fix: Map subclass loses @type when serialized with ValueFilter (#3984) (#4029)

* fix: Map subclass loses @type when serialized with ValueFilter (#3984)

ObjectWriterImplMap.writeWithFilter() was missing the writeTypeInfo
call that write() had, causing @type to be silently dropped when
any filter was present with WriteClassName enabled.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* test: use exact class name and occurrence count in assertions

Address review feedback: assert against full qualified type name
via CustomMap.class.getName() and count exact @type occurrences
instead of substring checks.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* test: use structural JSON assertions instead of substring matching

Parse serialized JSON and assert @type via JSONObject.getString()
rather than substring matching, making tests resilient to
formatting/order changes.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: wenshao

core/src/main/java/com/alibaba/fastjson2/writer/ObjectWriterImplMap.java

@@ -617,6 +617,12 @@ public void writeWithFilter(JSONWriter jsonWriter, Object object, Object fieldNa
         jsonWriter.startObject();
         Map map = (Map) object;
 
+        boolean writeTypeInfo = (fieldType == this.objectType && jsonWriter.isWriteMapTypeInfo(object, this.objectClass, features))
+                || jsonWriter.isWriteTypeInfo(object, fieldType, features);
+        if (writeTypeInfo) {
+            writeTypeInfo(jsonWriter);
+        }
+
         features |= jsonWriter.getFeatures();
         if ((features & (MapSortField.mask | SortMapEntriesByKeys.mask)) != 0) {
             if (!(map instanceof SortedMap)

core/src/test/java/com/alibaba/fastjson2/issues_3900/Issue3984.java

@@ -0,0 +1,70 @@
+package com.alibaba.fastjson2.issues_3900;
+
+import com.alibaba.fastjson2.JSON;
+import com.alibaba.fastjson2.JSONObject;
+import com.alibaba.fastjson2.JSONWriter;
+import com.alibaba.fastjson2.filter.ValueFilter;
+import org.junit.jupiter.api.Test;
+
+import java.util.HashMap;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class Issue3984 {
+    static final String CUSTOM_MAP_TYPE = CustomMap.class.getName();
+
+    public static class CustomMap extends HashMap<String, Object> {
+    }
+
+    @Test
+    public void testWriteClassNameWithValueFilter() {
+        CustomMap map = new CustomMap();
+        map.put("key1", "value1");
+
+        ValueFilter valueFilter = (object, name, value) -> value;
+
+        String json = JSON.toJSONString(map,
+                new ValueFilter[]{valueFilter},
+                JSONWriter.Feature.WriteClassName);
+
+        JSONObject parsed = JSON.parseObject(json);
+        assertEquals(CUSTOM_MAP_TYPE, parsed.getString("@type"));
+        assertEquals("value1", parsed.getString("key1"));
+    }
+
+    @Test
+    public void testWriteClassNameWithoutFilter() {
+        CustomMap map = new CustomMap();
+        map.put("key1", "value1");
+
+        String json = JSON.toJSONString(map, JSONWriter.Feature.WriteClassName);
+
+        JSONObject parsed = JSON.parseObject(json);
+        assertEquals(CUSTOM_MAP_TYPE, parsed.getString("@type"));
+        assertEquals("value1", parsed.getString("key1"));
+    }
+
+    @Test
+    public void testNestedMapWithValueFilter() {
+        CustomMap inner = new CustomMap();
+        inner.put("msg", 123);
+
+        CustomMap outer = new CustomMap();
+        outer.put("key1", "value1");
+        outer.put("nested", inner);
+
+        ValueFilter valueFilter = (object, name, value) -> value;
+
+        String json = JSON.toJSONString(outer,
+                new ValueFilter[]{valueFilter},
+                JSONWriter.Feature.WriteClassName);
+
+        JSONObject parsed = JSON.parseObject(json);
+        assertEquals(CUSTOM_MAP_TYPE, parsed.getString("@type"));
+        assertEquals("value1", parsed.getString("key1"));
+
+        JSONObject nestedParsed = parsed.getJSONObject("nested");
+        assertEquals(CUSTOM_MAP_TYPE, nestedParsed.getString("@type"));
+        assertEquals(123, nestedParsed.getIntValue("msg"));
+    }
+}