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ARRAY or OBJECT in Retrofit on Android using TypeAdapter, in a two depth level

I'm struggling with TypeAdapter. Indeed for a json field, I can have an Array (when it's empty) or an Object (when it's not empty). This can't be changed.
Here is the JSON received :
{
"notifications": [
{
...
}
],
"meta": {
"pagination": {
"total": 13,
"count": 13,
"per_page": 20,
"current_page": 1,
"total_pages": 1,
"links": []
}
}
}
The field concerned is links, as you can see the field is inside pagination, which is inside meta. And that's my issue, I don't know how the TypeAdapter has to handle links in a two depth level.
I used this reply to start building a solution. Here it is :
My Custom TypeAdapter class :
public class PaginationTypeAdapter extends TypeAdapter<Pagination> {
private Gson gson = new Gson();
#Override
public void write(JsonWriter out, Pagination pagination) throws IOException {
gson.toJson(pagination, Links.class, out);
}
#Override
public Pagination read(JsonReader jsonReader) throws IOException {
Pagination pagination;
jsonReader.beginObject();
if (jsonReader.peek() == JsonToken.BEGIN_ARRAY) {
pagination = new Pagination((Links[]) gson.fromJson(jsonReader, Links[].class));
} else if(jsonReader.peek() == JsonToken.BEGIN_OBJECT) {
pagination = new Pagination((Links) gson.fromJson(jsonReader, Links.class));
} else {
throw new JsonParseException("Unexpected token " + jsonReader.peek());
}
return pagination;
}
}
My Pagination class :
public class Pagination {
private int total;
private int count;
#SerializedName("per_page")
private int perPage;
#SerializedName("current_page")
private int currentPage;
#SerializedName("total_pages")
private int totalPages;
private Links links;
Pagination(Links ... links) {
List<Links> linksList = Arrays.asList(links);
this.links = linksList.get(0);
}
}
And I'm building my Gson object like that :
Gson gson = new GsonBuilder().registerTypeAdapter(Pagination.class, new PaginationTypeAdapter()).create();
For now my error is : com.google.gson.JsonParseException: Unexpected token NAME
So I know I'm not doing it right, because I'm building my Gson with pagination. But I don't know how it should be handle. Using a TypeAdapter with meta ?
Any help will be welcome, thanks !

When you implement a custom type adapter, make sure that your type adapter has balanced token reading and writing: if you open a composite token pair like [ and ], you have to close it (applies for both JsonWriter and JsonReader). You just don't need this line to fix your issue:
jsonReader.beginObject();
because it moves the JsonReader instance to the next token, so the next token after BEGIN_OBJECT is either NAME or END_OBJECT (the former in your case sure).
Alternative option #1
I would suggest also not to use ad-hoc Gson object instatiation -- this won't share the configuration between Gson instances (say, your "global" Gson has a lot of custom adapters registered, but this internal does not have any thus your (de)serialization results might be very unexpected). In order to overcome this, just use TypeAdapterFactory that is more context-aware than a "free" Gson instance.
final class PaginationTypeAdapterFactory
implements TypeAdapterFactory {
private static final TypeAdapterFactory paginationTypeAdapterFactory = new PaginationTypeAdapterFactory();
private PaginationTypeAdapterFactory() {
}
static TypeAdapterFactory getPaginationTypeAdapterFactory() {
return paginationTypeAdapterFactory;
}
#Override
public <T> TypeAdapter<T> create(final Gson gson, final TypeToken<T> typeToken) {
// Classes can be compared using == and !=
if ( typeToken.getRawType() != Pagination.class ) {
// Not Pagination? Let Gson pick up the next best-match
return null;
}
// Here we get the references for two types adapters:
// - this is what Gson.fromJson does under the hood
// - we save some time for the further (de)serialization
// - you classes should not ask more than they require
final TypeAdapter<Links> linksTypeAdapter = gson.getAdapter(Links.class);
final TypeAdapter<Links[]> linksArrayTypeAdapter = gson.getAdapter(Links[].class);
#SuppressWarnings("unchecked")
final TypeAdapter<T> typeAdapter = (TypeAdapter<T>) new PaginationTypeAdapter(linksTypeAdapter, linksArrayTypeAdapter);
return typeAdapter;
}
private static final class PaginationTypeAdapter
extends TypeAdapter<Pagination> {
private final TypeAdapter<Links> linksTypeAdapter;
private final TypeAdapter<Links[]> linksArrayTypeAdapter;
private PaginationTypeAdapter(final TypeAdapter<Links> linksTypeAdapter, final TypeAdapter<Links[]> linksArrayTypeAdapter) {
this.linksTypeAdapter = linksTypeAdapter;
this.linksArrayTypeAdapter = linksArrayTypeAdapter;
}
#Override
public void write(final JsonWriter out, final Pagination pagination)
throws IOException {
linksTypeAdapter.write(out, pagination.links);
}
#Override
public Pagination read(final JsonReader in)
throws IOException {
final JsonToken token = in.peek();
// Switches are somewhat better: you can let your IDE or static analyzer to check if you covered ALL the cases
switch ( token ) {
case BEGIN_ARRAY:
return new Pagination(linksArrayTypeAdapter.read(in));
case BEGIN_OBJECT:
return new Pagination(linksTypeAdapter.read(in));
case END_ARRAY:
case END_OBJECT:
case NAME:
case STRING:
case NUMBER:
case BOOLEAN:
case NULL:
case END_DOCUMENT:
// MalformedJsonException, not sure, might be better, because it's an IOException and the read method throws IOException
throw new MalformedJsonException("Unexpected token: " + token + " at " + in);
default:
// Maybe some day Gson adds something more here... Let be prepared
throw new AssertionError(token);
}
}
}
}
Alternative option #2
You can annotate your private Links links; with #JsonAdapter and bind a type adapter factory directly to links: Gson will "inject" links objects directly to Pagination instances, so you don't even need a constructor there.

