Allow Enum self as field when Enum with JsonFormat.Shape.OBJECT (#4565)

Author: JooHyukKim

release-notes/CREDITS-2.x

@@ -1738,3 +1738,7 @@ Stephane Bailliez (sbailliez@github)
  * Reported #4409: Deserialization of enums with name defined with different cases
    leads to `InvalidDefinitionException`: Multiple fields representing property
   (2.16.2)
+
+Guillaume Jardillier (Mugiwara84@github)
+ * Reported #4564: Possible 2.16.0 Enum-as-JSON-Object serialization regression
+  (2.16.3)

release-notes/VERSION-2.x

@@ -8,6 +8,12 @@ Project: jackson-databind
 
 -
 
+2.16.3 (not yet released)
+
+#4564: Possible 2.16.0 Enum-as-JSON-Object serialization regression
+ (reported by Guillaume J)
+ (fix contributed by Joo-Hyuk K)
+
 2.16.2 (09-Mar-2024)
 
 #4302: Problem deserializing some type of Enums when using `PropertyNamingStrategy`

src/main/java/com/fasterxml/jackson/databind/introspect/AnnotatedFieldCollector.java

@@ -124,18 +124,18 @@ private void _addFieldMixIns(Class<?> mixInCls, Class<?> targetClass,
 
     private boolean _isIncludableField(Field f)
     {
-        // [databind#2787]: Allow `Enum` mixins
+        // [databind#2787]: To allow `Enum` mixins, need to include Enum constants even
+        //  to they are static fields, not instance ones.
         if (f.isEnumConstant()) {
             return true;
         }
         // Most likely synthetic fields, if any, are to be skipped similar to methods
         if (f.isSynthetic()) {
             return false;
         }
-        // Static fields are never included. Transient are (since 2.6), for
-        // purpose of propagating removal
-        int mods = f.getModifiers();
-        if (Modifier.isStatic(mods)) {
+        // Static fields are never included (except for above-mentioned Enum constants.
+        // Transient are (since 2.6), for  purpose of propagating removal
+        if (Modifier.isStatic(f.getModifiers())) {
             return false;
         }
         return true;

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

@@ -1208,7 +1208,7 @@ protected JsonSerializer<?> buildEnumSerializer(SerializationConfig config,
             ((BasicBeanDescription) beanDesc).removeProperty("declaringClass");
             // [databind#2787]: remove self-referencing enum fields introduced by annotation flattening of mixins
             if (type.isEnumType()){
-                _removeEnumSelfReferences(((BasicBeanDescription) beanDesc));
+                _removeEnumSelfReferences(beanDesc);
             }
             // returning null will mean that eventually BeanSerializer gets constructed
             return null;
@@ -1226,24 +1226,28 @@ protected JsonSerializer<?> buildEnumSerializer(SerializationConfig config,
     }
 
     /**
-     * Helper method used for serialization {@link Enum} as {@link JsonFormat.Shape#OBJECT}. Removes any 
-     * self-referencing properties from its bean description before it is transformed into a JSON Object 
-     * as configured by {@link JsonFormat.Shape#OBJECT}.
+     * Helper method used for serialization {@link Enum} as {@link JsonFormat.Shape#OBJECT}.
+     * Removes any  self-referencing properties from its bean description before it is
+     * transformed into a JSON Object as configured by {@link JsonFormat.Shape#OBJECT}.
      * <p>
-     * Internally, this method iterates through {@link BeanDescription#findProperties()} and removes self.
+     * Internally, this method iterates through {@link BeanDescription#findProperties()}
+     * and removes self references.
      *
      * @param beanDesc the bean description to remove Enum properties from.
      *
      * @since 2.16
      */
-    private void _removeEnumSelfReferences(BasicBeanDescription beanDesc) {
+    private void _removeEnumSelfReferences(BeanDescription beanDesc) {
         Class<?> aClass = ClassUtil.findEnumType(beanDesc.getBeanClass());
         Iterator<BeanPropertyDefinition> it = beanDesc.findProperties().iterator();
         while (it.hasNext()) {
             BeanPropertyDefinition property = it.next();
             JavaType propType = property.getPrimaryType();
             // is the property a self-reference?
-            if (propType.isEnumType() && propType.isTypeOrSubTypeOf(aClass)) {
+            if (propType.isEnumType() && propType.isTypeOrSubTypeOf(aClass)
+                    // [databind#4564] Since 2.16.3, Enum's should allow self as field, so let's remove only if static.
+                    && property.getAccessor().isStatic())
+            {
                 it.remove();
             }
         }

src/test/java/com/fasterxml/jackson/databind/ser/enums/EnumAsFormatObject4564Test.java

@@ -0,0 +1,61 @@
+package com.fasterxml.jackson.databind.ser.enums;
+
+import com.fasterxml.jackson.annotation.JsonFormat;
+import com.fasterxml.jackson.annotation.JsonInclude;
+import com.fasterxml.jackson.core.JsonProcessingException;
+import com.fasterxml.jackson.core.type.TypeReference;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.json.JsonMapper;
+import org.junit.jupiter.api.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+// [databind#4564] Fix Enum-asJSON-Object serialization with self as field.
+public class EnumAsFormatObject4564Test
+{
+
+    @JsonFormat(shape = JsonFormat.Shape.OBJECT)
+    @JsonInclude(JsonInclude.Include.NON_NULL)
+    public enum Level {
+        LEVEL1("level1"),
+        LEVEL2("level2"),
+        LEVEL3("level3", Level.LEVEL1);
+
+        public String label;
+        public Level sublevel;
+
+        Level(String level2) {
+            this.label = level2;
+        }
+
+        Level(String level3, Level level) {
+            this.label = level3;
+            this.sublevel = level;
+        }
+    }
+
+    private final ObjectMapper MAPPER = new JsonMapper();
+
+    @Test
+    public void testEnumAsFormatObject() throws JsonProcessingException {
+        List<Level> levels = new ArrayList<>();
+        levels.add(Level.LEVEL1);
+        levels.add(Level.LEVEL2);
+        levels.add(Level.LEVEL3);
+
+        String JSON = MAPPER.writerFor(new TypeReference<List<Level>>() {
+        }).writeValueAsString(levels);
+
+        // Fails, because we get [{"label":"level1"},{"label":"level2"},{"label":"level3"}]
+        assertEquals(
+                "["
+                        + "{\"label\":\"level1\"},"
+                        + "{\"label\":\"level2\"},"
+                        + "{\"label\":\"level3\",\"sublevel\":{\"label\":\"level1\"}}"
+                        + "]",
+                JSON);
+    }
+}