Json映射为Map，避免定义过多pojo类

我们在开发过程中经常会遇到解析Json字符串的情况，这时候采用开源工具可以快速将json字符串映射为pojo对象。而在某些业务场景中，往往为了获得json字符串中某个属性的值而不得不定义一个pojo类，从而形成与json字符串的对应。一旦json格式发生改变。pojo类也不得不作相应修改。
这时候可以考虑将json与Map映射。只要知道具体的属性名即可获取属性值。

假设有如下Json字符串

{
    "id": 100,
    "name": "scott",
    "birthday": 1480785217693,
    "org": {
    "id": 1,
    "name": "销售部"
    }
}

由于Json格式都是键值对形式存在，所以直接映射Map类型即可。这里采用Gson作为Json解析工具。

Gson gson = new GsonBuilder().create();

String json = "{"id":100,"name":"scott","birthday":"Dec 4, 2016 1:10:33 AM","org":{"id":1,"name":"销售部"}}";

Map map = gson.fromJson(json, Map.class);

如何取值？传统循环方式

这里用.作为属性名之间的分隔符。

定义解析Map的方法

  private <T> T getValueByMap(String path,Map<String, Object> map,Class<T> clazz,Object defaultVal){

      String[] params = path.split("\.");
      for(int i=0;i<params.length-1;i++){
          map = (Map)map.get(params[i]);
      }
      Object result = null;
      result = map.get(params[params.length-1]);

      return result == null ? (T) defaultVal : (T) result;
  }

调用上述方法

  String name = getValueByMap("org.name",map,String.class,"");
  System.out.println(name);

  double birthday = getValueByMap("birthday",map,double.class,"");
  System.out.println(new Double(birthday).longValue());

Ognl方式,采用该方式需引入Ognl依赖

<dependency>
    <groupId>ognl</groupId>
    <artifactId>ognl</artifactId>
    <version>3.1.12</version>
</dependency>

代码实现

private <T> T getValueByMap(String path, Map<String, Object> map, Class<T> clazz, Object defaultVal) throws OgnlException {

    OgnlContext context = new OgnlContext();
    context.putAll(map);
    Object value = Ognl.getValue(path, context);
    return value == null ? (T) defaultVal : (T) value;
}

Json数组处理

假设json字符串如下：

{
    "id": 1,
    "name": "销售部",
        "empSet": [
        {
            "id": 2,
            "name": "scott 2",
            "birthday": 1480787444646
        },
        {
            "id": 3,
                "name": "scott 3",
            "birthday": 1480787444646
        },
        {
            "id": 4,
            "name": "scott 4",
            "birthday": 1480787444646
        },
        {
            "id": 0,
            "name": "scott 0",
            "birthday": 1480787444646
        },
        {
            "id": 1,
            "name": "scott 1",
            "birthday": 1480787444646
        }
    ]
}

用List与Json字符串中的数组部门映射即可。

  Gson gson = new GsonBuilder().create();

  String json = "{"id":1,"name":"销售部","empSet":[{"id":2,"name":"scott 2","birthday":1480787444646},{"id":3,"name":"scott 3","birthday":1480787444646},{"id":4,"name":"scott 4","birthday":1480787444646},{"id":0,"name":"scott 0","birthday":1480787444646},{"id":1,"name":"scott 1","birthday":1480787444646}]}";

  Map map = gson.fromJson(json, Map.class);

  List emps = getValueByMap("empSet", map, List.class, "");
  System.out.println(emps);