问题
I'm attempting to parse json from the GitHub API with play-json, and encountering a problem with the merge_commit_sha field on Pull Requests (incidentally, I know this field is deprecated, but don't want to discuss that in this parsing problem!). Unfortunately merge_commit_sha field comes back as the empty string in some cases:
"merge_commit_sha": ""
This is how the field is declared in my case class:
merge_commit_sha: Option[ObjectId],
I have an implicit Format[ObjectId], which does not tolerate the empty string, because that's not a valid value for a Git hash id. I'm also using a play-json macro-generated Read[PullRequest], which I'd like to keep on using, in preference to individually declaring reads for every single field on pull requests.
As I've declared the field to be an Option, I'd like "merge_commit_sha": "" to be read as the value None, but this is not what currently happens - a string is present, so the Format[ObjectId] is invoked, and returns a JsFailure.
One thing I tried was declaring an implicit Format[Option[ObjectId]] with the required behaviour, but it didn't seem to get used by the macro-generated Read[PullRequest].
回答1:
You can define a custom Reads and Writes yourself.
Using Json.format[MyType] uses a Scala macro. You may be able to hook into that. Although, 'extending' a macro for this one case class just seems wrong.
Custom Reads and Writes might be a little 'boilerplate-like' and boring, but they have their upsides.
For example if your json has a bunch of new fields on it, you wont get a JsError when validating or transforming it to a case class. You only take what you need from the JSON and create objects. It also allows for a separation between your internal model and what you're consuming, which in some cases is preferred.
I hope this helps,
Rhys
EDIT
After using some other JSON libs I may have found what you are looking for. I know the question was asking specifically after Play JSON.
If you're able to move away from Play JSON, Look at spray-json-shapeless specifically JsNullBehaviour and JsNullNotNone REF.
来源：https://stackoverflow.com/questions/37442144/make-play-json-read-the-empty-string-as-none-for-a-type-of-optiont