collection deserializer should create empty collection instead of null

Author: pjfanning

src/main/java/com/fasterxml/jackson/module/scala/deser/ContainerDeserializerWithNullValueAsEmpty.java

@@ -0,0 +1,21 @@
+package com.fasterxml.jackson.module.scala.deser;
+
+import com.fasterxml.jackson.databind.DeserializationContext;
+import com.fasterxml.jackson.databind.JavaType;
+import com.fasterxml.jackson.databind.JsonMappingException;
+import com.fasterxml.jackson.databind.deser.std.ContainerDeserializerBase;
+
+/**
+ * Internal Usage only
+ */
+abstract class ContainerDeserializerWithNullValueAsEmpty<T> extends ContainerDeserializerBase<T> {
+
+    protected ContainerDeserializerWithNullValueAsEmpty(JavaType selfType) {
+        super(selfType);
+    }
+
+    @Override
+    public T getNullValue(DeserializationContext ctxt) throws JsonMappingException {
+        return (T) getEmptyValue(ctxt);
+    }
+}

src/main/scala/com/fasterxml/jackson/module/scala/deser/GenericFactoryDeserializerResolver.scala

@@ -4,7 +4,7 @@ import java.util
 import com.fasterxml.jackson.core.JsonParser
 import com.fasterxml.jackson.databind._
 import com.fasterxml.jackson.databind.`type`.CollectionLikeType
-import com.fasterxml.jackson.databind.deser.std.{CollectionDeserializer, ContainerDeserializerBase, StdValueInstantiator}
+import com.fasterxml.jackson.databind.deser.std.{CollectionDeserializer, StdValueInstantiator}
 import com.fasterxml.jackson.databind.deser.{ContextualDeserializer, Deserializers, ValueInstantiator}
 import com.fasterxml.jackson.databind.jsontype.TypeDeserializer
 
@@ -105,7 +105,7 @@ abstract class GenericFactoryDeserializerResolver[CC[_], CF[X[_]]] extends Deser
   }
 
   private class Deserializer[A](collectionType: JavaType, containerDeserializer: CollectionDeserializer)
-    extends ContainerDeserializerBase[CC[A]](collectionType)
+    extends ContainerDeserializerWithNullValueAsEmpty[CC[A]](collectionType)
       with ContextualDeserializer {
 
     def this(collectionType: JavaType, valueDeser: JsonDeserializer[Object], valueTypeDeser: TypeDeserializer, valueInstantiator: ValueInstantiator) = {

src/main/scala/com/fasterxml/jackson/module/scala/deser/GenericMapFactoryDeserializerResolver.scala

@@ -4,7 +4,7 @@ import com.fasterxml.jackson.core.{JsonParser, StreamReadCapability}
 import com.fasterxml.jackson.databind._
 import com.fasterxml.jackson.databind.`type`.MapLikeType
 import com.fasterxml.jackson.databind.deser.{ContextualDeserializer, Deserializers, ValueInstantiator}
-import com.fasterxml.jackson.databind.deser.std.{ContainerDeserializerBase, MapDeserializer, StdValueInstantiator}
+import com.fasterxml.jackson.databind.deser.std.{MapDeserializer, StdValueInstantiator}
 import com.fasterxml.jackson.databind.jsontype.TypeDeserializer
 
 import scala.collection.mutable.ListBuffer
@@ -124,7 +124,7 @@ abstract class GenericMapFactoryDeserializerResolver[CC[K, V], CF[X[_, _]]] exte
   }
 
   private class Deserializer[K, V](mapType: MapLikeType, containerDeserializer: MapDeserializer)
-    extends ContainerDeserializerBase[CC[K, V]](mapType) with ContextualDeserializer {
+    extends ContainerDeserializerWithNullValueAsEmpty[CC[K, V]](mapType) with ContextualDeserializer {
 
     def this(mapType: MapLikeType, valueInstantiator: ValueInstantiator,
              keyDeser: KeyDeserializer, valueDeser: JsonDeserializer[_], valueTypeDeser: TypeDeserializer) = {

src/test/scala/com/fasterxml/jackson/module/scala/deser/CaseClassDeserializerTest.scala

@@ -200,7 +200,8 @@ class CaseClassDeserializerTest extends DeserializerTest {
   it should "support deserializing null input for list as empty list" in {
     val input = """{}"""
     val result = deserialize(input, classOf[ListHolder[String]])
-    result.list shouldBe null // ideally should be empty list, Scala users expect no nulls
+    // result.list used to be null until v2.19.0
+    result.list shouldBe List.empty
   }
 
   it should "support deserializing null input for list as empty list (JsonSetter annotation)" in {