Gson使用方法之Databinding

Databinding

Google json提供了一些方法用于将json字符串转换为java对象，包括继承的java对象。Gson使用名字作为关键字将json的成员匹配为java的成员。

有两种转换方法：

1. 实例化com.google.gson.Gson class，使用fromJson方法

public <T> T fromJson(JsonElement json, Type typeOfT);

2. 使用com.google.gson.GsonBuilder class，可以通过这个类来设定转换的参数和属性，然后得到Gson class进行转换

这篇教程中的json字符串来源于free music archive，其中包含了Albums class和Dataset class，每一个Dataset都是一个Album

先看一下Albums转换为json字符串的长相，定义一个Albums类

class Albums {
public String title;
public String message;
public String[] errors = new String[]{};
public String total;
public int total_pages;
public int page;
public String limit;
}

public class GsonTest {
public static void main(String[] args) {
Albums albums = new Albums();
albums.title = "Free Music Archive - Albums";
albums.message = "";
albums.total = "11259";
albums.total_pages = 2252;
albums.page = 1;
albums.limit = "5";
GsonBuilder builder = new GsonBuilder();
Gson gson = builder.create();
System.out.println(gson.toJson(albums));
}
}

得到的json字符串

{"title":"Free Music Archive - Albums","message":"","errors":[],"total":"11259","total_pages":2252,"page":1,"limit":"5"}

定义Dataset类和AlbumImages类

class Dataset {
public String album_id;
public String album_title;
}
class AlbumImages {
public String image_id;
public String user_id;
}

Dataset dataset = new Dataset();
dataset.album_id = "7596";
dataset.album_title = "Album 1";
System.out.println(gson.toJson(dataset));

AlbumImages image = new AlbumImages();
image.image_id = "1";
System.out.println(gson.toJson(image));

得到的json字符串

{"album_id":"7596","album_title":"Album 1"}
{"image_id":"1"}

在builder中设置在json字符串输出null

builder.serializeNulls();

{"album_id":"7596","album_title":"Album 1"}
{"image_id":"1","user_id":null}

将在dataset中加入image的list，并且在albums中加入dataset的list

在builder中设置更pretty的输出格式

builder.setPrettyPrinting().serializeNulls();

输出的结果为

{
"title": "Free Music Archive - Albums",
"message": "",
"errors": [],
"total": "11259",
"total_pages": 2252,
"page": 1,
"limit": "5",
"datasets": [
{
"album_id": "7596",
"album_title": "Album 1",
"images": [
{
"image_id": "1",
"user_id": null
}
]
}
]
}

利用@SerializedName("name")在序列化时设定特定的名字，这里讲images改为album_images

@SerializedName("album_images")
public List<AlbumImages> images = new ArrayList<AlbumImages>();

利用setFieldNamingStrategy()方法来修改序列化后的名字

builder.setFieldNamingStrategy(new FieldNamingStrategy() {
@Override
public String translateName(java.lang.reflect.Field f) {
if (f.getName().equals("albumId"))
return "album_id";
else
return f.getName();
}
});

完整的代码如下：

public class GsonTest {
public static void main(String[] args) {
Albums albums = new Albums();
albums.title = "Free Music Archive - Albums";
albums.message = "";
albums.total = "11259";
albums.total_pages = 2252;
albums.page = 1;
albums.limit = "5";
GsonBuilder builder = new GsonBuilder();
builder.setPrettyPrinting().serializeNulls();
builder.setFieldNamingStrategy(new FieldNamingStrategy() {
@Override
public String translateName(java.lang.reflect.Field f) {
if (f.getName().equals("albumId"))
return "album_id";
else
return f.getName();
}
});
Gson gson = builder.create();
for (int i = 0; i < 2; i++)
{
Dataset dataset = new Dataset();
dataset.album_id = Integer.toString(i*10+i);
dataset.album_title = "Album " + Integer.toString(i);
AlbumImages image = new AlbumImages();
image.image_id = "Image_" + Integer.toString(i);
dataset.images.add(image);
}
String json = gson.toJson(albums);
Albums albumsCopy = gson.fromJson(json, Albums.class);
System.out.println(albumsCopy.title);
System.out.println(albumsCopy.page);
System.out.println(albumsCopy.datasets.size());
}
}

原文中只是将java对象序列化，在反序列化时如果遇到arraylist，直接调用fromJson()是不行的，需要用到JsonArray这个类，详情见Gson使用方法之Serializing
list

原文： Java Gson
- Convert json to a java object