Gson转换 — json数据转换为Object实体

最近在做远程接口调用时，需要对数据进行加解密，因此返回的参数统一为以下json格式

{
    "code":0,
    "msg":"成功",
    "sign":"abcdefghijkl",
    "timestamp":1491897441000,
    "data":{}
}

首先，我们假设传输过程中的对象实体是

user.java

public class User {

    private int id;

    private String name;

    ...省略get set...
    
    @Override
    public String toString() {
        return "User [id=" + id + ", name=" + name + "]";
    }

}

当返回的数据转成对象时，可能存在三种情况：

1. User 对象
2. List<User> list 对象
3. Page page

Page.java

public class Page<T> {

    private int page;

    private int size;

    private int total;

    private List<T> data;

    ...省略 get set...
    
        @Override
    public String toString() {
        return "User [page=" + page + ", size=" + size + ", total=" + total + ", data=" + data + "]";
    }
    
}

因此返回data的json可能为以下三种：

1.对象实体的json json -> User

{
    "id":1,
    "name":"eric"
}

2.对象实体列表的json json -> List<User> list

[
    {
        "id":1,
        "name":"eric"
    },
    {
        "id":2,
        "name":"john"
    }
]

3.分页的实体json json -> Page<User> page

{
    "page":1,
    "size":10,
    "total":2,
    "data":[
        {
            "id":1,
            "name":"eric"
        },
        {
            "id":2,
            "name":"john"
        }
    ]
}

因此我们需要写一个公共类解决上述转换问题

GsonUtils.java

public class GsonUtils {

    private GsonUtils() {

    }

    public static <T>T fromJson(String json,Class<T> type){
        Gson gson = new Gson();
        return gson.fromJson(json,type);

    }

    public static <T> List<T> listFromJson(String json){
        Gson gson = new Gson();
        return gson.fromJson(json, new TypeToken<List<T>>(){}.getType());
    }

}

测试：

public class Main {

    public static void main(String[] args) {

        String json1 = "{\"id\":1,\"name\":\"eric\"}";
        String json2 = "[{\"id\":1,\"name\":\"eric\"},{\"id\":2,\"name\":\"john\"}]";
        String json3 = "{\"page\":1,\"size\":10,\"total\":2,\"data\":[{\"id\":1,\"name\":\"eric\"},{\"id\":2,\"name\":\"john\"}]}";

        User user = GsonUtils.fromJson(json1, User.class);
        List<User> lists = GsonUtils.listFromJson(json2);
        Page<User> page = GsonUtils.fromJson(json3, Page.class);

        System.out.println("user:" + user);
        System.out.println("lists:" + lists);
        System.out.println("page:" + page);

    }

}

运行后的结果：

user:User [id=1, name=eric]
lists:[{id=1.0, name=eric}, {id=2.0, name=john}]
page:User [page=1, size=10, total=2, list=[{id=1.0, name=eric}, {id=2.0, name=john}]]

此时已经成功将三种格式的json转化为Object，但是这里有个问题如下：如果我们用一下方法获取user，就会报错，因为处理Java反射TypeToken泛型运行时类型擦除

User user = lists.get(0);

Exception in thread "main" java.lang.ClassCastException: com.google.gson.internal.LinkedTreeMap cannot be cast to swun.test.entity.User
    at swun.test.Main.main(Main.java:27)

因此我们将方法listFromJson修改为

    public static <T> List<T> listFromJson(String json,Type type){
        Gson gson = new Gson();
        return gson.fromJson(json, type);
    }

在调用的时候，将具体的Type传入

public class Main {

    public static void main(String[] args) {

        String json1 = "{\"id\":1,\"name\":\"eric\"}";
        String json2 = "[{\"id\":1,\"name\":\"eric\"},{\"id\":2,\"name\":\"john\"}]";
        String json3 = "{\"page\":1,\"size\":10,\"total\":2,\"data\":[{\"id\":1,\"name\":\"eric\"},{\"id\":2,\"name\":\"john\"}]}";

        User user = GsonUtils.fromJson(json1, User.class);
        List<User> lists = GsonUtils.listFromJson(json2,new TypeToken<List<User>>(){}.getType());
        Page<User> page = GsonUtils.fromJson(json3, Page.class);

        System.out.println("user:" + user);
        System.out.println("lists:" + lists);
        System.out.println("page:" + page);

        User user1 = lists.get(0);
        System.out.println("user1:" + user1);

    }

}

运行结果如下：

user:User [id=1, name=eric]
lists:[User [id=1, name=eric], User [id=2, name=john]]
page:User [page=1, size=10, total=2, data=[{id=1.0, name=eric}, {id=2.0, name=john}]]
user1:User [id=1, name=eric]