Deserialize in-string JSON using Gson

I am using Retrofit 2 with GsonConverter. The problem is I have this response:
"responseData": {
"data": "<json array>"
}
As you can see one of the parameters is a JSON array, but it is a string. Should I use TypeAdapter and override the read and write methods? If so can you show how I can do this?

As you can see one of the parameters is a JSON array, but it is a string.
If the response generator is under your control, you should definitely change the response format (both for well-formedness and performance (Gson does not allow to read/write string literals as raw values)).
Should I use TypeAdapter and override the read and write methods?
If you cannot control your server response, you have to implement a custom type adapter with the read method implemented only. To align with that response format, you could define custom mappings like these:
final class Response<T> {
final ResponseData<T> responseData = null;
}
final class ResponseData<T> {
// This is where we're telling Gson to apply the special read strategy, not to all types
#JsonAdapter(RawJsonTypeAdapterFactory.class)
final T data = null;
}
As you can see, you just have to bind a custom type adapter to a specific field only. Despite the JsonAdapter annotation accepts TypeAdapter classes as well, you cannot bind TypeAdapter directly because you need Gson and Type instances.
final class RawJsonTypeAdapterFactory
implements TypeAdapterFactory {
// Gson will instantiate it itself without any issues
private RawJsonTypeAdapterFactory() {
}
#Override
public <T> TypeAdapter<T> create(final Gson gson, final TypeToken<T> typeToken) {
// No additional checks here, we're assuming the necessary fields are properly annotated
final Type type = typeToken.getType();
return new TypeAdapter<T>() {
#Override
public void write(final JsonWriter out, final T value) {
throw new UnsupportedOperationException();
}
#Override
public T read(final JsonReader in)
throws IOException {
return gson.fromJson(in.nextString(), type);
}
}.nullSafe(); // And making the type adapter null-safe
}
}
Now JSON documents like
{
"responseData": {
"data": "[1,2,3]"
}
}
can be easily parsed in plain Java:
private static final Type intArrayResponseType = new TypeToken<Response<int[]>>() {
}.getType();
private static final Gson gson = new Gson();
...
try ( final JsonReader jsonReader = getPackageResourceJsonReader(Q43456942.class, "stringified.json") ) {
final Response<int[]> response = gson.fromJson(jsonReader, intArrayResponseType);
System.out.println(Arrays.toString(response.responseData.data));
}
Output:
[1, 2, 3]
Your Retrofit-bound service might have a method declared this (no type tokens necessary since Retrofit is smart enough to pass the Call parameterization to the underlying Gson converter):
Call<Response<int[]>> getIntArray();

