SafeBooleanTypeAdapter for Kotlin

BooleanTypeAdapterTest.kt

class BooleanTypeAdapterTest {

    @Test
    fun testBooleanTypeAdapter() {
        val gson = GsonBuilder()
            .registerTypeAdapter(Boolean::class.javaObjectType, BooleanObjectTypeAdapter())
            .registerTypeAdapter(Boolean::class.javaPrimitiveType, BooleanPrimitiveTypeAdapter())
            .create()

        assertEquals(null, gson.fromJson("""{"isBoolean"=null}""", Foo::class.java).isBoolean)
        assertEquals(false, gson.fromJson("""{"isBoolean"=false}""", Foo::class.java).isBoolean)
        assertEquals(true, gson.fromJson("""{"isBoolean"=true}""", Foo::class.java).isBoolean)
        assertEquals(true, gson.fromJson("""{"isBoolean"="true"}""", Foo::class.java).isBoolean)
        assertEquals(true, gson.fromJson("""{"isBoolean"="TrUe"}""", Foo::class.java).isBoolean)
        assertEquals(false, gson.fromJson("""{"isBoolean"="false"}""", Foo::class.java).isBoolean)
        assertEquals(false, gson.fromJson("""{"isBoolean"="FaLse"}""", Foo::class.java).isBoolean)
        assertEquals(false, gson.fromJson("""{"isBoolean"=""}""", Foo::class.java).isBoolean)
        assertEquals(false, gson.fromJson("""{"isBoolean"="0"}""", Foo::class.java).isBoolean)
        assertEquals(true, gson.fromJson("""{"isBoolean"="1"}""", Foo::class.java).isBoolean)
        assertEquals(false, gson.fromJson("""{"isBoolean"="2"}""", Foo::class.java).isBoolean)
        assertEquals(false, gson.fromJson("""{"isBoolean"=0}""", Foo::class.java).isBoolean)
        assertEquals(true, gson.fromJson("""{"isBoolean"=1}""", Foo::class.java).isBoolean)
        assertEquals(false, gson.fromJson("""{"isBoolean"=2}""", Foo::class.java).isBoolean)
        assertEquals(false, gson.fromJson("""{"isBoolean"=[1,2]}""", Foo::class.java).isBoolean)
        assertEquals(false, gson.fromJson("""{"isBoolean"={}}""", Foo::class.java).isBoolean)
        assertEquals(false, gson.fromJson("""{"isBoolean"={"a"="b"}}""", Foo::class.java).isBoolean)

        assertEquals(false, gson.fromJson("""{"isBoolean"=null}""", Bar::class.java).isBoolean)
        assertEquals(false, gson.fromJson("""{"isBoolean"=false}""", Bar::class.java).isBoolean)
        assertEquals(true, gson.fromJson("""{"isBoolean"=true}""", Bar::class.java).isBoolean)
        assertEquals(true, gson.fromJson("""{"isBoolean"="true"}""", Bar::class.java).isBoolean)
        assertEquals(true, gson.fromJson("""{"isBoolean"="TrUe"}""", Bar::class.java).isBoolean)
        assertEquals(false, gson.fromJson("""{"isBoolean"="false"}""", Bar::class.java).isBoolean)
        assertEquals(false, gson.fromJson("""{"isBoolean"="FaLse"}""", Bar::class.java).isBoolean)
        assertEquals(false, gson.fromJson("""{"isBoolean"=""}""", Bar::class.java).isBoolean)
        assertEquals(false, gson.fromJson("""{"isBoolean"="0"}""", Bar::class.java).isBoolean)
        assertEquals(true, gson.fromJson("""{"isBoolean"="1"}""", Bar::class.java).isBoolean)
        assertEquals(false, gson.fromJson("""{"isBoolean"="2"}""", Bar::class.java).isBoolean)
        assertEquals(false, gson.fromJson("""{"isBoolean"=0}""", Bar::class.java).isBoolean)
        assertEquals(true, gson.fromJson("""{"isBoolean"=1}""", Bar::class.java).isBoolean)
        assertEquals(false, gson.fromJson("""{"isBoolean"=2}""", Bar::class.java).isBoolean)
        assertEquals(false, gson.fromJson("""{"isBoolean"=[1,2]}""", Foo::class.java).isBoolean)
        assertEquals(false, gson.fromJson("""{"isBoolean"={"a"="b"}}""", Foo::class.java).isBoolean)
    }

    data class Foo(var isBoolean: Boolean? = null)
    data class Bar(var isBoolean: Boolean = false)
}

GsonModule.kt

@Module
class GsonModule {
    
    @Singleton
    @Provides
    fun provideGson(): Gson {
        return GsonBuilder()
            .registerTypeAdapter(Boolean::class.javaObjectType, BooleanObjectTypeAdapter())
            .registerTypeAdapter(Boolean::class.javaPrimitiveType, BooleanPrimitiveTypeAdapter())
            .create()
    }
}

SafeBooleanTypeAdapter.kt

private val TRUE_STRINGS: Array<String> = arrayOf("true", "1")

class BooleanObjectTypeAdapter : JsonDeserializer<Boolean?>, JsonSerializer<Boolean?> {

    override fun serialize(src: Boolean?, typeOfSrc: Type?, context: JsonSerializationContext?): JsonElement {
        return if (src == null) {
            JsonNull.INSTANCE
        } else {
            JsonPrimitive(src)
        }
    }

    override fun deserialize(json: JsonElement?, typeOfT: Type?, context: JsonDeserializationContext?): Boolean? {
        if (json == null || json.isJsonNull) {
            return null
        }

        return when {
            json !is JsonPrimitive -> false
            json.isBoolean -> json.asBoolean
            json.isNumber -> json.asNumber.toInt() == 1
            json.isString -> TRUE_STRINGS.contains(json.asString.lowercase())
            else -> false
        }
    }
}

class BooleanPrimitiveTypeAdapter : JsonDeserializer<Boolean>, JsonSerializer<Boolean> {

    override fun serialize(src: Boolean?, typeOfSrc: Type?, context: JsonSerializationContext?): JsonElement {
        return if (src == null) {
            JsonPrimitive(false)
        } else {
            JsonPrimitive(src)
        }
    }

    override fun deserialize(json: JsonElement?, typeOfT: Type?, context: JsonDeserializationContext?): Boolean {
        if (json == null || json.isJsonNull) {
            return false
        }

        return when {
            json !is JsonPrimitive -> false
            json.isBoolean -> json.asBoolean
            json.isNumber -> json.asNumber.toInt() == 1
            json.isString -> TRUE_STRINGS.contains(json.asString.lowercase())
            else -> false
        }
    }
}