fix remove the field writer which only differ in first character that one is upper case the other is lower case

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

@@ -479,6 +479,20 @@ boolean record = BeanUtils.isRecord(objectClass);
                     if (origin != null && origin.compareTo(fieldWriter) > 0) {
                         fieldWriterMap.put(fieldName, fieldWriter);
                     }
+
+                    // the sameFieldName means only differ in first character that one is upper case the other is lower case
+                    if (origin == null) {
+                        String sameFieldName = null;
+                        char firstChar = fieldName.charAt(0);
+                        if (firstChar >= 'A' && firstChar <= 'Z') {
+                            sameFieldName = (char) (firstChar + 32) + fieldName.substring(1);
+                        } else if (firstChar >= 'a' && firstChar <= 'z') {
+                            sameFieldName = (char) (firstChar - 32) + fieldName.substring(1);
+                        }
+                        if (sameFieldName != null && fieldWriterMap.containsKey(sameFieldName)) {
+                            fieldWriterMap.remove(sameFieldName);
+                        }
+                    }
                 });
 
                 fieldWriters = new ArrayList<>(fieldWriterMap.values());

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

@@ -345,6 +345,20 @@ boolean record = BeanUtils.isRecord(objectClass);
                     if (origin != null && origin.compareTo(fieldWriter) > 0) {
                         fieldWriterMap.put(fieldName, fieldWriter);
                     }
+
+                    // the sameFieldName means only differ in first character that one is upper case the other is lower case
+                    if (origin == null) {
+                        String sameFieldName = null;
+                        char firstChar = fieldName.charAt(0);
+                        if (firstChar >= 'A' && firstChar <= 'Z') {
+                            sameFieldName = (char) (firstChar + 32) + fieldName.substring(1);
+                        } else if (firstChar >= 'a' && firstChar <= 'z') {
+                            sameFieldName = (char) (firstChar - 32) + fieldName.substring(1);
+                        }
+                        if (sameFieldName != null && fieldWriterMap.containsKey(sameFieldName)) {
+                            fieldWriterMap.remove(sameFieldName);
+                        }
+                    }
                 });
             }
         } else {

core/src/test/java/com/alibaba/fastjson2/issues_3200/Issue3220.java

@@ -0,0 +1,27 @@
+package com.alibaba.fastjson2.issues_3200;
+
+import com.alibaba.fastjson2.JSON;
+import lombok.Data;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class Issue3220 {
+    @Test
+    public void test() {
+        Param param = new Param();
+        param.setUserName("张三");
+        param.setCode("001");
+        param.setSort("1");
+        param.setStatus("正常");
+        assertEquals("{\"code\":\"001\",\"sort\":\"1\",\"status\":\"正常\",\"userName\":\"张三\"}", JSON.toJSONString(param));
+    }
+
+    @Data
+    private static class Param {
+        public String UserName;
+        String Code;
+        String Sort;
+        String Status;
+    }
+}

pom.xml

@@ -663,7 +663,8 @@
                                 com/alibaba/fastjson2/issues_1000/Issue1395.java,
                                 com/alibaba/fastjson2/issues_2100/Issue2164.java,
                                 com/alibaba/fastjson2/issues_2100/Issue2181.java,
-                                com/alibaba/fastjson/v2issues/Issue1432.java
+                                com/alibaba/fastjson/v2issues/Issue1432.java,
+                                com/alibaba/fastjson2/issues_3200/Issue3220.java
                             </excludes>
                         </configuration>
                     </execution>