Flutter Web调试模式下Freezed自定义模型fromJson调用失败

安全阿岳

2026-7-24

问题描述

在Flutter Web项目中定义了如下Freezed自定义模型：

@freezed
class User with _$User {
  const factory User({
    required int id,
    required String name,
    required Address address,
    @Default(null) Contact? contact,
  }) = _User;

  factory User.fromJson(Map<String, dynamic> json) => _$UserFromJson(json);
}

Address、Coordinates及Contact模型定义如下：

@Freezed()
class Address with _$Address {
  const factory Address({
    required String type,
    required String address,
    required String subDistrict,
    required String district,
    required String state,
    required String country,
    required String pincode,
    Coordinates? coordinates,
  }) = _Address;

  factory Address.fromJson(Map<String, dynamic> json) => _$AddressFromJson(json);
}

@Freezed()
class Coordinates with _$Coordinates {
  const factory Coordinates({
    required double latitude,
    required double longitude,
  }) = _Coordinates;

  factory Coordinates.fromJson(Map<String, dynamic> json) => _$CoordinatesFromJson(json);
}

@Freezed()
class Contact with _$Contact {
  const factory Contact({
    @Default([]) List<String> email,
    @Default([]) List<String> mobile,
    @Default([]) List<String> landline,
    @Default([]) List<String> fax,
  }) = _Contact;

  factory Contact.fromJson(Map<String, dynamic> json) => _$ContactFromJson(json);
}

调用User.fromJson从JSON创建实例时，抛出错误：

NoSuchMethodError: tried to call a non-function, such as null: 'user$.User.fromJson'

问题细节：

仅User模型出现该问题，移除@Default(null)后问题依旧
已确认传入的是Map类型而非字符串
仅调试模式触发，发布模式运行正常

环境及依赖版本：

Flutter 3.7.9 • channel stable • https://github.com/flutter/flutter.git
Framework • revision 62bd79521d (2 weeks ago) • 2023-03-30 10:59:36 -0700
Engine • revision ec975089ac
Tools • Dart 2.19.6 • DevTools 2.20.1

dependencies:
  freezed_annotation: ^2.2.0
  json_annotation: ^4.8.0

dev_dependencies:
  build_runner: ^2.3.3
  freezed: ^2.3.2
  json_serializable: ^6.6.1

堆栈追踪：

C:/b/s/w/ir/cache/builder/src/out/host_debug/dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 45:50   <fn>
C:/b/s/w/ir/cache/builder/src/out/host_debug/dart-sdk/lib/async/zone.dart 1660:54                                 runUnary
C:/b/s/w/ir/cache/builder/src/out/host_debug/dart-sdk/lib/async/future_impl.dart 147:18                           handleValue
C:/b/s/w/ir/cache/builder/src/out/host_debug/dart-sdk/lib/async/future_impl.dart 767:44                           handleValueCallback
C:/b/s/w/ir/cache/builder/src/out/host_debug/dart-sdk/lib/async/future_impl.dart 796:13                           _propagateToListeners
C:/b/s/w/ir/cache/builder/src/out/host_debug/dart-sdk/lib/async/future_impl.dart 567:5                            [_completeWithValue]
C:/b/s/w/ir/cache/builder/src/out/host_debug/dart-sdk/lib/async/future_impl.dart 640:7                            callback
C:/b/s/w/ir/cache/builder/src/out/host_debug/dart-sdk/lib/async/schedule_microtask.dart 40:11                     _microtaskLoop
C:/b/s/w/ir/cache/builder/src/out/host_debug/dart-sdk/lib/async/schedule_microtask.dart 49:5                      _startMicrotaskLoop
C:/b/s/w/ir/cache/builder/src/out/host_debug/dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 166:15  <fn>

原因分析及解决方案

这个问题是Flutter Web调试模式下的代码生成延迟/未正确加载导致的。调试模式采用JIT编译，Freezed生成的_$UserFromJson方法没有被提前初始化加载，调用时就会出现null无法执行的错误；而发布模式是AOT编译，所有代码（包括生成代码）都会被提前打包，因此不会出现这个问题。

具体解决步骤：

强制预加载生成代码
在调用User.fromJson之前，提前触发生成代码的加载，比如在初始化代码里添加：

// 引用生成文件中的方法，触发加载
import 'user.g.dart' as user_gen;

void init() {
  // 仅触发加载，无需实际调用
  user_gen._$UserFromJson;
}

或者直接创建一个空的_$User实例：

_$User(const User(id: 0, name: '', address: Address(type: '', address: '', subDistrict: '', district: '', state: '', country: '', pincode: '')));

彻底清理并重新生成代码
执行以下命令，清除旧的生成文件并重新构建：
```
flutter clean
flutter pub get
flutter pub run build_runner build --delete-conflicting-outputs
```
检查导入与代码结构
确保所有模型的.g.dart生成文件都被正确导入，且没有循环导入的情况。
禁用Web延迟加载
如果以上方法无效，可在pubspec.yaml中关闭Flutter Web的延迟加载：
```
flutter:
  web:
    defer_js: false
```

内容的提问来源于stack exchange，提问作者Damien Leon