CODEWITHSUNDEEP
javajsongenericsgson
Rodrigo Tavares
Rodrigo Tavares

Reputation: 369

Gson deserialize generic list into a generic method

When I have a list inside a generic method Gson returns a list of Object instead a list of generic type. I've seen a lot of threads with no solution, and if I dont use a generic method would have to create a method for each bean.

Is there any one with any idea what do I have to do so solve it?

PS: For a while I've created loop into the list to serialize entity by entity, splitting returned String and deserializing entity by entity, but clearly it's a workaround

Creating a generic list and serializing to JSON (this is a webservices method):

public String listEntity(String nomeClasse) throws WsException {
    // Identifying the entity class
    Class<?> clazz = Class.forName(nomeClasse);

    // Querying with Hibernate
    List<?> lst = getDao().listEntity(clazz);

    // Check if is null
    if (lst == null) {
        return "[]";
    }

    return gson.toJson(lst);
}

Consuming the Webservice method:

public <T> List<T> listEntity(Class<T> clazz)
        throws WsIntegracaoException {
    try {
        // Consuming remote method
        String strJson = getService().listEntity(clazz.getName());

        Type type = new TypeToken<List<T>>() {}.getType();

        // HERE IS THE PROBLEM
        List<T> lst =  GSON.fromJson(strJson, type);
        // RETURNS IS A LIST OF OBJECT INSTEAD OF A LIST OF <T>

        return lst;

    } catch (Exception e) {
        throw new WsIntegracaoException(
                "WS method error [listEntity()]", e);
    }
}

Invoking the generic method:

List<City> list =  listEntity(City.class);
// Here I get a ClassCastException
fillTable(list);

List Element (wrong):

java.lang.Object@23f6b8

Exception:

java.lang.ClassCastException: java.lang.Object cannot be cast to java.io.Serializable

Upvotes: 1

Views: 1767

Answers (1)

Rodrigo Tavares
Rodrigo Tavares

Reputation: 369

SOLUTION - THIS WORKED FOR ME: Gson TypeToken with dynamic ArrayList item type

public <T> List<T> listEntity(Class<T> clazz)
        throws WsIntegracaoException {
    try {
        // Consuming remote method
        String strJson = getService().listEntity(clazz.getName());

        JsonParser parser = new JsonParser();
        JsonArray array = parser.parse(strJson).getAsJsonArray();

        List<T> lst =  new ArrayList<T>();
        for(final JsonElement json: array){
            T entity = GSON.fromJson(json, clazz);
            lst.add(entity);
        }

        return lst;

    } catch (Exception e) {
        throw new WsIntegracaoException(
                "WS method error [listEntity()]", e);
    }
}

Upvotes: 4

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