Java_Gson json的学习

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转载请注明出处：http://blog.csdn.net/y22222ly/article/details/52213043

Gson基本使用

下面对json做json->object;object->json操作：

{    "code": 200,    "msg": "success",    "newslist": [        {            "ctime": "2016-03-31",            "title": "奇虎360宣布通过私有化决议",            "description": "互联网头条",            "picUrl": "http://t1.qpic.cn/mblogpic/f01a972dbcc1060fd456/2000",            "url": "http://mp.weixin.qq.com/s?__biz=MjM5OTMyODA2MA==&idx=1&mid=402594468&sn=5cd644536b472a283cc1d3f5124a0cab"        },        {            "ctime": "2016-03-31",            "title": "小本生意做什么挣钱十七大小本生意推荐",            "description": "创业最前线",            "picUrl": "http://zxpic.gtimg.com/infonew/0/wechat_pics_-4225297.jpg/640",            "url": "http://mp.weixin.qq.com/s?__biz=MzA3NjgzNDUwMQ==&idx=2&mid=401864059&sn=cfa082e38ba38c7e673b1ce0a075faee"        }    ]}

首先需要写一个Object类，作为这个json的实体类。这里我使用的IDE为Android Studio,使用GsonFormat插件能一键转换为Object对象。自己写的时候，顺着符号{``[来写即可，遇到{表示该字段为对象，遇到[表明该字段为数组或是集合。
转换后的对象如下
Weather.java

package com.raise.raisestudy.gson.entity;import java.util.List;/** * Created by raise.yang on 16/08/15. */public class NewsInfo {    /**     * code : 200     * msg : success     * newslist : [{"ctime":"2016-03-31","title":"奇虎360宣布通过私有化决议","description":"互联网头条","picUrl":"http://t1.qpic.cn/mblogpic/f01a972dbcc1060fd456/2000","url":"http://mp.weixin.qq.com/s?__biz=MjM5OTMyODA2MA==&idx=1&mid=402594468&sn=5cd644536b472a283cc1d3f5124a0cab"},{"ctime":"2016-03-31","title":"小本生意做什么挣钱十七大小本生意推荐","description":"创业最前线","picUrl":"http://zxpic.gtimg.com/infonew/0/wechat_pics_-4225297.jpg/640","url":"http://mp.weixin.qq.com/s?__biz=MzA3NjgzNDUwMQ==&idx=2&mid=401864059&sn=cfa082e38ba38c7e673b1ce0a075faee"}]     */    private int code;    private String msg;    /**     * ctime : 2016-03-31     * title : 奇虎360宣布通过私有化决议     * description : 互联网头条     * picUrl : http://t1.qpic.cn/mblogpic/f01a972dbcc1060fd456/2000     * url : http://mp.weixin.qq.com/s?__biz=MjM5OTMyODA2MA==&idx=1&mid=402594468&sn=5cd644536b472a283cc1d3f5124a0cab     */    private List<NewslistBean> newslist;    public int getCode() {        return code;    }    public void setCode(int code) {        this.code = code;    }    public String getMsg() {        return msg;    }    public void setMsg(String msg) {        this.msg = msg;    }    public List<NewslistBean> getNewslist() {        return newslist;    }    public void setNewslist(List<NewslistBean> newslist) {        this.newslist = newslist;    }    public static class NewslistBean {        private String ctime;        private String title;        private String description;        private String picUrl;        private String url;        public String getCtime() {            return ctime;        }        public void setCtime(String ctime) {            this.ctime = ctime;        }        public String getTitle() {            return title;        }        public void setTitle(String title) {            this.title = title;        }        public String getDescription() {            return description;        }        public void setDescription(String description) {            this.description = description;        }        public String getPicUrl() {            return picUrl;        }        public void setPicUrl(String picUrl) {            this.picUrl = picUrl;        }        public String getUrl() {            return url;        }        public void setUrl(String url) {            this.url = url;        }    }}

准备工作已做好，下面看Gson如何解析：

json,object互转

        String json = ...;        Gson gson = new Gson();        //json -> object        NewsInfo newsInfo = gson.fromJson(json, NewsInfo.class);        //object ->json        gson.toJson(newsInfo);

数组的处理

Gson gson = new Gson();int[] ints = {1, 2, 3, 4, 5};String[] strings = {"abc", "def", "ghi"};// Serializationgson.toJson(ints);     // ==> [1,2,3,4,5]gson.toJson(strings);  // ==> ["abc", "def", "ghi"]// Deserializationint[] ints2 = gson.fromJson("[1,2,3,4,5]", int[].class); // ==> ints2 will be same as ints

