Add @JsonTypeInfo.writeTypeIdForDefaultImpl to allow skipping writing of type id for values of default type

release-notes/VERSION-2.x

@@ -17,6 +17,8 @@ NOTE: Jackson 3.x components rely on 2.x annotations; there are no separate
 2.22 (not yet released)
 
 #339: Add `OptBoolean` valued property "order" in `@JsonIncludeProperties`
+#342: Add `@JsonTypeInfo.writeTypeIdForDefaultImpl` to allow skipping
+  writing of type id for values of default type
 
 2.21 (18-Jan-2026)
 

src/main/java/com/fasterxml/jackson/annotation/JsonTypeInfo.java

@@ -67,9 +67,9 @@
 public @interface JsonTypeInfo
 {
     /*
-    /**********************************************************
+    /**********************************************************************
     /* Value enumerations used for properties
-    /**********************************************************
+    /**********************************************************************
      */
 
     /**
@@ -256,9 +256,9 @@ public enum As {
     }
 
     /*
-    /**********************************************************
+    /**********************************************************************
     /* Annotation properties
-    /**********************************************************
+    /**********************************************************************
      */
 
     /**
@@ -339,23 +339,6 @@ public enum As {
      */
     public boolean visible() default false;
 
-    /*
-    /**********************************************************
-    /* Helper classes
-    /**********************************************************
-     */
-
-    /**
-     * This marker class that is only to be used with <code>defaultImpl</code>
-     * annotation property, to indicate that there is no default implementation
-     * specified.
-     *
-     * @deprecated Since 2.5, use any Annotation type (such as {@link JsonTypeInfo}),
-     *    if such behavior is needed; this is rarely necessary.
-     */
-    @Deprecated // since 2.5
-    public abstract static class None {}
-
     /**
      * Specifies whether the type ID should be strictly required during polymorphic
      * deserialization of its subtypes.
@@ -374,6 +357,44 @@ public abstract static class None {}
      */
     public OptBoolean requireTypeIdForSubtypes() default OptBoolean.DEFAULT;
 
+    /**
+     * Property that defines whether serialization of type id should be done
+     * when the runtime type of the value is the same as {@link #defaultImpl()}.
+     * Skipping write can be useful since during deserialization, if no type id is present,
+     * {@code defaultImpl} is used as the fallback type -- so the type id is redundant
+     * for that type.
+     *<p>
+     * When disabled ({@link OptBoolean#FALSE}), the type id will NOT be written
+     * if the actual runtime class of the value exactly matches {@code defaultImpl}.
+     * Subclasses of {@code defaultImpl} will still have their type id written.
+     *<p>
+     * NOTE: support for this feature is only added in Jackson 3.x, specifically
+     * 3.2. It is not supported by Jackson 2.x.
+     *<p>
+     * Default value is {@link OptBoolean#DEFAULT} (which means {@code TRUE}),
+     * preserving backwards-compatible behavior of always writing type id.
+     *
+     * @since 2.22
+     */
+    public OptBoolean writeTypeIdForDefaultImpl() default OptBoolean.DEFAULT;
+
+    /*
+    /**********************************************************************
+    /* Helper classes
+    /**********************************************************************
+     */
+
+    /**
+     * This marker class that is only to be used with <code>defaultImpl</code>
+     * annotation property, to indicate that there is no default implementation
+     * specified.
+     *
+     * @deprecated Since 2.5, use any Annotation type (such as {@link JsonTypeInfo}),
+     *    if such behavior is needed; this is rarely necessary.
+     */
+    @Deprecated // since 2.5
+    public abstract static class None {}
+
     /*
     /**********************************************************************
     /* Value class used to enclose information, allow for
@@ -399,25 +420,49 @@ public static class Value
         protected final boolean _idVisible;
         protected final Boolean _requireTypeIdForSubtypes;
 
+        /**
+         * @since 2.22
+         */
+        protected final Boolean _writeTypeIdForDefaultImpl;
+
         /*
         /**********************************************************************
         /* Construction
         /**********************************************************************
          */
 
+        /**
+         * @since 2.22
+         */
         protected Value(Id idType, As inclusionType,
-                String propertyName, Class<?> defaultImpl, boolean idVisible, Boolean requireTypeIdForSubtypes)
+                String propertyName, Class<?> defaultImpl, boolean idVisible,
+                Boolean requireTypeIdForSubtypes, Boolean writeTypeIdForDefaultImpl)
         {
             _defaultImpl = defaultImpl;
             _idType = idType;
             _inclusionType = inclusionType;
             _propertyName = propertyName;
             _idVisible = idVisible;
             _requireTypeIdForSubtypes = requireTypeIdForSubtypes;
+            _writeTypeIdForDefaultImpl = writeTypeIdForDefaultImpl;
         }
 
