使用Gson将JSON转换为Java对象

Question

I am trying to convert JSON string to simple java object but it is returning null. Below are the class details.

JSON String:

   {"menu": 
    {"id": "file",
     "value": "File",
     }
   }

This is parsable class:

public static void main(String[] args) {
try {
    Reader r = new
         InputStreamReader(TestGson.class.getResourceAsStream("testdata.json"), "UTF-8");
    String s = Helper.readAll(r);
    Gson gson = new Gson();
    Menu m = gson.fromJson(s, Menu.class);
    System.out.println(m.getId());
    System.out.println(m.getValue());
    } catch (IOException e) {
    e.printStackTrace();
    }
}

Below are th model class:

public class Menu {

    String id;
    String value;

    public String getId() {
        return id;
    }
    public void setId(String id) {
        this.id = id;
    }
    public String getValue() {
        return value;
    }
    public void setValue(String value) {
        this.value = value;
    }

    public String toString() {
        return String.format("id: %s, value: %d", id, value);
    }

}

Everytime i am getting null. Can anyone please help me?

Answer 1

Your JSON is an object with a field menu .

If you add the same in your Java it works:

class MenuWrapper {
    Menu menu;
    public Menu getMenu() { return menu; }
    public void setMenu(Menu m) { menu = m; }
}

And an example:

public static void main(String[] args) {
    String json =  "{\"menu\": {\"id\": \"file\", \"value\": \"File\"} }";

    Gson gson = new Gson();
    MenuWrapper m = gson.fromJson(json, MenuWrapper.class);
    System.out.println(m.getMenu().getId());
    System.out.println(m.getMenu().getValue());

}

It will print:

file
File

And your JSON: {"menu": {"id": "file", "value": "File", } } has an error, it has an extra comma. It should be:

{"menu": {"id": "file", "value": "File" } }

Answer 2

我发现Gson有用的是创建一个类的实例，在其上调用toJson()并将生成的字符串与我试图解析的字符串进行比较。