holder readers
override `getExtensionType()` to return `holder.type()` — the
`ExtensionHolder.type()` method added in #892 already returns the right
ArrowType (UuidType.INSTANCE, VariantType.INSTANCE).
ComplexCopier call `reader.getExtensionType()` instead of
`reader.getField().getType()`. The new method goes on ExtensionReader rather
than AbstractFieldReader to sit next to the other extension-specific methods
already there (read(ExtensionHolder), readObject(), isSet()). Adding a default
method is source- and binary-compatible.
Closes #1109 .
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