Deserializing Generic Types with GSON

Question

I have some problems with implementation of Json Deserialization in my Android application (with Gson library)

I've made class like this

public class MyJson<T>{     public List<T> posts; } 

And Deserialization call is:

public class JsonDownloader<T> extends AsyncTask<Void, Void, MyJson<T>> { ... protected MyJson<T> doInBackground(Void... params) {   ...     Reader reader = new InputStreamReader(content);     GsonBuilder gson = new GsonBuilder();     Type collectionType = new TypeToken<MyJson<T>>() {}.getType();     result = gson.create().fromJson(reader, collectionType);   ...   } } 

Problem is that result.posts list after call holds one Array of LinkedTreeMap Objects(with correct values so problem is Deserialization) instead of MyJson Objects. When I use MyObject instead of T everything is running fine and MyObject is correct.

So is there any way to implement deserialization call without creating custom deserializer?

Answer 1

You have to specify the type of T at the time of deserialization. How would your List of posts get created if Gson didn't know what Type to instantiate? It can't stay T forever. So, you would provide the type T as a Class parameter.

Now assuming, the type of posts was String you would deserialize MyJson<String> as (I've also added a String json parameter for simplicity; you would read from your reader as before):

doInBackground(String.class, "{posts: [\"article 1\", \"article 2\"]}");  protected MyJson<T> doInBackground(Class<T> type, String json, Void... params) {      GsonBuilder gson = new GsonBuilder();     Type collectionType = new TypeToken<MyJson<T>>(){}.getType();      MyJson<T> myJson = gson.create().fromJson(json, collectionType);      System.out.println(myJson.getPosts()); // ["article 1", "article 2"]     return myJson; } 

Similarly, to deserialize a MyJson of Boolean objects

doInBackground(Boolean.class, "{posts: [true, false]}");  protected MyJson<T> doInBackground(Class<T> type, String json, Void... params) {      GsonBuilder gson = new GsonBuilder();     Type collectionType = new TypeToken<MyJson<T>>(){}.getType();      MyJson<T> myJson = gson.create().fromJson(json, collectionType);      System.out.println(myJson.getPosts()); // [true, false]     return myJson; } 

I've assumed MyJson<T> for my examples to be as

public class MyJson<T> {      public List<T> posts;      public List<T> getPosts() {         return posts;     } } 

So, if you were looking for to deserialize a List<MyObject> you would invoke the method as

// assuming no Void parameters were required MyJson<MyObject> myJson = doInBackground(MyObject.class); 

Answer 2

Have you tried?

gson.create().fromJson(reader, MyJson.class); 

EDIT

After reading this post it seems that you use of Type is correct. I believe your issue is the use of T. You must remember that with Java there is type-erasure. This means that at runtime all instances of T are replaced with Object. Therefore at runtime what you are passing GSON is really MyJson<Object>. If you tried this with a concrete class in place of <T> I believe it would work.

Google Gson - deserialize list<class> object? (generic type)