Kotlinx Serialization - Custom serializer to ignore null value - kotlin

Question

Let's say I'm having a class like:
#Serializable
data class MyClass(
#SerialName("a") val a: String?,
#SerialName("b") val b: String
)
Assume the a is null and b's value is "b value", then Json.stringify(MyClass.serializer(), this) produces:
{ "a": null, "b": "b value" }
Basically if a is null, I wanted to get this:
{ "b": "b value" }
From some research I found this is currently not doable out of the box with Kotlinx Serialization so I was trying to build a custom serializer to explicitly ignore null value. I followed the guide from here but couldn't make a correct one.
Can someone please shed my some light? Thanks.

Answer 1

You can use explicitNulls = false
example:
#OptIn(ExperimentalSerializationApi::class)
val format = Json { explicitNulls = false }
#Serializable
data class Project(
val name: String,
val language: String,
val version: String? = "1.3.0",
val website: String?,
)
fun main() {
val data = Project("kotlinx.serialization", "Kotlin", null, null)
val json = format.encodeToString(data)
println(json) // {"name":"kotlinx.serialization","language":"Kotlin"}
}
https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/json.md#explicit-nulls

Answer 2

Use encodeDefaults = false property in JsonConfiguration and it won't serialize nulls (or other optional values)

Answer 3

Try this (not tested, just based on adapting the example):
#Serializable
data class MyClass(val a: String?, val b: String) {
#Serializer(forClass = MyClass::class)
companion object : KSerializer<MyClass> {
override val descriptor: SerialDescriptor = object : SerialClassDescImpl("MyClass") {
init {
addElement("a")
addElement("b")
}
}
override fun serialize(encoder: Encoder, obj: MyClass) {
encoder.beginStructure(descriptor).run {
obj.a?.let { encodeStringElement(descriptor, 0, obj.a) }
encodeStringElement(descriptor, 1, obj.b)
endStructure(descriptor)
}
}
override fun deserialize(decoder: Decoder): MyClass {
var a: String? = null
var b = ""
decoder.beginStructure(descriptor).run {
loop# while (true) {
when (val i = decodeElementIndex(descriptor)) {
CompositeDecoder.READ_DONE -> break#loop
0 -> a = decodeStringElement(descriptor, i)
1 -> b = decodeStringElement(descriptor, i)
else -> throw SerializationException("Unknown index $i")
}
}
endStructure(descriptor)
}
return MyClass(a, b)
}
}
}

Answer 4

Since I was also struggling with this one let me share with you the solution I found that is per property and does not require to create serializer for the whole class.
class ExcludeIfNullSerializer : KSerializer<String?> {
override fun deserialize(decoder: Decoder): String {
return decoder.decodeString()
}
override val descriptor: SerialDescriptor
get() = PrimitiveSerialDescriptor("ExcludeNullString", PrimitiveKind.STRING)
override fun serialize(encoder: Encoder, value: String?) {
if (value != null) {
encoder.encodeString(value)
}
}
}
will work as expected with the following class
#Serializable
class TestDto(
#SerialName("someString")
val someString: String,
#SerialName("id")
#EncodeDefault(EncodeDefault.Mode.NEVER)
#Serializable(with = ExcludeIfNullSerializer::class)
val id: String? = null
)
Note the #EncodeDefault(EncodeDefault.Mode.NEVER) is crucial here in case you using JsonBuilder with encodeDefaults = true, as in this case the serialization library will still add the 'id' json key even if the value of id field is null unless using this annotation.

Answer 5

JsonConfiguration is deprecated in favor of Json {} builder since kotlinx.serialization 1.0.0-RC according to its changelog.
Now you have to code like this:
val json = Json { encodeDefaults = false }
val body = json.encodeToString(someSerializableObject)

Answer 6

As of now, for anyone seeing this pos today, default values are not serialized (see https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/basic-serialization.md#defaults-are-not-encoded-by-default)
So you simply add to set a default null value, and it will not be serialized.