Gson deserialization of List into realmList

Question

I'm using retrofit with gson to deserialize my json into realm objects. This works very well for the most part. Trouble arises when dealing with

RealmList(String(or any other basic data type))

Since Realm doesnt support RealmList where E doesnt extend Realm object, I wrapped String in a RealmObject.

public class RealmString extends RealmObject {
  private String val;

  public String getValue() {
    return val;
  }

  public void setValue(String value) {
    this.val = value;
  }
}

My realm Object is as below

    public class RealmPerson extends RealmObject {
    @PrimaryKey
    private String userId;
    ...
    private RealmList<RealmString> stringStuff;
    private RealmList<SimpleRealmObj> otherStuff;

    <setters and getters>
   }

SimpleRealmObj works fine as it only has String elements

    public class SimpleRealmObj extends RealmObject {
    private String foo;
    private String bar;
       ...
    }

How can I deserialize stringStuff? I tried using a gson TypeAdapter

public class RealmPersonAdapter extends TypeAdapter<RealmPerson> {
    @Override
    public void write(JsonWriter out, RealmPerson value) throws IOException {
        out.beginObject();
        Log.e("DBG " + value.getLastName(), "");
        out.endObject();
    }

    @Override
    public RealmPerson read(JsonReader in) throws IOException {
        QLRealmPerson rList = new RealmPerson();
        in.beginObject();
        while (in.hasNext()) {
            Log.e("DBG " + in.nextString(), "");
        }
        in.endObject();

        return rList;
    }

However I still hit the IllegalStateException

2334-2334/com.qualcomm.qlearn.app E//PersonService.java:71﹕ main com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected a string but was NAME at line 1 column 3 path $.

I tried RealmList, RealmString adapter earlier to no avail. The only workaround I managed to find so far is https://github.com/realm/realm-java/issues/620#issuecomment-66640786 Any better options?

Answer 1

It is better to use JsonSerializer and JsonDeserializer rather than TypeAdapter for your RealmObject, because of 2 reasons:

1. They allow you to delegate (de)serialization for your RealmObject to the default Gson (de)serializer, which means you don't need to write the boilerplate yourself.

2. There's a weird bug in Gson 2.3.1 that might cause a StackOverflowError during deserialization (I tried the TypeAdapter approach myself and encountered this bug).

Here's how (replace Tag with your RealmObject class):

(NOTE that context.serialize and context.deserialize below are equivalent to gson.toJson and gson.fromJson, which means we don't need to parse the Tag class ourselves.)

Parser + serializer for RealmList<Tag>:

public class TagRealmListConverter implements JsonSerializer<RealmList<Tag>>,
        JsonDeserializer<RealmList<Tag>> {

    @Override
    public JsonElement serialize(RealmList<Tag> src, Type typeOfSrc,
                                 JsonSerializationContext context) {
        JsonArray ja = new JsonArray();
        for (Tag tag : src) {
            ja.add(context.serialize(tag));
        }
        return ja;
    }

    @Override
    public RealmList<Tag> deserialize(JsonElement json, Type typeOfT,
                                      JsonDeserializationContext context)
            throws JsonParseException {
        RealmList<Tag> tags = new RealmList<>();
        JsonArray ja = json.getAsJsonArray();
        for (JsonElement je : ja) {
            tags.add((Tag) context.deserialize(je, Tag.class));
        }
        return tags;
    }

}

Tag class:

@RealmClass
public class Tag extends RealmObject {
    private String value;

    public String getValue() {
        return value;
    }

    public void setValue(String value) {
        this.value = value;
    }
}

Then register your converter class with Gson:

Gson gson = new GsonBuilder()
        .registerTypeAdapter(new TypeToken<RealmList<Tag>>() {}.getType(),
                new TagRealmListConverter())
        .create();

Answer 2

The error message "Expected a string but was NAME" can be solved by retrieving the name of the json object in the JsonReader before the actual json object (which is a String in your case).

You can take a look at the Android documentation for JsonReader. It has detailed explanation and code snippet. You can also take a look at the readMessage method in the sample code snippet in the documentation.

I have modified your read method to what I think it should be. NOTE: I didn't test the code, so there may be some minor errors in it.

@Override
public RealmPerson read(JsonReader in) throws IOException {
    RealmPerson rList = new RealmPerson();
    in.beginObject();
    String name = "";
    while (in.hasNext()) {
        name = in.nextName();

        if (name.equals("userId")) {
            String userId = in.nextString();
            // update rList here 
        } else if (name.equals("otherStuff")) {
            // since otherStuff is a RealmList of RealmStrings,
            // your json data would be an array
            // You would need to loop through the array to retrieve 
            // the json objects
            in.beginArray();
            while (in.hasNext()) {
                // begin each object in the array
                in.beginObject();
                name = in.nextName();
                // the RealmString object has just one property called "value"
                // (according to the code snippet in your question)
                if (name.equals("val")) {
                    String val = in.nextString();
                     // update rList here 
                } else {
                    in.skipValue();
                }
                in.endObject();
            }
            in.endArray();
        } else {
            in.skipValue();
        }
    }
    in.endObject();


    return rList;
}

Let me know if this helps.

Answer 3

My gson typeAdapter was the culprit. The above error was seen as I wasnt deserializing the json into RealmPerson correctly, the first field is not a String, hence

in.nextString()

was borking.

I looked at some example code and it hit me, I didnt have to use

in.beginObject() and in.endObject()

to deserialize a String. The below code works.

public class QLRealmStringAdapter extends TypeAdapter<QLRealmString> {
@Override
public void write(JsonWriter out, QLRealmString value) throws IOException {
    Log.e("DBG " + value.getValue(), "");
    out.value(value.getValue());
}

@Override
public RealmString read(JsonReader in) throws IOException {
    RealmString rString = new RealmString();
    if (in.hasNext()) {
        String nextStr = in.nextString();
        System.out.println("DBG " + nextStr);
        rString.setValue(nextStr);
    }

    return rString;
}

}

Hope this helps someone.

Answer 4

i need a jackson serializer and deserializer for the Converting Arraylist to RealmList