Is there any clever way to handle null in a JSON body lens I

Question

Is there any clever way to handle null in a JSON body lens I want a Kotlin `null` value to be mapped to from a JSON `null` value

dave · Answer

Assuming you mean Jackson then this is controlled by the underlying configuration of the mapper

Mikael Ståldal · Answer

I am actually using Gson

dave · Answer

Of all of the libraries GSON plays the worst with Kotin I d recommend Moshi if you have a choice upside down face

Mikael Ståldal · Answer

I am using Gson since it is already used in the project Moshi is not and I want to avoid bring in more dependencies than necessary But if Moshi can solve this problem better than Gson then I can consider it Do you have an example on how to do it with Moshi I have not used Moshi before

dave · Answer

+1 on the dependencies slightly smiling face Are we talking about null fields or an entirely null body

Mikael Ståldal · Answer

entirely null body

dave · Answer

and we re talking about an empty string when we say null that s quite a curious response instead of a 404

Mikael Ståldal · Answer

No a JSON null value encoded as string null Otherwise an JSON object

dave · Answer

it s a tricky one for sure I take it you re not in charge of that API it seems pretty broken to me The problem with the auto marshalling is that they are bound to `Any` and not `Any `

dave · Answer

You could write a filter to detect the content length and turn it into a 404

Mikael Ståldal · Answer

Yes I am converting an existing API with existing clients from another framework SparkJava to http4k The API is manually documented here <https gitlab com chromaway postchain blob dev doc restapi postchain restapi yaml>

dave · Answer

``` val client = Filter next gt next it let if it header Content Length == 6 Response NOT FOUND else it then JavaHttpClient ```

dave · Answer

then you can pretend that their API isn t utterly broken joy

Mikael Ståldal · Answer

Well I can t change the existing clients that easily shrug We have clients in JavaScript and other languages as well

dave · Answer

sorry are you trying to recreate this null or consume it

Mikael Ståldal · Answer

Recreate I am working on the server side

dave · Answer

ok you definitely can t do it nicely with the standard lens system because the lens is designed to do nothing if it encounters a null value You ll have to do something manual I m afraid although you can obviously wrap the behaviour up in an reusable extension function somewhere and use that in those endpoints

dave · Answer

The reason why is this line of code in case you re interested slightly smiling face <https github com http4k http4k blob b77bcb2b0788e45437c7b319d100b0d93269846b http4k core src main kotlin org http4k lens body kt L84>

Mikael Ståldal · Answer

Ah OK

Mikael Ståldal · Answer

I did it like this with GSON ```val fooBody = Body auto lt Foo gt toLens val nullBody = Body json null JSON map require it isJsonNull null JSON expected com google gson JsonNull INSTANCE toLens ``` ``` foo fooId bind GET to request gt val fooId = fooIdPath request val foo Foo = fooDB getFoo fooId if foo = null Response OK with fooBody of foo else Response OK with nullBody of Unit ```

Mikael Ståldal · Answer

And it works I even managed to wrap it in a `ContentNegotiation` since I have another format than JSON to serve as well with similar `null` semantics