Gson serialize null for specific class or field

Question

I want to serialize nulls for a specific field or class.

In GSON, the option serializeNulls() applies to the whole JSON.

Example:

class MainClass {     public String id;     public String name;     public Test test; }  class Test {     public String name;     public String value;     }   MainClass mainClass = new MainClass(); mainClass.id = "101" // mainClass has no name. Test test = new Test(); test.name = "testName"; test.value = null; mainClass.test = test;     

Creating JSON using GSON:

GsonBuilder builder = new GsonBuilder().serializeNulls(); Gson gson = builder.create(); System.out.println(gson.toJson(mainClass)); 

Current ouput:

{     "id": "101",     "name": null,     "test": {         "name": "testName",         "value": null     } } 

Desired output:

{     "id": "101",     "test": {         "name": "testName",         "value": null     } } 

How to achieve the desired output?

Preferred solution would have the following properties:

• Do NOT serialize nulls by default,
• Serialize nulls for fields with a specific annotation.

Answer 1

I have a solution similar to the one of Aleksey but that can be applied to one or more fields in any class (example in Kotlin):

Create a new annotation for fields that should be serialized as null:

@Retention(AnnotationRetention.RUNTIME) @Target(AnnotationTarget.FIELD) annotation class SerializeNull 

Create a TypeAdapterFactory that checks if a class has fields annotated with this annotation and removes the fields that are null and not annotated with the annotation from the JsonTree when writing the object:

class SerializableAsNullConverter : TypeAdapterFactory {      override fun <T : Any?> create(gson: Gson, type: TypeToken<T>): TypeAdapter<T>? {         fun Field.serializedName() = declaredAnnotations             .filterIsInstance<SerializedName>()             .firstOrNull()?.value ?: name         val declaredFields = type.rawType.declaredFields         val nullableFieldNames = declaredFields             .filter { it.declaredAnnotations.filterIsInstance<SerializeNull>().isNotEmpty() }             .map { it.serializedName() }         val nonNullableFields = declaredFields.map { it.serializedName() } - nullableFieldNames          return if (nullableFieldNames.isEmpty()) {             null         } else object : TypeAdapter<T>() {             private val delegateAdapter = gson.getDelegateAdapter(this@SerializableAsNullConverter, type)             private val elementAdapter = gson.getAdapter(JsonElement::class.java)              override fun write(writer: JsonWriter, value: T?) {                 val jsonObject = delegateAdapter.toJsonTree(value).asJsonObject                 nonNullableFields                     .filter { jsonObject.get(it) is JsonNull }                     .forEach { jsonObject.remove(it) }                 val originalSerializeNulls = writer.serializeNulls                 writer.serializeNulls = true                 elementAdapter.write(writer, jsonObject)                 writer.serializeNulls = originalSerializeNulls             }              override fun read(reader: JsonReader): T {                 return delegateAdapter.read(reader)             }         }     } } 

Register the adapter with your Gson instance:

val builder = GsonBuilder().registerTypeAdapterFactory(SerializableAsNullConverter()) 

And annotate the fields you would like to be nullable:

class MyClass(val id: String?, @SerializeNull val name: String?) 

Serialization result:

val myClass = MyClass(null, null) val gson = builder.create() val json = gson.toJson(myClass) 

json:

{     "name": null }