Java_Gson json的学习

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"};

// Serialization
gson.toJson(ints);     // ==> [1,2,3,4,5]
gson.toJson(strings);  // ==> ["abc", "def", "ghi"]

// Deserialization
int[] 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