Support deserialization of top-level nullable types

formats/hocon/src/main/kotlin/kotlinx/serialization/hocon/Hocon.kt

@@ -131,6 +131,12 @@ public sealed class Hocon(
             return !conf.getIsNull(tag)
         }
 
+        override fun decodeNotNullMark(): Boolean {
+            // Tag might be null for top-level deserialization
+            val currentTag = currentTagOrNull ?: return !conf.isEmpty
+            return decodeTaggedNotNullMark(currentTag)
+        }
+
         override fun beginStructure(descriptor: SerialDescriptor): CompositeDecoder =
             when {
                 descriptor.kind.listLike -> ListConfigReader(conf.getList(currentTag))

formats/hocon/src/test/kotlin/kotlinx/serialization/hocon/HoconValuesTest.kt

@@ -5,6 +5,7 @@
 package kotlinx.serialization.hocon
 
 import kotlinx.serialization.*
+import kotlinx.serialization.builtins.*
 import org.junit.*
 import org.junit.Assert.*
 
@@ -79,6 +80,19 @@ class HoconValuesTest {
         assertEquals(null, obj.s)
     }
 
+    @Test
+    fun `deserialize nullable types with nullable serializer`() {
+        val obj = deserializeConfig("i = 10, s = null", WithNullable.serializer().nullable)!!
+        assertEquals(10, obj.i)
+        assertEquals(null, obj.s)
+    }
+
+    @Test
+    fun testDeserializerTopLevelNullableType() {
+        val value = deserializeConfig("", WithNullable.serializer().nullable)
+        assertNull(value)
+    }
+
     @Test
     fun `deserialize complex nullable values`() {
         val configString = "i1 = [1,null,3], i2=null, i3 = [null, {i: 10, s: bar}]"

runtime/api/kotlinx-serialization-core.api

@@ -1043,23 +1043,17 @@ public class kotlinx/serialization/internal/Migration : kotlinx/serialization/de
 
 public abstract class kotlinx/serialization/internal/NamedValueDecoder : kotlinx/serialization/internal/TaggedDecoder {
 	public fun <init> ()V
-	public fun <init> (Ljava/lang/String;)V
-	public synthetic fun <init> (Ljava/lang/String;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
 	protected fun composeName (Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;
 	protected fun elementName (Lkotlinx/serialization/descriptors/SerialDescriptor;I)Ljava/lang/String;
-	protected final fun getRootName ()Ljava/lang/String;
 	public synthetic fun getTag (Lkotlinx/serialization/descriptors/SerialDescriptor;I)Ljava/lang/Object;
 	protected final fun getTag (Lkotlinx/serialization/descriptors/SerialDescriptor;I)Ljava/lang/String;
 	protected final fun nested (Ljava/lang/String;)Ljava/lang/String;
 }
 
 public abstract class kotlinx/serialization/internal/NamedValueEncoder : kotlinx/serialization/internal/TaggedEncoder {
 	public fun <init> ()V
-	public fun <init> (Ljava/lang/String;)V
-	public synthetic fun <init> (Ljava/lang/String;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
 	protected fun composeName (Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;
 	protected fun elementName (Lkotlinx/serialization/descriptors/SerialDescriptor;I)Ljava/lang/String;
-	protected final fun getRootName ()Ljava/lang/String;
 	public synthetic fun getTag (Lkotlinx/serialization/descriptors/SerialDescriptor;I)Ljava/lang/Object;
 	protected final fun getTag (Lkotlinx/serialization/descriptors/SerialDescriptor;I)Ljava/lang/String;
 	protected final fun nested (Ljava/lang/String;)Ljava/lang/String;
@@ -1229,7 +1223,7 @@ public abstract class kotlinx/serialization/internal/TaggedDecoder : kotlinx/ser
 	public final fun decodeIntElement (Lkotlinx/serialization/descriptors/SerialDescriptor;I)I
 	public final fun decodeLong ()J
 	public final fun decodeLongElement (Lkotlinx/serialization/descriptors/SerialDescriptor;I)J
-	public final fun decodeNotNullMark ()Z
+	public fun decodeNotNullMark ()Z
 	public final fun decodeNull ()Ljava/lang/Void;
 	public fun decodeNullableSerializableElement (Lkotlinx/serialization/descriptors/SerialDescriptor;ILkotlinx/serialization/DeserializationStrategy;)Ljava/lang/Object;
 	public final fun decodeNullableSerializableElement (Lkotlinx/serialization/descriptors/SerialDescriptor;ILkotlinx/serialization/DeserializationStrategy;Ljava/lang/Object;)Ljava/lang/Object;

runtime/commonMain/src/kotlinx/serialization/internal/NullableSerializer.kt

@@ -13,6 +13,7 @@ import kotlinx.serialization.encoding.*
  * @suppress internal API
  */
 @PublishedApi
+@OptIn(ExperimentalSerializationApi::class)
 internal class NullableSerializer<T : Any>(private val serializer: KSerializer<T>) : KSerializer<T?> {
     override val descriptor: SerialDescriptor = SerialDescriptorForNullable(serializer.descriptor)
 