Android Retrofit + Realm + Gson: Serializer not called

I'm having a bit of an issue with my custom serializers working with Retrofit, Realm and Gson. Here I am registering type adapters for my three classes:
GsonBuilder builder = new GsonBuilder();
// Register type adapters to modify outgoing json according to server requirements.
try {
builder
.registerTypeAdapter(Class.forName("io.realm.UserProxy"), new UserSerializer())
.registerTypeAdapter(Class.forName("io.realm.FavoriteSongProxy"), new FavoriteSongSerializer())
.registerTypeAdapter(Class.forName("io.realm.ArtistProxy"), new ArtistSerializer())
} catch (ClassNotFoundException e) {
e.printStackTrace();
return null;
}
Gson gson = builder.create();
return new Retrofit.Builder()
.baseUrl("http://10.0.3.2:8080/myUrl/rest/v1/")
.addConverterFactory(GsonConverterFactory.create(gson))
.client(httpClient.build())
.build();
A user has a FavoriteSong, and a FavoriteSong has an Artist. My serializers are set up very similar, here is an example for the FavoriteSongSerializer.
public class FavoriteSongSerializer implements JsonSerializer<FavoriteSong> {
#Override
public JsonElement serialize(FavoriteSong src, Type typeOfSrc, JsonSerializationContext context) {
Log.v("qwer", "serializingFavoriteSong");
final JsonObject jsonObject = new JsonObject();
jsonObject.addProperty("id", src.getId());
jsonObject.add("song", context.serialize(src.getSong()));
return jsonObject;
}
}
Here is the User serializer:
public class UserSerializer implements JsonSerializer<User> {
#Override
public JsonElement serialize(User src, Type typeOfSrc, JsonSerializationContext context) {
Log.v("qwer", "serializingUser");
final JsonObject jsonObject = new JsonObject();
jsonObject.addProperty("id", src.getId());
jsonObject.add("favoriteSong", context.serialize(src.getFavoriteSong()));
return jsonObject;
}
}
The weird part is, both the FavoriteSong and Song serializers are called when I send my request (I can see the log statements), but the UserSerializer is not. Anyone know why this might be happening, or how I might troubleshoot?
/** Edit **/
So as a test I created a TestModel that just contains an id, created a serializer for it, registered a type adapter, and added it to my User model. And it also fails to call the TestModelSerializer (even though it serializes according to the default method).

Try with registerTypeHierarchyAdapter instead of registerTypeAdapter:
.registerTypeHierarchyAdapter(Class.forName("io.realm.UserProxy"), new UserSerializer())
EDIT:
builder
.setExclusionStrategies(new ExclusionStrategy() {
#Override
public boolean shouldSkipField(FieldAttributes f) {
return f.getDeclaringClass().equals(RealmObject.class);
}
#Override
public boolean shouldSkipClass(Class<?> clazz) {
return false;
}
})
.registerTypeAdapter(Class.forName("io.realm.UserProxy"), new UserSerializer())
.registerTypeAdapter(Class.forName("io.realm.FavoriteSongProxy"), new FavoriteSongSerializer())
.registerTypeAdapter(Class.forName("io.realm.ArtistProxy"), new ArtistSerializer())

So if register my User type adapter as:
registerTypeAdapter(User.class, new UserSerializer())
instead of:
.registerTypeAdapter(Class.forName("io.realm.UserProxy"), new UserSerializer())
the User serializer is called. I still have no idea why, but it works.

Retrofit returns object with null members

When I try to parse the following JSON with Retrofit, I end up with null member objects.
Parsing:
RestAdapter restAdapter = new RestAdapter.Builder()
.setEndpoint(CallerInfo.API_URL)
.setLogLevel(RestAdapter.LogLevel.FULL)
.build();
InGameInfo igi = restAdapter.create(InGameInfo.class);
Game game = igi.fetchInGameInfo("EUW", "sasquatching");
Log.d("Cancantest", "Game " + game); //Not null
Log.d("Cancantest", "Team one " + game.getTeamOne()); //Null
Game Class:
#SerializedName("teamTwo")
#Expose private Team teamTwo;
#SerializedName("teamOne")
#Expose private Team teamOne;
public void setTeamOne(Team teamOne) {
this.teamOne = teamOne;
}
public void setTeamTwo(Team teamTwo) {
this.teamTwo = teamTwo;
}
public Team getTeamOne() {
return teamOne;
}
public Team getTeamTwo() {
return teamTwo;
}
Team Class:
#SerializedName("array")
#Expose private TeamMember[] teamMembers;
public void setTeamMembers(TeamMember[] teamMembers) {
this.teamMembers = teamMembers;
}
public TeamMember[] getTeamMembers() {
return teamMembers;
}
Example JSON:
{
"game":{
"teamTwo":{
"array":[]
},
"teamOne":{
"array":[]
}
}
}

