Dart/Flutter JSON转类映射失败，FutureBuilder无数据

在Flutter项目中使用FutureBuilder时遇到无数据返回的问题，经过排查定位到LUKETXT字段映射错误，具体细节如下：

后端数据获取函数 #

Future<List<OrderReceipt>> getOrderReceipt(orderno) async {
  final response = await HttpService.order_receipt(orderno);
  var decoded = json.decode(response.body)["qry"].cast<Map<String,dynamic>>();
  print(decoded);
  var OrderR = await decoded.map<OrderReceipt>((json) => OrderReceipt.fromJson(json)).toList();
  return OrderR;
}

自定义类及JSON映射方法 #

class OrderReceipt {
  final String LUKEBLART;
  final String LUKEBLOART;
  final String LUKEBLONR;
  final String LUKEBLVC;
  final String LUKEBLBNR;
  final String LUKEBLDRAKT;
  final String LUKEBLADS;
  final String LUKEBLDATUM;
  final String LUKEBLPGM;
  final String LUKETIME;
  final String LUKESART;
  final String LUKEPAGE;
  final String LUKELINE;
  final String LUKELNR;
  final String LUKETXT;

  OrderReceipt({
    required this.LUKEBLART,
    required this.LUKEBLOART,
    required this.LUKEBLONR,
    required this.LUKEBLVC,
    required this.LUKEBLBNR,
    required this.LUKEBLDRAKT,
    required this.LUKEBLADS,
    required this.LUKEBLDATUM,
    required this.LUKEBLPGM,
    required this.LUKETIME,
    required this.LUKESART,
    required this.LUKEPAGE,
    required this.LUKELINE,
    required this.LUKELNR,
    required this.LUKETXT,
  });

  factory OrderReceipt.fromJson(Map<String, dynamic> json) {
    return OrderReceipt(
        LUKEBLART: json['LUKEBLART'] as String ?? '', 
        LUKEBLOART: json['LUKEBLOART'] as String ?? '',   
        LUKEBLONR: json['LUKEBLONR'] as String ?? '',  
        LUKEBLVC: json['LUKEBLVC'] as String ?? '', 
        LUKEBLBNR: json['LUKEBLBNR'] as String ?? '', 
        LUKEBLDRAKT: json['LUKEBLDRAKT'] as String ?? '', 
        LUKEBLADS: json['LUKEBLADS'] as String ?? '', 
        LUKEBLDATUM: json['LUKEBLDATUM'] as String ?? '',  
        LUKEBLPGM: json['LUKEBLPGM'] as String ?? '',
        LUKETIME: json['LUKETIME'] as String ?? '',
        LUKESART: json['LUKESART'] as String ?? '', 
        LUKEPAGE: json['LUKEPAGE'] as String ?? '', 
        LUKELINE: json['LUKELINE'] as String ?? '', 
        LUKELNR: json['LUKELNR'] as String ?? '', 
        LUKETXT: json['LUKETX'] as String ?? '', 
    );}
}

JSON数据示例（单条） #

[{
  "LUKEBLADS": "222222", 
  "LUKEBLART": "W ", 
  "LUKEBLBNR": "333333", 
  "LUKEBLDATUM": "20230113", 
  "LUKEBLDRAKT": "FX  ", 
  "LUKEBLOART": "FX  ", 
  "LUKEBLONR": "4444", 
  "LUKEBLPGM": "XXXKG    ", 
  "LUKEBLVC": "X1 ", 
  "LUKELINE": "41", 
  "LUKELNR": "36", 
  "LUKEPAGE": "1", 
  "LUKESART": "1", 
  "LUKETIME": "Fri, 13 Jan 2023 09:54:02 GMT", 
  "LUKETXT": "                                    ExampleText                                      USD         66,20                                                                                          "
}]

问题表现 #

• getOrderReceipt函数能正常打印解码后的decoded数据，但无法完成映射返回OrderReceipt列表
• FutureBuilder处理后无数据返回，且无任何报错提示
• 项目中其他两个FutureBuilder功能正常

排查过程 #

• 移除.cast<Map<String,dynamic>>()和await后，问题依旧存在
• 添加try-catch捕获到错误：type 'null' is not a subtype of type 'string' in type cast
• 注释模型中的LUKETXT字段后，功能恢复正常

错误原因及解决方法 #

错误原因 #

在OrderReceipt.fromJson工厂方法中，LUKETXT字段的映射使用了错误的键名：json['LUKETX']，而JSON数据中的实际键名是LUKETXT（少了一个字母T），导致获取到null，执行as String类型转换时抛出类型不匹配错误，最终导致整个列表映射失败，FutureBuilder无数据返回。

解决方法 #

将工厂方法中LUKETXT的映射代码修改为正确的键名：

LUKETXT: json['LUKETXT'] as String ?? '', 

同时，建议优化类型转换的安全性，避免直接使用as String导致崩溃，可以改用更稳妥的写法：

LUKETXT: (json['LUKETXT'] ?? '').toString(),

内容的提问来源于stack exchange，提问作者Lukas Fürst