jhrotko commented on code in PR #892:
URL: https://github.com/apache/arrow-java/pull/892#discussion_r2514056901
##########
vector/src/main/codegen/templates/PromotableWriter.java:
##########
@@ -325,6 +325,9 @@ protected boolean requiresArrowType(MinorType type) {
@Override
protected FieldWriter getWriter(MinorType type, ArrowType arrowType) {
+ if(type == MinorType.EXTENSIONTYPE) {
Review Comment:
This was required to provide the interface shown in tests for all vector
types. This was specially more tricky for Structs and Union writers where they
own more than one writer because the fields can be different types (lists,
ints, nested, etc). Given the way, writers interface works, where first it is
specified which "type" is going to be written to get the type:
```java
writer.extension(UuidType.INSTANCE);
writer.writeExtension(new UUID(...));
```
The best way to understand the flow is to run the test
`TestPromotableWriter.testPromoteToUnion`
```
mvn test -Dtest=TestPromotableWriter#testPromoteToUnion \
-Dcheckstyle.skip=true -Dmaven.surefire.debug="-Xdebug
-Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=5005" \
-Derrorprone.skip=true
```
we will hit the if condition:
The `lastExtensionType` is set in `NullableStructWriter` in:
```java
@Override
public ExtensionWriter extension(String name, ArrowType arrowType) {
String finalName = handleCase(name);
FieldWriter writer = fields.get(finalName);
if(writer == null){
...
} else {
if (writer instanceof PromotableWriter) {
// ensure writers are initialized
((PromotableWriter)writer).getWriter(MinorType.EXTENSIONTYPE,
arrowType); <---
}
}
return (ExtensionWriter) writer;
}
```
which will "save" the current "active" Extension type in the line where you
pointed out your comment.
Then when writing the extension, it is necessary to know why extension type
it's required because UnionWriter has several writers (list, map, etc) and the
extension types can be many (besides UUID). This is the method call at this
point of execution:
```java
@Override
public void writeExtension(Object value) {
FieldWriter writer = getWriter(MinorType.EXTENSIONTYPE,
lastExtensionType);
if(writer instanceof UnionWriter) { <---
((UnionWriter) writer).writeExtension(value, lastExtensionType);
} else {
writer.writeExtension(value);
}
}
```
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