@@ -42,6 +43,7 @@ internal class NullableSerializer<T : Any>(private val serializer: KSerializer<T
     }
 }
 
+@OptIn(ExperimentalSerializationApi::class)
 internal class SerialDescriptorForNullable(internal val original: SerialDescriptor) : SerialDescriptor by original {
     override val serialName: String = original.serialName + "?"
     override val isNullable: Boolean

runtime/commonMain/src/kotlinx/serialization/internal/Tagged.kt

@@ -10,9 +10,10 @@ import kotlinx.serialization.descriptors.*
 import kotlinx.serialization.encoding.*
 import kotlinx.serialization.encoding.updateModeDeprecated
 import kotlinx.serialization.modules.*
+
 /*
  * These classes are intended to be used only within the kotlinx.serialization.
- * They neither do have stable API, not internal invariants and are changed without any warnings.
+ * They neither do have stable API, nor internal invariants and are changed without any warnings.
  */
 @InternalSerializationApi
 public abstract class TaggedEncoder<Tag : Any?> : Encoder, CompositeEncoder {
@@ -32,7 +33,6 @@ public abstract class TaggedEncoder<Tag : Any?> : Encoder, CompositeEncoder {
 
     protected open fun encodeTaggedNotNullMark(tag: Tag) {}
     protected open fun encodeTaggedNull(tag: Tag): Unit = throw SerializationException("null is not supported")
-
     protected open fun encodeTaggedInt(tag: Tag, value: Int): Unit = encodeTaggedValue(tag, value)
     protected open fun encodeTaggedByte(tag: Tag, value: Byte): Unit = encodeTaggedValue(tag, value)
     protected open fun encodeTaggedShort(tag: Tag, value: Short): Unit = encodeTaggedValue(tag, value)
@@ -167,9 +167,10 @@ public abstract class TaggedEncoder<Tag : Any?> : Encoder, CompositeEncoder {
 }
 
 @InternalSerializationApi
-public abstract class NamedValueEncoder(protected val rootName: String = "") : TaggedEncoder<String>() {
+@OptIn(ExperimentalSerializationApi::class)
+public abstract class NamedValueEncoder : TaggedEncoder<String>() {
     final override fun SerialDescriptor.getTag(index: Int): String = nested(elementName(this, index))
-    protected fun nested(nestedName: String): String = composeName(currentTagOrNull ?: rootName, nestedName)
+    protected fun nested(nestedName: String): String = composeName(currentTagOrNull ?: "", nestedName)
     protected open fun elementName(descriptor: SerialDescriptor, index: Int): String = descriptor.getElementName(index)
     protected open fun composeName(parentName: String, childName: String): String =
         if (parentName.isEmpty()) childName else "$parentName.$childName"
@@ -214,7 +215,13 @@ public abstract class TaggedDecoder<Tag : Any?> : Decoder,
 
     // ---- Implementation of low-level API ----
 
-    final override fun decodeNotNullMark(): Boolean = decodeTaggedNotNullMark(currentTag)
+    // TODO this method should be overridden by any sane format that supports top-level nulls
+    override fun decodeNotNullMark(): Boolean {
+        // Tag might be null for top-level deserialization
+        val currentTag = currentTagOrNull ?: return false
+        return decodeTaggedNotNullMark(currentTag)
+    }
+
     final override fun decodeNull(): Nothing? = null
 
     final override fun decodeBoolean(): Boolean = decodeTaggedBoolean(popTag())
@@ -327,10 +334,11 @@ public abstract class TaggedDecoder<Tag : Any?> : Decoder,
 }
 
 @InternalSerializationApi
-public abstract class NamedValueDecoder(protected val rootName: String = "") : TaggedDecoder<String>() {
+@OptIn(ExperimentalSerializationApi::class)
+public abstract class NamedValueDecoder : TaggedDecoder<String>() {
     final override fun SerialDescriptor.getTag(index: Int): String = nested(elementName(this, index))
 