The JSON contains a top level "game" entry so you cannot directly deserialize an instance of game. You need another type which has a field of type Game that represents the response.
public class Response {
public final Game game;
public Response(Game game) {
this.game = game;
}
}
You can put your JSON in a string and use Gson directly to test how the response will be deserialized. This behavior has almost nothing to do with Retrofit and all to do with the behavior of Gson.
String data = "...";
Game game = gson.fromJson(data, Game.class);
Response response = gson.fromJson(data, Response.class);

There can be one more reason for somewhat similar behavior: in this case debugger actually has no field members for the response returned from Retrofit.
And the reason for that is proguard. If you are using minifyEnabled true, make sure you explicitly tell it to keep your POJOs. It can be something like that:
#save model classes
-keep class com.example.app.**.model.** {*; }

Serializing a SparseArray<T> with GSON

I want to serialize a custom Java object, so I can use SharedPreferences to store it and retreive it in another Activity. I don't need persistant storage, the SharedPreferences, I wipe them when my application is closed. I'm currently using GSON for this, but it doesn't seem to work well with Android's SparseArray type.
My objects:
public class PartProfile {
private int gameId;
// Some more primitives
private SparseArray<Part> installedParts = new SparseArray<Part>();
// ...
}
public class Part {
private String partName;
// More primitives
}
Serialization:
Type genericType = new TypeToken<PartProfile>() {}.getType();
String serializedProfile = Helpers.serializeWithJSON(installedParts, genericType);
preferences.edit().putString("Parts", serializedProfile).commit();
serializeWithJSON():
public static String serializeWithJSON(Object o, Type genericType) {
Gson gson = new Gson();
return gson.toJson(o, genericType);
}
Deserialization:
Type genericType = new TypeToken<PartProfile>() {}.getType();
PartProfile parts = gson.fromJson(preferences.getString("Parts", "PARTS_ERROR"), genericType);
SparseArray<Part> retreivedParts = parts.getInstalledParts();
int key;
for (int i = 0; i < retreivedParts.size(); i++) {
key = retreivedParts.keyAt(i);
// Exception here:
Part part = retreivedParts.get(key);
// ...
}
Exception:
java.lang.ClassCastException: com.google.gson.internal.LinkedTreeMap cannot be cast to com.mypackage.objects.Part
I don't understand why Gson wants to cast a LinkedTreeMap to my object, I never use one in my entire program. I used to have a HashMap<Integer,Part> before I switched to the SparseArray<Part>, and never had issues with that. Are SparseArrays not supported by Gson, or is there an error on my side?
Edit: It seems that the SparseArray gets deserialized correctly, but not the objects inside. Instead of LinkedTreeMaps, these should be of type Part.

Really there is a way to serialize any kind of SparseArray, here is an example code:
public class SparseArrayTypeAdapter<T> extends TypeAdapter<SparseArray<T>> {
private final Gson gson = new Gson();
private final Class<T> classOfT;
private final Type typeOfSparseArrayOfT = new TypeToken<SparseArray<T>>() {}.getType();
private final Type typeOfSparseArrayOfObject = new TypeToken<SparseArray<Object>>() {}.getType();
public SparseArrayTypeAdapter(Class<T> classOfT) {
this.classOfT = classOfT;
}
#Override
public void write(JsonWriter jsonWriter, SparseArray<T> tSparseArray) throws IOException {
if (tSparseArray == null) {
jsonWriter.nullValue();
return;
}
gson.toJson(gson.toJsonTree(tSparseArray, typeOfSparseArrayOfT), jsonWriter);
}
#Override
public SparseArray<T> read(JsonReader jsonReader) throws IOException {
if (jsonReader.peek() == JsonToken.NULL) {
jsonReader.nextNull();
return null;
}
SparseArray<Object> temp = gson.fromJson(jsonReader, typeOfSparseArrayOfObject);
SparseArray<T> result = new SparseArray<T>(temp.size());
int key;
JsonElement tElement;
for (int i = 0; i < temp.size(); i++) {
key = temp.keyAt(i);
tElement = gson.toJsonTree(temp.get(key));
result.put(key, gson.fromJson(tElement, classOfT));
}
return result;
}
}
and to use it you need to register it in your Gson object, like this:
Type sparseArrayType = new TypeToken<SparseArray<MyCustomClass>>() {}.getType();
Gson gson = new GsonBuilder()
.registerTypeAdapter(sparseArrayType, new SparseArrayTypeAdapter<MyCustomClass>(MyCustomClass.class))
.create();
you can find this example in this gist.
P.S.: I know it's not optimized at all, but it's only an example to give an idea on how to achieve what you need.

