GSON: Expected a string but was BEGIN_OBJECT?

Question

I'm trying to use GSON to parse some very simple JSON. Here's my code:

    Gson gson = new Gson();

    InputStreamReader reader = new InputStreamReader(getJsonData(url));
    String key = gson.fromJson(reader, String.class);

Here's the JSON returned from the url:

{
  "access_token": "abcdefgh"
}

I'm getting this exception:

E/AndroidRuntime(19447): com.google.gson.JsonSyntaxException:     java.lang.IllegalStateException: Expected a string but was BEGIN_OBJECT at line 1 column 2

Any ideas? I'm new to GSON.

Answer 1

Another "low level" possibility using the Gson JsonParser:

package stackoverflow.questions.q11571412;

import com.google.gson.*;

public class GsonFooWithParser
{
  public static void main(String[] args)
  {
    String jsonInput = "{\"access_token\": \"abcdefgh\"}";

    JsonElement je = new JsonParser().parse(jsonInput);

    String value = je.getAsJsonObject().get("access_token").getAsString();
    System.out.println(value);
  }
}

If one day you'll write a custom deserializer, JsonElement will be your best friend.

Answer 2

The JSON structure is an object with one element named "access_token" -- it's not just a simple string. It could be deserialized to a matching Java data structure, such as a Map, as follows.

import java.util.Map;

import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;

public class GsonFoo
{
  public static void main(String[] args)
  {
    String jsonInput = "{\"access_token\": \"abcdefgh\"}";

    Map<String, String> map = new Gson().fromJson(jsonInput, new TypeToken<Map<String, String>>() {}.getType());
    String key = map.get("access_token");
    System.out.println(key);
  }
}

Another common approach is to use a more specific Java data structure that matches the JSON. For example:

import com.google.gson.Gson;
import com.google.gson.annotations.SerializedName;

public class GsonFoo
{
  public static void main(String[] args)
  {
    String jsonInput = "{\"access_token\": \"abcdefgh\"}";

    Response response = new Gson().fromJson(jsonInput, Response.class);
    System.out.println(response.key);
  }
}

class Response
{
  @SerializedName("access_token")
  String key;
}