Gson解析JSON中动态未知字段key的方法

       前面一篇文章我介绍了Gson的解析的基本方法。但我们在享受Gson解析的高度封装带来的便利时，有时可能会遇到一些特殊情况，比如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
}

实体类中放上set、get和toString方法就太长了，这里就没有加上去。

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 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;
}

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 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 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。

下面是最后打印出来的结果。

工程源码的下载地址点击打开链接