1.引入json_annotation依赖

pubspec.yaml

dependencies:
  # Your other regular dependencies here
  json_annotation: ^2.0.0

dev_dependencies:
  # Your other dev_dependencies here
  build_runner: ^1.0.0
  json_serializable: ^2.0.0

2.根据json字符串的结构和字段构造实体类（model），注意字段类型

一般步骤为：

• 新建类和属性（字段）以及带参构造方法，注意属性类型。
• 引入json_annotation，即插入import 'package:json_annotation/json_annotation.dart';。
• 指定此类的生成代码，part '类名.g.dart';。
• 添加序列化标注，@JsonSerializable()，注意括号。
• 添加反序列化和序列化方法fromJson和toJson，即factory 类名.fromJson(Map<String, dynamic> json) =>_$类名FromJson(json);和Map<String, dynamic> toJson() => _$类名ToJson(this);。

例：BaiduSNComputePosition.dart

//引入json_annotation
import 'package:json_annotation/json_annotation.dart';
import 'package:flutter_app/model/SnResult.dart';
//此类的生成代码，part '类名.g.dart';
part 'BaiduSNComputePosition.g.dart';
//序列化标注
@JsonSerializable()
class BaiduSNComputePosition {
  //不加任何JsonKey默认允许空json字段
  int status;
  /*SnResult是我自定义的实体类，这个类同样需要执行上述的“一般步骤“*/
  SnResult result;

  //构造方法
  BaiduSNComputePosition(this.status, this.result);

  //反序列化,factory *.fromJson(Map<String, dynamic> json) =>_$*FromJson(json);
  factory BaiduSNComputePosition.fromJson(Map<String, dynamic> json) =>
      _$BaiduSNComputePositionFromJson(json);

  //序列化,Map<String, dynamic> toJson() => _$*ToJson(this);
  Map<String, dynamic> toJson() => _$BaiduSNComputePositionToJson(this);

  @override
  String toString() {
    return '{status: $status, result: $result}';
  }
}

3.生成对应的.g.dart文件

忽略报错，使用命令flutter packages pub run build_runner build进行一次性构建，构建过程中flutter会使用Model类的源文件（包含@JsonSerializable标注的）来生成对应的.g.dart文件。

另外，也可以使用命令flutter packages pub run build_runner watch，这个命令可以实现文件监听，自动地为你后续创建的实体类生成对应的.g.dart文件。
这里给出上述实体类例子的对应.g.dart文件：BaiduSNComputePosition.g.dart

// GENERATED CODE - DO NOT MODIFY BY HAND

part of 'BaiduSNComputePosition.dart';

// **************************************************************************
// JsonSerializableGenerator
// **************************************************************************

BaiduSNComputePosition _$BaiduSNComputePositionFromJson(
    Map<String, dynamic> json) {
  return BaiduSNComputePosition(
      json['status'] as int,
      json['result'] == null
          ? null
          : SnResult.fromJson(json['result'] as Map<String, dynamic>));
}

Map<String, dynamic> _$BaiduSNComputePositionToJson(
        BaiduSNComputePosition instance) =>
    <String, dynamic>{'status': instance.status, 'result': instance.result};

4.使用

使用json.encode和json.decode，需要引入import 'dart:convert';。

//序列化为json字符串，变量modelObject是实体类对象
String jsonStr = json.encode(modelObject);
//反序列化为实体类，变量jsonString是与实体类完全匹配的json字符串，BaiduWebPosition是实体类名
var modelObj= BaiduWebPosition.fromJson(json.decode(jsonString));

以下是我在单元测试中的例子:

import 'dart:convert';
import 'package:flutter_app/model/Address_detail.dart';
import 'package:flutter_app/model/BWPContent.dart';
import 'package:flutter_app/model/BaiduSNComputePosition.dart';
import 'package:flutter_app/model/BaiduWebPosition.dart';
import 'package:flutter_app/model/Bdpoint.dart';
import 'package:flutter_app/model/SnLocation.dart';
import 'package:flutter_app/model/SnResult.dart';

void main() {

  String ipjsonString =
      '{"address":"CN|\u5929\u6d25|\u5929\u6d25|None|UNICOM|0|0","content":{"address":"\u5929\u6d25\u5e02","address_detail":{"city":"\u5929\u6d25\u5e02","city_code":332,"district":"","province":"\u5929\u6d25\u5e02","street":"","street_number":""},"point":{"x":"117.21081309","y":"39.14392990"}},"status":0}';
  String snjsonString =
      '{"status":0,"result":{"location":{"lng":117.04521735270664,"lat":39.11214414784875},"precise":1,"confidence":80,"comprehension":92,"level":"地产小区"}}';
  var address = 'CN|\u5929\u6d25|\u5929\u6d25|None|UNICOM|0|0';
  var addr = '\u5929\u6d25\u5e02';
  var ipp = new BaiduWebPosition(
      address,
      new BWPContent(
          addr,
          new Address_detail(
              "\u5929\u6d25\u5e02", 332, "", "\u5929\u6d25\u5e02", "", ""),
          new Bdpoint("117.21081309", "39.14392990")),
      0);
  String ipjsonStr = json.encode(ipp);
  print("new ip-json=" + ipjsonStr);
  var ippAfter = BaiduWebPosition.fromJson(json.decode(ipjsonString));
  //我的实体类重写了toString方法哦
  print("反序列化toString=" + ippAfter.toString());

  var snp = new BaiduSNComputePosition(
      0,
      new SnResult(new SnLocation(117.04521735270665, 39.11214414784875), 1, 80,
          92, "地产小区"));
  String snjsonStr = json.encode(snp);
  print("new sn-json=" + snjsonStr);
  var snpAfter2 = BaiduSNComputePosition.fromJson(json.decode(snjsonString));
  print("反序列化toString=" + snpAfter2.toString());
}

参考资料：
flutter demo （三）：json处理
Json Model