泛型的处理

泛型的解析，需要通过Gson提供的TypeToken类，获取一个Type.

class Foo<T> {  T value;}Gson gson = new Gson();Foo<Bar> foo = new Foo<Bar>();Type fooType = new TypeToken<Foo<Bar>>() {}.getType();gson.toJson(foo,fooType); gson.fromJson(json, fooType);

null对象的处理

当一个对象的某个属性值为null时，Gson在生成json时会忽略掉这个属性。
Persion对象如下：

public class Person {    public double height;    public String name;    public int age;    public Person(double height, String name, int age) {        this.height = height;        this.name = name;        this.age = age;    }    //...get(),set()}

比如：

        Gson gson = new Gson();        Person person = new Person(1.72,null,10);        Log.d(TAG,gson.toJson(person));// -->{"age":10,"height":1.72}

GsonBuilder

Gson对象有一个很强大的工厂类GsonBuilder，它可以在创建一个特定的Gson对象，用来制定转换规则。
若需要显示 null 对象：

        Gson gson = new GsonBuilder().serializeNulls().create();        Person person = new Person(1.72,null,10);        Log.d(TAG,gson.toJson(person));// -->{"age":10,"height":1.72,"name":null}

日期的处理

在Person类增加了下例字段：

    public Date birthday;

默认日期格式：

        Gson gson = new GsonBuilder().serializeNulls().create();        Person person = new Person(1.72,null,10,new Date());        Log.d(TAG,gson.toJson(person));// -->{"age":10,"birthday":"Aug 15, 2016 19:42:52","height":1.72,"name":null}

使用GsonBuilder格式化后的日志格式：

        Gson gson = new GsonBuilder().                serializeNulls()                .setDateFormat("yyyyMMdd")                .create();        Person person = new Person(1.72,null,10,new Date());        Log.d(TAG,gson.toJson(person));// -->{"age":10,"birthday":"20160815","height":1.72,"name":null}

修改映射key名称

后端人员有时下发json也不安套路出牌，若后端下发给我们的是以下json:

{"age":10,"birthday":"20160815","height":1.72,"person_name":null}

但是我们移动端根本不需要这么详细的定义，还是想像之前一样使用name来表示姓名，那可不可以呢，当然可以，使用注解@SerializedName即可，这样Person修改为：

    public double height;    @SerializedName("person_name")    public String name;    public int age;    public Date birthday;

@SerializedName可以指定转换时，字段的key。

        Gson gson = new GsonBuilder().                serializeNulls()                .setDateFormat("yyyyMMdd")                .create();        String person_json = "{\"age\":10,\"birthday\":\"20160815\",\"height\":1.72,\"person_name\":null}";        Person person = gson.fromJson(person_json, Person.class);        Log.d(TAG,person.toString());// -->Person{height=1.72, name='null', age=10, birthday=20160815}

忽略某些字段

当我们收到Person信息时，为了持久化数据，应该保存在数据库中，因此会增加一个id字段，Person字段修改如下：

    public double _id;//新增    public double height;    @SerializedName("person_name")    public String name;    public int age;    public Date birthday;

当然，从后端下发下来的json不带_id字段也可以直接使用gson.fromJson()自动转换为Person对象，但是当我们将Person转换为json上传到后端时，如果不做处理，会自动的增加_id这个字段,如下：

        Gson gson = new GsonBuilder().                serializeNulls()                .setDateFormat("yyyyMMdd")                .create();        Person person = new Person(1,1.70,"raise",10, new Date());        Log.d(TAG,gson.toJson(person));// -->{"_id":1.0,"age":10,"birthday":"20160816","height":1.7,"person_name":"raise"}

很明显，我们应该在生成json时，想办法踢掉这个_id字段，这是Gson提供了新的注解@Expose，用来显示哪些字段需要转换。
使用了新的注解后，记得在GsonBuilder时增加excludeFieldsWithoutExposeAnnotation()方法，转换的json就不带_id字段：
Person

    public double _id;    @Expose //需要显示转换的字段    public double height;    @Expose    @SerializedName("person_name")    public String name;    @Expose    public int age;    @Expose    public Date birthday;

        Gson gson = new GsonBuilder()                .serializeNulls()                .excludeFieldsWithoutExposeAnnotation()//只转换带@Expose注解的字段                .setDateFormat("yyyyMMdd")                .create();        Person person = new Person(1,1.70,"raise",10, new Date());        Log.d(TAG,gson.toJson(person));// -->{"age":10,"birthday":"20160816","height":1.7,"person_name":"raise"}

参考资料：
http://www.studytrails.com/java/json/java-google-json-parse-json-to-java.jsp
http://blog.csdn.net/lk_blog/article/details/7685169

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