How to Create a Custom JSON Deserializer for Gson?

Custom JSON Deserializer for Gson

When receiving JSON data with custom structures, Gson's default deserialization process may not suffice. In such cases, a custom deserializer is necessary.

Consider the following Java class representing a user:

public class User {
    int id;
    String name;
    Timestamp updateDate;
}

And a JSON structure representing a list of users:

[{"id":1,"name":"Jonas","update_date":"1300962900226"},
{"id":5,"name":"Test","date_date":"1304782298024"}]

To deserialize this JSON into User instances, a custom deserializer can be defined:

import com.google.gson.JsonDeserializationContext;
import com.google.gson.JsonDeserializer;
import com.google.gson.JsonElement;
import com.google.gson.JsonParseException;
import java.lang.reflect.Type;
import java.sql.Timestamp;

public class UserDeserializer implements JsonDeserializer<User> {
    @Override
    public User deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context)
        throws JsonParseException {
        long time = json.getAsJsonPrimitive().getAsLong();
        return new User(
            json.get("id").getAsInt(),
            json.get("name").getAsString(),
            new Timestamp(time));
    }
}

This deserializer parses the JSON into a User instance, taking into account the specified field names and custom timestamp parsing.

To apply this deserializer to the Gson instance, use the registerTypeAdapter method:

GsonBuilder gsonBuilder = new GsonBuilder();
gsonBuilder.registerTypeAdapter(User.class, new UserDeserializer());
Gson gson = gsonBuilder.create();

Now, the JSON can be deserialized into a list of User instances using the modified Gson instance.

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