Empty String deserialized as `null` instead of empty string

[iulianrosca]

Hello,

I have the following issue/question for which I was hoping to find a easy solution.

I have inherited a large codebase where I am trying to migrate from jackson 2.6.4 to 2.9.8. The project contains multiple YAML files which are deserialized using com.fasterxml.jackson.dataformat.yaml.YAMLMapper. Most of these files contain empty fields e.g.

emptyStringField:

With version 2.6.4 these fields were being decoded to empty strings, while after migrating to 2.9.8 the field evaluated to null. I have managed to trace the diverging behavior in com.fasterxml.jackson.dataformat.yaml.YAMLParser#_decodeScalar.
The 2.6 version was evaluating the field to JsonToken.VALUE_STRING while the 2.9 version is using SnakeYaml implicit resolvers and evaluates to JsonToken.VALUE_NULL.

What is the desirable way to keep the behavior from 2.6, deserialize empty fields to empty string instead of null, after migrating to 2.9?

[cowtowncoder]

Hmmh. I do not have a good answer here, but "Null replacement/error/skipping" introduced in 2.9:

https://medium.com/@cowtowncoder/jackson-2-9-features-b2a19029e9ff

would at least allow coercing null into "" in general. It can be enabled for String valued fields, or specific fields.

[stefanleh]

Please also check ... #116

[cowtowncoder]

Finally got around to have another look: fix seems easy, but timing is bit challenging as we are past last RC for 2.12.0. I think, however, that I can add a YAMLParser.Feature, defaulting to false (for 2.12) to allow restoring behavior.

[cowtowncoder]

Fixed this by adding a new feature, YAMLParser.Feature.EMPTY_STRING_AS_NULL, which defaults to true for backwards-compatibility with Jackson 2.11.
It can be disabled to prevent coercion from empty String into null in 2.12; we may consider changing default behavior in a later 2.x version as well as 3.0.