-        public static Value construct(Id idType, As inclusionType,
+        /**
+         * @deprecated Since 2.22 use the 7-argument overload
+         */
+        @Deprecated
+        protected Value(Id idType, As inclusionType,
                 String propertyName, Class<?> defaultImpl, boolean idVisible, Boolean requireTypeIdForSubtypes)
+        {
+            this(idType, inclusionType, propertyName, defaultImpl, idVisible, requireTypeIdForSubtypes, null);
+        }
+
+        /**
+         * @since 2.22
+         */
+        public static Value construct(Id idType, As inclusionType,
+                String propertyName, Class<?> defaultImpl, boolean idVisible,
+                Boolean requireTypeIdForSubtypes, Boolean writeTypeIdForDefaultImpl)
         {
             // couple of overrides we need to apply here. First: if no propertyName specified,
             // use Id-specific property name
@@ -433,15 +478,29 @@ public static Value construct(Id idType, As inclusionType,
             if ((defaultImpl == null) || defaultImpl.isAnnotation()) {
                 defaultImpl = null;
             }
-            return new Value(idType, inclusionType, propertyName, defaultImpl, idVisible, requireTypeIdForSubtypes);
+            return new Value(idType, inclusionType, propertyName, defaultImpl, idVisible,
+                    requireTypeIdForSubtypes, writeTypeIdForDefaultImpl);
+        }
+
+        /**
+         * @deprecated Since 2.22 use the 7-argument overload
+         */
+        @Deprecated
+        public static Value construct(Id idType, As inclusionType,
+                String propertyName, Class<?> defaultImpl, boolean idVisible,
+                Boolean requireTypeIdForSubtypes)
+        {
+            return construct(idType, inclusionType, propertyName, defaultImpl, idVisible, requireTypeIdForSubtypes, null);
         }
 
         public static Value from(JsonTypeInfo src) {
             if (src == null) {
                 return null;
             }
             return construct(src.use(), src.include(),
-                    src.property(), src.defaultImpl(), src.visible(), src.requireTypeIdForSubtypes().asBoolean());
+                    src.property(), src.defaultImpl(), src.visible(),
+                    src.requireTypeIdForSubtypes().asBoolean(),
+                    src.writeTypeIdForDefaultImpl().asBoolean());
         }
 
         /*
@@ -452,32 +511,47 @@ public static Value from(JsonTypeInfo src) {
 
         public Value withDefaultImpl(Class<?> impl) {
             return (impl == _defaultImpl) ? this :
-                new Value(_idType, _inclusionType, _propertyName, impl, _idVisible, _requireTypeIdForSubtypes);
+                new Value(_idType, _inclusionType, _propertyName, impl, _idVisible,
+                        _requireTypeIdForSubtypes, _writeTypeIdForDefaultImpl);
         }
 
         public Value withIdType(Id idType) {
             return (idType == _idType) ? this :
-                new Value(idType, _inclusionType, _propertyName, _defaultImpl, _idVisible, _requireTypeIdForSubtypes);
+                new Value(idType, _inclusionType, _propertyName, _defaultImpl, _idVisible,
+                        _requireTypeIdForSubtypes, _writeTypeIdForDefaultImpl);
         }
 
         public Value withInclusionType(As inclusionType) {
             return (inclusionType == _inclusionType) ? this :
-                new Value(_idType, inclusionType, _propertyName, _defaultImpl, _idVisible, _requireTypeIdForSubtypes);
+                new Value(_idType, inclusionType, _propertyName, _defaultImpl, _idVisible,
+                        _requireTypeIdForSubtypes, _writeTypeIdForDefaultImpl);
         }
 
         public Value withPropertyName(String propName) {
             return (propName == _propertyName) ? this :
-                new Value(_idType, _inclusionType, propName, _defaultImpl, _idVisible, _requireTypeIdForSubtypes);
+                new Value(_idType, _inclusionType, propName, _defaultImpl, _idVisible,
+                        _requireTypeIdForSubtypes, _writeTypeIdForDefaultImpl);
         }
 
         public Value withIdVisible(boolean visible) {
             return (visible == _idVisible) ? this :
-                new Value(_idType, _inclusionType, _propertyName, _defaultImpl, visible, _requireTypeIdForSubtypes);
+                new Value(_idType, _inclusionType, _propertyName, _defaultImpl, visible,
+                        _requireTypeIdForSubtypes, _writeTypeIdForDefaultImpl);
         }
-        
+
         public Value withRequireTypeIdForSubtypes(Boolean requireTypeIdForSubtypes) {
             return (_requireTypeIdForSubtypes == requireTypeIdForSubtypes) ? this :
-                new Value(_idType, _inclusionType, _propertyName, _defaultImpl, _idVisible, requireTypeIdForSubtypes);
+                new Value(_idType, _inclusionType, _propertyName, _defaultImpl, _idVisible,
+                        requireTypeIdForSubtypes, _writeTypeIdForDefaultImpl);
+        }
+
+        /**
+         * @since 2.22
+         */
+        public Value withWriteTypeIdForDefaultImpl(Boolean writeTypeIdForDefaultImpl) {
+            return (_writeTypeIdForDefaultImpl == writeTypeIdForDefaultImpl) ? this :
+                new Value(_idType, _inclusionType, _propertyName, _defaultImpl, _idVisible,
+                        _requireTypeIdForSubtypes, writeTypeIdForDefaultImpl);
         }
 
