readValue() for empty YAML returns null, not empty map

[damianszczepanik]

When passing empty YAML file into ObjectMapper.readValue() then null is returned, not empty map.
Example:
https://github.com/damianszczepanik/silencio/blob/master/src/main/java/pl/szczepanik/silencio/processors/YAMLProcessor.java#L44
Input file:
https://github.com/damianszczepanik/silencio/blob/master/src/test/resources/yaml/empty.yaml

When passing empty JSON {} this returns empty map so I would except to have same for YAML

[cowtowncoder]

Case of { } is little bit different, since that is explicit Object, whereas I am not even sure what empty content for YAML should be. But I can see how it could be interpreted as empty Document.
I wonder what SnakeYAML itself sees such content as.

But it is possible that existing Jackson YAML backend is handling it before. So I hope to look into this in near future to see what is going on.

[cowtowncoder]

I am not sure there is something to change here: empty Map or POJO would be serialized as

--- {}

(or without document start indicator --- if so configured)

and empty content is parsed by SnakeYAML so that there is Stream start, Stream end events, but no Document events to indicate existence of document.
So to me returning null seems like the right thing.

[damianszczepanik]

But then it should be consistent with JSON mapper which AFAIR is not

[cowtowncoder]

@damianszczepanik How so? Passing empty file to JSON will not give you equivalent of { } but either exception (if trying to bind content into POJO), or null with streaming read using JsonParser.

[rehevkor5]

Appears to me it doesn't even give null, it throws an exception:

com.fasterxml.jackson.databind.exc.MismatchedInputException: No content to map due to end-of-input
 at [Source: (StringReader); line: 1, column: 1]

	at [email protected]/com.fasterxml.jackson.databind.exc.MismatchedInputException.from(MismatchedInputException.java:59)
	at [email protected]/com.fasterxml.jackson.databind.ObjectMapper._initForReading(ObjectMapper.java:4688)
	at [email protected]/com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4586)
	at [email protected]/com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3548)
	at [email protected]/com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3531)
	
...

Seems to me like an empty YAML should be valid.

[cowtowncoder]

@rehevkor5 this is unfortunately quite tricky an area; but I would strongly suggest that anyone who needs to produce empty documents makes sure that YAML document start/separator marker (---) is used.