Fix #2077

release-notes/CREDITS-2.x

@@ -341,6 +341,9 @@ Sadayuki Furuhashi (frsyuki@github)
   * Reported #941: Deserialization from "{}" to ObjectNode field causes
     "out of END_OBJECT token" error
    (2.6.3)
+  * Reported #2077: `JsonTypeInfo` with a subtype having `JsonFormat.Shape.ARRAY`
+    and no fields generates `{}` not `[]`
+   (2.10.0)
 
 David Haraburda (dharaburda@github)
   * Contributed #918: Add `MapperFeature.ALLOW_EXPLICIT_PROPERTY_RENAMING`

release-notes/VERSION-2.x

@@ -6,6 +6,9 @@ Project: jackson-databind
 
 2.10.0 (not yet released)
 
+#2077: `JsonTypeInfo` with a subtype having `JsonFormat.Shape.ARRAY` and
+  no fields generates `{}` not `[]`
+ (reported by Sadayuki F)
 #2466: Didn't find class "java.nio.file.Path" below Android api 26
  (reported by KevynBct@github)
 #2467: Accept `JsonTypeInfo.As.WRAPPER_ARRAY` with no second argument to

src/main/java/com/fasterxml/jackson/databind/ser/BeanSerializer.java

@@ -75,14 +75,25 @@ protected BeanSerializer(BeanSerializerBase src, Set<String> toIgnore) {
      */
 
     /**
-     * Method for constructing dummy bean serializer; one that
-     * never outputs any properties
+     * @deprecated Since 2.10
      */
+    @Deprecated
     public static BeanSerializer createDummy(JavaType forType)
     {
         return new BeanSerializer(forType, null, NO_PROPS, null);
     }
 
+    /**
+     * Method for constructing dummy bean serializer; one that
+     * never outputs any properties
+     *
+     * @since 2.10
+     */
+    public static BeanSerializer createDummy(JavaType forType, BeanSerializerBuilder builder)
+    {
+        return new BeanSerializer(forType, builder, NO_PROPS, null);
+    }
+
     @Override
     public JsonSerializer<Object> unwrappingSerializer(NameTransformer unwrapper) {
         return new UnwrappingBeanSerializer(this, unwrapper);

src/main/java/com/fasterxml/jackson/databind/ser/BeanSerializerBuilder.java

@@ -223,7 +223,8 @@ public JsonSerializer<?> build()
      * type information)
      */
     public BeanSerializer createDummy() {
-        return BeanSerializer.createDummy(_beanDesc.getType());
+        // 20-Sep-2019, tatu: Can not skimp on passing builder  (see [databind#2077])
+        return BeanSerializer.createDummy(_beanDesc.getType(), this);
     }
 }
 

src/main/java/com/fasterxml/jackson/databind/ser/std/BeanSerializerBase.java

@@ -114,6 +114,8 @@ protected BeanSerializerBase(JavaType type, BeanSerializerBuilder builder,
         _props = properties;
         _filteredProps = filteredProperties;
         if (builder == null) { // mostly for testing
+            // 20-Sep-2019, tatu: Actually not just that but also "dummy" serializer for
+            //     case of no bean properties, too
             _typeId = null;
             _anyGetterWriter = null;
             _propertyFilterId = null;
@@ -414,7 +416,7 @@ public JsonSerializer<?> createContextual(SerializerProvider provider,
         final AnnotatedMember accessor = (property == null || intr == null)
                 ? null : property.getMember();
         final SerializationConfig config = provider.getConfig();
-        
+
         // Let's start with one big transmutation: Enums that are annotated
         // to serialize as Objects may want to revert
         JsonFormat.Value format = findFormatOverrides(provider, property, handledType());

src/test/java/com/fasterxml/jackson/failing/TestPOJOAsArrayPolymorphic2077.java → src/test/java/com/fasterxml/jackson/databind/struct/TestPOJOAsArrayPolymorphic.java

@@ -1,12 +1,12 @@
-package com.fasterxml.jackson.failing;
+package com.fasterxml.jackson.databind.struct;
 
 import com.fasterxml.jackson.annotation.JsonFormat;
 import com.fasterxml.jackson.annotation.JsonSubTypes;
 import com.fasterxml.jackson.annotation.JsonTypeInfo;
 import com.fasterxml.jackson.databind.BaseMapTest;
 import com.fasterxml.jackson.databind.ObjectMapper;
 
-public class TestPOJOAsArrayPolymorphic2077 extends BaseMapTest
+public class TestPOJOAsArrayPolymorphic extends BaseMapTest
 {
     // [databind#2077]
     @JsonTypeInfo(