         /*
@@ -498,6 +572,18 @@ public Class<JsonTypeInfo> valueFor() {
         public boolean getIdVisible() { return _idVisible; }
         public Boolean getRequireTypeIdForSubtypes() { return _requireTypeIdForSubtypes; }
 
+        /**
+         * @since 2.22
+         */
+        public Boolean getWriteTypeIdForDefaultImpl() { return _writeTypeIdForDefaultImpl; }
+
+        /**
+         * @since 2.22
+         */
+        public boolean shouldWriteTypeIdForDefaultImpl() {
+            return (_writeTypeIdForDefaultImpl == null) || _writeTypeIdForDefaultImpl.booleanValue();
+        }
+
         /**
          * Static helper method for simple(r) checking of whether there's a Value instance
          * that indicates that polymorphic handling is (to be) enabled.
@@ -516,11 +602,11 @@ public static boolean isEnabled(JsonTypeInfo.Value v) {
 
         @Override
         public String toString() {
-            return String.format("JsonTypeInfo.Value(idType=%s,includeAs=%s,propertyName=%s,defaultImpl=%s,idVisible=%s" 
-                            + ",requireTypeIdForSubtypes=%s)",
+            return String.format("JsonTypeInfo.Value(idType=%s,includeAs=%s,propertyName=%s,defaultImpl=%s,idVisible=%s"
+                            + ",requireTypeIdForSubtypes=%s,writeTypeIdForDefaultImpl=%s)",
                     _idType, _inclusionType, _propertyName,
                     ((_defaultImpl == null) ? "NULL" : _defaultImpl.getName()),
-                    _idVisible, _requireTypeIdForSubtypes);
+                    _idVisible, _requireTypeIdForSubtypes, _writeTypeIdForDefaultImpl);
         }
 
         @Override
@@ -530,8 +616,9 @@ public int hashCode() {
             hashCode = 31 * hashCode + (_inclusionType != null ? _inclusionType.hashCode() : 0);
             hashCode = 31 * hashCode + (_propertyName != null ? _propertyName.hashCode() : 0);
             hashCode = 31 * hashCode + (_defaultImpl != null ? _defaultImpl.hashCode() : 0);
-            hashCode = 31 * hashCode + (_requireTypeIdForSubtypes ? 11 : -17);
+            hashCode = 31 * hashCode + Objects.hashCode(_requireTypeIdForSubtypes);
             hashCode = 31 * hashCode + (_idVisible ? 11 : -17);
+            hashCode = 31 * hashCode + Objects.hashCode(_writeTypeIdForDefaultImpl);
             return hashCode;
         }
 
@@ -551,6 +638,7 @@ private static boolean _equals(Value a, Value b)
                     && (a._idVisible == b._idVisible)
                     && Objects.equals(a._propertyName, b._propertyName)
                     && Objects.equals(a._requireTypeIdForSubtypes, b._requireTypeIdForSubtypes)
+                    && Objects.equals(a._writeTypeIdForDefaultImpl, b._writeTypeIdForDefaultImpl)
             ;
         }
     }

src/test/java/com/fasterxml/jackson/annotation/JsonIncludePropertiesTest.java

@@ -70,15 +70,15 @@ public void testWithOverridesAll() {
     @Test
     public void testWithOverridesEmpty() {
         JsonIncludeProperties.Value v = JsonIncludeProperties.Value.from(Bogus.class.getAnnotation(JsonIncludeProperties.class));
-        v = v.withOverrides(new JsonIncludeProperties.Value(Collections.<String>emptySet()));
+        v = v.withOverrides(new JsonIncludeProperties.Value(Collections.<String>emptySet(), false));
         Set<String> included = v.getIncluded();
         assertEquals(0, included.size());
     }
 
     @Test
     public void testWithOverridesMerge() {
         JsonIncludeProperties.Value v = JsonIncludeProperties.Value.from(Bogus.class.getAnnotation(JsonIncludeProperties.class));
-        v = v.withOverrides(new JsonIncludeProperties.Value(_set("foo")));
+        v = v.withOverrides(new JsonIncludeProperties.Value(_set("foo"), false));
         Set<String> included = v.getIncluded();
         assertEquals(1, included.size());
         assertEquals(_set("foo"), included);