How to read JSON to a list of generic objects in java?

Question

I have to get data from a web service, I'm using Jackson but I have the same problem using Gson, I have no problem with single objects but when I receive several objects list it is not that easy for me.

JSON received are like this:

{"country":
[
{"code":"AD","nombre":"Andorra","name":"Andorra"},
{"code":"AE","nombre":"Emiratos Árabes Unidos","name":"United Arab Emirates"}
]
}

This is a list of my own class CountryWSType, I have several classes like this and need a way that can get the list of any type of them. I've tried parse it like a list:

List<MyClass> myObjects = mapper.readValue(jsonInput, mapper.getTypeFactory().constructCollectionType(List.class, MyClass.class));

Also trying creating an own list type:

    public class ListWSType<T> implements List<T>{

    private List<T> listaInterna;

    //implents methods
    }

But I always get a JsonMappingException and I have no more ideas of how to do it.

I hope someone can help me.

As some people ask here is the class i was trying to parse from a JSON:

@XmlRootElement(name="country")
@XmlType(propOrder={"code", "nombre", "name"})
public class CountryWSType {

    /** CÃ³digo ISO */
    @XmlElement(name="code")
    public String code;

    /** Nombre en espaÃ±ol */
    @XmlElement(name="nombre")
    public String nombre;

    /** Nombre en inglÃ©s */
    @XmlElement(name="name")
    public String name;

    /** Constructor sin parÃ¡metros. No inicializa nada...
     * EstÃ¡ para que funcione el marshall/unmarshall.
     */
    public CountryWSType() {}
}

Also notice than when i put MyClass it means CountryWSType class, sorry for the missunderstood.

I'm guessing MyClass is your class backing the JSON objects? Maybe if we could see this class and the error stack trace it would be easier to find out what's the problem. — Micho
– Micho, Commented Jun 4, 2015 at 2:41
A List<MyClass> would be [{...}, {...}, ...]. You have {"country": [{...}, {...}, ...]}. That's not a list, it's an object with a field country which is a list. — Stack Exchange Broke The Law
– Stack Exchange Broke The Law, Commented Jun 4, 2015 at 3:16
You have, at the outermost layer, a JSON object, not a JSON array. This translates to a Map, not a List. — Hot Licks
– Hot Licks, Commented Jun 4, 2015 at 3:46
well, yes i think thats why i can't take the list like an array directly or something like this, i saw in other solutions that it will get it implemeting a class with an atribute named country as a list<CountryWSType>, but i need a way that will work also with other types, not only with CountryWSType. Dont know if i explain correctly... — José Alejandro Flores Roch
– José Alejandro Flores Roch, Commented Jun 4, 2015 at 3:48

Community · Accepted Answer · 2017-05-23 11:45:58Z

0

May be this will help:

//first convert input string to json array
JSONObject jsonobject = new JSONObject(countriesAsJsonString);

JSONArray jsonArray = jsnobject.getJSONArray("countries");

//then get the type for list and parse using gson as
Type listType = new TypeToken<List<MyClass>>(){}.getType();
List<MyClass> countriesList = new Gson().fromJson(jsonArray, listType);

If you are saying your json arrary elements can be of different types, than the mapper will not know which type of class to instantiate and you will need to supply additional attribute in your json to indicate that, an ugly approach specially if the json is external facing. But if this is what you want than it is explained in this SO post.

edited May 23, 2017 at 11:45

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answered Jun 4, 2015 at 3:21

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3 Comments

José Alejandro Flores Roch Over a year ago

yes, its external facing. But the thing is i wont get diferent types in the same Json just need a solution that will work with different classes, for example i cant use an own class like this public class ListCountryWSType { private List<CountryWSType> countries; } because it will only work with CountryWSType and i need it for other types too.

José Alejandro Flores Roch Over a year ago

Also checked your answer and I'm having problems with List<MyClass> countriesList = new Gson().fromJson(jsonArray, listType); i think its because JSONObject and JSONArray are classes from org.json and ´Gson().fromJson´ expect owns gson classes? Thanks for the answer anyway.

José Alejandro Flores Roch Over a year ago

Well thank you, i change some of your code and it works perfectly, thats what i changed: List<MyClass> countriesList = new Gson().fromJson(jsonArray.toString(), listType); Thank you again.

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