Gson解析JSON数据中动态未知字段key的方法

有时在解析json数据中的字段key是动态可变的时候，由于Gson是使用静态注解的方式来设置实体对象的，因此我们很难直接对返回的类型来判断。但Gson在解析过程中如果不知道解析的字段，就会将所有变量存储在一个Map中，我们只要实例化这个map就能动态地取出key和value了。

先给出一段jsondata，这是天气预报的数据，其中day_20151002这种key是随日期而变化的，在实体类中就不能当做静态变量来处理，我们就通过map来取出其映射对象。

{    "resultcode": "200",    "reason": "successed!",    "result": {        "sk": {            "temp": "24",            "wind_direction": "东北风",            "wind_strength": "2级",            "humidity": "28%",            "time": "17:38"        },        "today": {            "temperature": "15℃~26℃",            "weather": "多云转晴",            "wind": "东北风微风",            "week": "星期日",            "city": "桂林",            "date_y": "2015年10月11日",            "dressing_index": "舒适",            "dressing_advice": "建议着长袖T恤、衬衫加单裤等服装。年老体弱者宜着针织长袖衬衫、马甲和长裤。",            "uv_index": "弱",            "comfort_index": "",            "wash_index": "较适宜",            "travel_index": "较适宜",            "exercise_index": "较适宜",            "drying_index": ""        },        "future": {            "day_20151011": {                "temperature": "15℃~26℃",                "weather": "多云转晴",                "wind": "东北风微风",                "week": "星期日",                "date": "20151011"            },            "day_20151012": {                "temperature": "16℃~27℃",                "weather": "晴转多云",                "wind": "微风",                "week": "星期一",                "date": "20151012"            },            "day_20151013": {                "temperature": "16℃~26℃",                "weather": "多云转晴",                ,                "wind": "微风",                "week": "星期二",                "date": "20151013"            },            "day_20151014": {                "temperature": "17℃~27℃",                "weather": "晴",                "wind": "北风微风",                "week": "星期三",                "date": "20151014"            },            "day_20151015": {                "temperature": "17℃~28℃",                "weather": "晴",                "wind": "北风微风",                "week": "星期四",                "date": "20151015"            },            "day_20151016": {                "temperature": "17℃~30℃",                "weather": "晴",                "wind": "北风微风",                "week": "星期五",                "date": "20151016"            },            "day_20151017": {                "temperature": "17℃~30℃",                "weather": "晴",                "wind": "北风微风",                "week": "星期六",                "date": "20151017"            }        }    },    "error_code": 0}

相关的实体类如下：

public class FutureDay {    private String temperature;    private String weather;    private String wind;    private String week;    private String date;}

public class Result {    private Sk sk;    private Today today;    private Map<String,FutureDay> future;}

public class Sk {    private String temp;    private String wind_direction;    private String wind_strength;    private String humidity;    private String time;}

public class Today {    private String temperature;    private String weather;    private String week;    private String city;    private String date_y;    private String dressing_index;    private String dressing_advice;    private String uv_index;    private String comfort_index;    private String wash_index;    private String travel_index;    private String exercise_index;    private String drying_index;}

public class Response {    private String resultcode;    private String reason;    private String error_code;    private Result result;}

具体解析过程如下代码所示：

import java.io.BufferedReader;import java.io.FileReader;import java.util.Map;import weather.*;import com.google.gson.Gson;public class GsonParseDynamicKey {    public static  void main( String args []){        String jsondata = readJsonFile();//从文件中读取出json字符串，并打印出来        Gson gson = new Gson();        System.out.println("Start Gson parse jsondata");           Response response = gson.fromJson(jsondata, Response.class);                System.out.println(response.toString());        System.out.println(response.getResult().getSk().toString());        System.out.println(response.getResult().getToday().toString());        Map<String, FutureDay> future = response.getResult().getFuture(); //对动态的key，来创建map，间接从中取出实体类futrue。        System.out.println("Keyset method");                     //这里取出value的方法有两种keySet() entrySet().都给出了遍历的方法        for (String key:future.keySet()){                        //遍历取出key，再遍历map取出value。            System.out.println("key:"+key);             System.out.println(future.get(key).toString());        }        System.out.println("Entryset method");        for (Map.Entry<String,FutureDay> pair:future.entrySet()){//遍历取出键值对，调用getkey()，getvalue()取出key和value。             System.out.println("key:"+pair.getKey());             System.out.println(pair.getValue().toString());       }    }

这里顺便一提遍历Map的两种方法keySet()，entrySet()的差别。
keySet()方法返回的是key的集合set，entrySet()返回的是键值对的集合set。虽然两者从set遍历取出元素的方法是一样的，但是根据这个元素取出value的效率有些不同。前者取出的元素是key，还要去原map中遍历取出value。
后者取出的元素是键值对，直接调用getkey()，getvalue()方法就能快速取出key和value。显然在map中存在大量键值对时，使用entrySet()来取出value的效率更高。

本文参考：http://blog.csdn.net/Chaosminds/article/details/49049455