fix fastjson 1.x compatible, for issue #3649

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

@@ -369,7 +369,12 @@ public class ObjectWriterCreatorASM
                             sameFieldName = (char) (firstChar - 32) + fieldName.substring(1);
                         }
                         if (sameFieldName != null) {
-                            fieldWriterMap.remove(sameFieldName);
+                            FieldWriter sameNameFieldWriter = fieldWriterMap.get(sameFieldName);
+                            if (sameNameFieldWriter != null
+                                    && sameNameFieldWriter.method != null
+                                    && sameNameFieldWriter.method.equals(method)) {
+                                fieldWriterMap.remove(sameFieldName);
+                            }
                         }
                     }
                 });

core/src/test/java/com/alibaba/fastjson2/issues_3600/Issue3649.java

@@ -1,6 +1,7 @@
 package com.alibaba.fastjson2.issues_3600;
 
 import com.alibaba.fastjson2.JSON;
+import com.alibaba.fastjson2.JSONObject;
 import com.alibaba.fastjson2.annotation.JSONField;
 import lombok.Data;
 import org.junit.jupiter.api.Test;
@@ -12,8 +13,10 @@ public class Issue3649 {
     public void test() {
         UserDTO user = new UserDTO();
         user.setUserName("张三");
-        assertEquals("{\"UserName\":\"张三\"}",
-                JSON.toJSONString(user));
+        JSONObject jsonObject = JSON.parseObject(JSON.toJSONString(user));
+        assertEquals(2, jsonObject.size());
+        assertEquals(user.getUserName(), jsonObject.getString("UserName"));
+        assertEquals(user.getUserName(), jsonObject.getString("userName"));
     }
 
     @Data
@@ -24,5 +27,9 @@ public class Issue3649 {
         public String getTheUserName() {
             return userName;
         }
+
+        public String getUserName() {
+            return userName;
+        }
     }
 }