It seems that the SparseArray gets deserialized correctly, but not the
objects inside. Instead of LinkedTreeMaps, these should be of type
Part.
Your observation is correct, since SparseArray contains Object (not Part), Gson won't have any clue to make Part as your object type. Hence it map your list as its infamous internal type LinkedTreeMap.
To solve it, I think you won't be able to use SparseArray... Or you may try retreivedParts.get(key).toString(), then use gson to parse the object again. But I don't think it's efficient to do that

As pointed out in the other answers SparseArray's internal implementation uses an Object[] to store the values so Gson cannot deserialize it correctly.
This can be solved by creating a custom Gson TypeAdapterFactory:
import java.io.IOException;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import android.util.SparseArray;
import com.google.gson.Gson;
import com.google.gson.TypeAdapter;
import com.google.gson.TypeAdapterFactory;
import com.google.gson.reflect.TypeToken;
import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonWriter;
public class SparseArrayTypeAdapterFactory implements TypeAdapterFactory {
public static final SparseArrayTypeAdapterFactory INSTANCE = new SparseArrayTypeAdapterFactory();
private SparseArrayTypeAdapterFactory() { }
#Override
public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
// This factory only supports (de-)serializing SparseArray
if (type.getRawType() != SparseArray.class) {
return null;
}
// Get the type argument for the element type parameter `<E>`
// Note: Does not support raw SparseArray type (i.e. without type argument)
Type elementType = ((ParameterizedType) type.getType()).getActualTypeArguments()[0];
TypeAdapter<?> elementAdapter = gson.getAdapter(TypeToken.get(elementType));
// This is safe because check at the beginning made sure type is SparseArray
#SuppressWarnings("unchecked")
TypeAdapter<T> adapter = (TypeAdapter<T>) new SparseArrayTypeAdapter<>(elementAdapter);
// call nullSafe() to make adapter automatically handle `null` SparseArrays
return adapter.nullSafe();
}
private static class SparseArrayTypeAdapter<E> extends TypeAdapter<SparseArray<E>> {
private final TypeAdapter<E> elementTypeAdapter;
public SparseArrayTypeAdapter(TypeAdapter<E> elementTypeAdapter) {
this.elementTypeAdapter = elementTypeAdapter;
}
#Override
public void write(JsonWriter out, SparseArray<E> sparseArray) throws IOException {
out.beginObject();
int size = sparseArray.size();
for (int i = 0; i < size; i++) {
out.name(Integer.toString(sparseArray.keyAt(i)));
elementTypeAdapter.write(out, sparseArray.valueAt(i));
}
out.endObject();
}
#Override
public SparseArray<E> read(JsonReader in) throws IOException {
in.beginObject();
SparseArray<E> sparseArray = new SparseArray<>();
while (in.hasNext()) {
int key = Integer.parseInt(in.nextName());
E value = elementTypeAdapter.read(in);
// Use `append(...)` here because SparseArray is serialized in ascending
// key order so `key` will be > previously added key
sparseArray.append(key, value);
}
in.endObject();
return sparseArray;
}
}
}
This factory serializes SparseArrays as JSON objects with the key as JSON property name and the value serialized with the respective adapter as JSON value, e.g.:
new SparseArray<List<String>>().put(5, Arrays.asList("Hello", "World"))
    ↓ JSON
{"5": ["Hello", "World"]}
You then use this TypeAdapterFactory by creating your Gson instance using a GsonBuilder on which you register the TypeAdapterFactory:
Gson gson = new GsonBuilder()
.registerTypeAdapterFactory​(SparseArrayTypeAdapterFactory.INSTANCE)
.create();