fix fastjson 1.x compatible, for issue #3649

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

@@ -515,7 +515,11 @@ public class ObjectWriterCreator {
                             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/main/java/com/alibaba/fastjson2/writer/ObjectWriterCreatorASM.java

@@ -371,8 +371,7 @@ public class ObjectWriterCreatorASM
                         if (sameFieldName != null) {
                             FieldWriter sameNameFieldWriter = fieldWriterMap.get(sameFieldName);
                             if (sameNameFieldWriter != null
-                                    && sameNameFieldWriter.method != null
-                                    && sameNameFieldWriter.method.equals(method)) {
+                                    && (sameNameFieldWriter.method == null || sameNameFieldWriter.method.equals(method))) {
                                 fieldWriterMap.remove(sameFieldName);
                             }
                         }

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

@@ -32,4 +32,89 @@ public class Issue3649 {
             return userName;
         }
     }
+
+    @Test
+    public void test1() {
+        UserDTO1 user = new UserDTO1();
+        user.setUserName("张三");
+        JSONObject jsonObject = JSON.parseObject(JSON.toJSONString(user));
+        assertEquals(2, jsonObject.size());
+        assertEquals(user.getUserName(), jsonObject.getString("UserName"));
+        assertEquals(user.getUserName(), jsonObject.getString("userName"));
+    }
+
+    @Data
+    public static class UserDTO1 {
+        private String userName;
+
+        @JSONField(name = "UserName")
+        public String getTheUserName() {
+            return userName;
+        }
+        private String field1;
+        private String field2;
+        private String field3;
+        private String field4;
+        private String field5;
+        private String field6;
+        private String field7;
+        private String field8;
+        private String field9;
+        private String field10;
+        private String field11;
+        private String field12;
+        private String field13;
+        private String field14;
+        private String field15;
+        private String field16;
+        private String field17;
+        private String field18;
+        private String field19;
+        private String field20;
+        private String field21;
+        private String field22;
+        private String field23;
+        private String field24;
+        private String field25;
+        private String field26;
+        private String field27;
+        private String field28;
+        private String field29;
+        private String field30;
+        private String field31;
+        private String field32;
+        private String field33;
+        private String field34;
+        private String field35;
+        private String field36;
+        private String field37;
+        private String field38;
+        private String field39;
+        private String field40;
+        private String field41;
+        private String field42;
+        private String field43;
+        private String field44;
+        private String field45;
+        private String field46;
+        private String field47;
+        private String field48;
+        private String field49;
+        private String field50;
+        private String field51;
+        private String field52;
+        private String field53;
+        private String field54;
+        private String field55;
+        private String field56;
+        private String field57;
+        private String field58;
+        private String field59;
+        private String field60;
+        private String field61;
+        //第62个序列化是2个
+        private String field62;
+        //第63个序列化是1个
+        private String field63;
+    }
 }