fix inconsistent behaviour when enable WriteClasName, for issue #2981

core/src/main/java/com/alibaba/fastjson2/JSON.java

@@ -3859,7 +3859,9 @@ static Object toJSON(Object object, JSONWriter.Feature... features) {
                 JSONFactory.createWriteContext() : JSONFactory.createWriteContext(features);
         Class<?> valueClass = object.getClass();
         ObjectWriter<?> objectWriter = writeContext.getObjectWriter(valueClass, valueClass);
-        if (objectWriter instanceof ObjectWriterAdapter && !writeContext.isEnabled(JSONWriter.Feature.ReferenceDetection)) {
+        if (objectWriter instanceof ObjectWriterAdapter
+                && !writeContext.isEnabled(JSONWriter.Feature.ReferenceDetection)
+                && (objectWriter.getFeatures() & JSONWriter.Feature.WriteClassName.mask) == 0) {
             ObjectWriterAdapter objectWriterAdapter = (ObjectWriterAdapter) objectWriter;
             return objectWriterAdapter.toJSONObject(object, writeContext.features);
         }

core/src/test/java/com/alibaba/fastjson2/issues_2900/Issue2981.java

@@ -0,0 +1,28 @@
+package com.alibaba.fastjson2.issues_2900;
+
+import com.alibaba.fastjson2.JSON;
+import com.alibaba.fastjson2.JSONWriter;
+import com.alibaba.fastjson2.annotation.JSONType;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class Issue2981 {
+    @Test
+    public void test() {
+        Shape.Circle circle = new Shape.Circle();
+        circle.setRadius(5);
+        assertEquals(JSON.parse(JSON.toJSONString(circle)), JSON.toJSON(circle));
+    }
+
+    @JSONType(typeKey = "type", seeAlso = {Shape.Circle.class})
+    public static class Shape {
+        @JSONType(typeKey = "type", typeName = "circle", serializeFeatures = JSONWriter.Feature.WriteClassName)
+        public static class Circle
+                extends Shape {
+            private int radius;
+            public int getRadius() { return radius; }
+            public void setRadius(int radius) { this.radius = radius; }
+        }
+    }
+}