-    protected fun nested(nestedName: String): String = composeName(currentTagOrNull ?: rootName, nestedName)
+    protected fun nested(nestedName: String): String = composeName(currentTagOrNull ?: "", nestedName)
     protected open fun elementName(desc: SerialDescriptor, index: Int): String = desc.getElementName(index)
     protected open fun composeName(parentName: String, childName: String): String =
         if (parentName.isEmpty()) childName else "$parentName.$childName"

runtime/commonMain/src/kotlinx/serialization/json/internal/TreeJsonDecoder.kt

@@ -78,6 +78,8 @@ private sealed class AbstractJsonTreeDecoder(
         // Nothing
     }
 
+    override fun decodeNotNullMark(): Boolean = currentObject() !is JsonNull
+
     protected open fun getValue(tag: String): JsonPrimitive {
         val currentElement = currentElement(tag)
         return currentElement as? JsonPrimitive ?: throw JsonDecodingException(

runtime/commonMain/src/kotlinx/serialization/json/internal/TreeJsonEncoder.kt

@@ -21,6 +21,7 @@ internal fun <T> Json.writeJson(value: T, serializer: SerializationStrategy<T>):
     return result
 }
 
+@ExperimentalSerializationApi
 private sealed class AbstractJsonTreeEncoder(
     final override val json: Json,
     val nodeConsumer: (JsonElement) -> Unit
@@ -186,9 +187,13 @@ private class JsonTreeListEncoder(json: Json, nodeConsumer: (JsonElement) -> Uni
     override fun getCurrent(): JsonElement = JsonArray(array)
 }
 
+@OptIn(ExperimentalSerializationApi::class)
 internal inline fun <reified T : JsonElement> cast(value: JsonElement, descriptor: SerialDescriptor): T {
     if (value !is T) {
-        throw JsonDecodingException(-1, "Expected ${T::class} as the serialized body of ${descriptor.serialName}, but had ${value::class}")
+        throw JsonDecodingException(
+            -1,
+            "Expected ${T::class} as the serialized body of ${descriptor.serialName}, but had ${value::class}"
+        )
     }
     return value
 }

runtime/commonTest/src/kotlinx/serialization/json/DecodeFromJsonElementTest.kt

@@ -0,0 +1,55 @@
+package kotlinx.serialization.json
+
+import kotlinx.serialization.*
+import kotlin.test.*
+
+class DecodeFromJsonElementTest {
+    @Serializable
+    data class A(val a: Int)
+
+    @Serializable
+    data class B(val a: A?)
+
+    @Test
+    fun testDecodeTopLevelNullable() {
+        val a = A(42)
+        val jsonElement = Json.encodeToJsonElement(a)
+        assertEquals(a, Json.decodeFromJsonElement<A?>(jsonElement))
+    }
+
+    @Test
+    fun topLevelNull() {
+        assertNull(Json.decodeFromJsonElement<A?>(JsonNull))
+    }
+
+    @Test
+    fun testInnerNullable() {
+        val b = B(A(42))
+        val json = Json.encodeToJsonElement(b)
+        assertEquals(b, Json.decodeFromJsonElement(json))
+    }
+
+    @Test
+    fun testInnerNullableNull() {
+        val b = B(null)
+        val json = Json.encodeToJsonElement(b)
+        assertEquals(b, Json.decodeFromJsonElement(json))
+    }
+
+    @Test
+    fun testPrimitive() {
+        assertEquals(42, Json.decodeFromJsonElement(JsonPrimitive(42)))
+        assertEquals(42, Json.decodeFromJsonElement<Int?>(JsonPrimitive(42)))
+        assertEquals(null, Json.decodeFromJsonElement<Int?>(JsonNull))
+    }
+
+    @Test
+    fun testNullableList() {
+        assertEquals(listOf(42), Json.decodeFromJsonElement<List<Int>?>(JsonArray(listOf(JsonPrimitive(42)))))
+        assertEquals(listOf(42), Json.decodeFromJsonElement<List<Int?>?>(JsonArray(listOf(JsonPrimitive(42)))))
+        assertEquals(listOf(42), Json.decodeFromJsonElement<List<Int?>>(JsonArray(listOf(JsonPrimitive(42)))))
+        // Nulls
+        assertEquals(null, Json.decodeFromJsonElement<List<Int>?>(JsonNull))
+        assertEquals(null, Json.decodeFromJsonElement<List<Int?>?>(JsonNull))
+    }
+}