Exception: Can not deserialize instance of java.util.ArrayList out of START_OBJECT token at

Question

I am trying to display data from a web service and getting this error : Exception in thread "main" com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of java.util.ArrayList out of START_OBJECT token at...

Find below my code :

Model object :

public class Commune implements Serializable{

    private static final long serialVersionUID = 1L;

    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Commune [name=" + name + "]";
    }
}

Main class:

public class Test {

    public static void main(String[] args) throws IOException {

        URL url = new URL("https://bj-decoupage-territorial.herokuapp.com/api/v1/towns");
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        connection.setRequestMethod("GET");
        connection.setRequestProperty("Accept", "application/json");

        if(connection.getResponseCode() != 200){
            throw new RuntimeException("Failed : Http code : "+connection.getResponseCode());
        }

        BufferedReader reader = new BufferedReader(new InputStreamReader(connection.getInputStream()));

        String output;

        while((output = reader.readLine()) != null){

            ObjectMapper mapper = new ObjectMapper();
            TypeReference<List<Commune>> mapType = new TypeReference<List<Commune>>() {};
            List<Commune> jsonToCommuneList = mapper.readValue(output, mapType);

            for(Commune c : jsonToCommuneList) {
                System.out.println(c.getName());
            }
        }
        connection.disconnect();        
    }
}

Here is the data getting from the url :

{"towns":
   [
      {"name":"BANIKOARA"},
      {"name":"GOGOUNOU"},
      {"name":"KANDI"},
      {"name":"KARIMAMA"}
   ]
}

Help me please to check what i did wrong.

Thank you

Answer 1

You are getting this error because you are trying to deserialize something that is not actually a JSON Array into a Collection

If you are able to change your JSON to send just a JSON Array format, it will look like this one below:

[
  {"name":"BANIKOARA"},
  {"name":"GOGOUNOU"},
  {"name":"KANDI"},
  {"name":"KARIMAMA"}
]

Otherwise, to parse it correctly you also can create a new class which will represent the town of your JSON and parse using it:

public class TownRecords implements Serializable {

    private List<Commune> communes;

    ... getters and setters
}

Answer 2

You're trying to deserialize a list of Commune, but in HTTP response you're getting an object, containing such a list, but not a list itself. So, you need another wrapper object for deserialisation:

Wrapper

public class Towns implements Serializable {
    private List<Commune> towns;

    public Towns() {}

    public List<Commune> getTowns() {
        return towns;
    }

    public void setTowns(List<Commune> towns) {
        this.towns = towns;
    }

    @Override
    public String toString() {
        return "Towns: " + towns.toString();
    }
}

main app

    TypeReference<Towns> mapType = new TypeReference<Towns>() {};
    Towns towns = mapper.readValue(output, mapType);
    System.out.println(towns);