JsonMappingException: Can not construct instance of... rest API in androidstudio

Question

I'm trying to implement the global magnet API in an android application. I do a rest API get request and put the json API data in a string (this string is called dataString. This string looks like this:

{
  declination: {
  units: "Deg",
  value: -4.8235602378845215
  },
  inclination: {
  units: "Deg",
  value: -30.085556030273438
  },
  total_intensity: {
  units: "nT",
  value: 31945.123046875
  }
}

I'm now trying to deserialize this string into an object. I've made the following classes:

    public class MagneticData {
        @JsonProperty("declination")
        public MagneticDataElement declination;
        @JsonProperty("grid_variation")
        public MagneticDataElement grid_variation;
        @JsonProperty("inclination")
        public MagneticDataElement inclination;
        @JsonProperty("total_intensity")
        public MagneticDataElement total_intensity;
    }

    public class MagneticDataElement {
        @JsonProperty("units")
        public String units;
        @JsonProperty("value")
        public double value;
    }

I now use the ObjectMapper.readValue() function to convert the dataString to an object of the type MagneticData but i get the following error:

com.fasterxml.jackson.databind.JsonMappingException: Can not construct instance of com.mobcom.apitest.MainActivity$MagneticData: no suitable constructor found, can not deserialize from Object value (missing default constructor or creator, or perhaps need to add/enable type information?)

What am i doing wrong?

Answer 1

I tested your code with revised JSON string and add toString() to your POJOs, everything works fine as follows, so maybe you have to provide more information for debugging such as how you deserialize it and which version of Jackson library you used.

Revised JSON string

{
  "declination": {
    "units": "Deg",
    "value": -4.8235602378845215
  },
  "inclination": {
    "units": "Deg",
    "value": -30.085556030273438
  },
  "total_intensity": {
    "units": "nT",
    "value": 31945.123046875
  }
}

Code snippet

ObjectMapper mapper = new ObjectMapper();
MagneticData magneticData = mapper.readValue(jsonStr, MagneticData.class);
System.out.println(magneticData.toString());

Console output

MagneticData [declination=MagneticDataElement [units=Deg, value=-4.8235602378845215], inclination=MagneticDataElement [units=Deg, value=-30.085556030273438], total_intensity=MagneticDataElement [units=nT, value=31945.123046875]]

Answer 2

I'm using jackson version 2.8.5:

    implementation 'com.fasterxml.jackson.core:jackson-databind:2.8.5'
    implementation 'com.fasterxml.jackson.core:jackson-core:2.8.5'
    implementation 'com.fasterxml.jackson.core:jackson-annotations:2.8.5'

This is my full code:

public class MainActivity extends AppCompatActivity {
    Button click;
    TextView data;

    @JsonInclude(JsonInclude.Include.NON_NULL)
    @JsonPropertyOrder({
            "units",
            "value"
    })

    public class MagneticData {
        @JsonProperty("declination")
        public MagneticDataElement declination;
        @JsonProperty("grid_variation")
        public MagneticDataElement grid_variation;
        @JsonProperty("inclination")
        public MagneticDataElement inclination;
        @JsonProperty("total_intensity")
        public MagneticDataElement total_intensity;

        MagneticData(MagneticDataElement d, MagneticDataElement g, MagneticDataElement i, MagneticDataElement t) {
            declination = d;
            grid_variation = g;
            inclination = i;
            total_intensity = t;
        }
    }

    public class MagneticDataElement {
        @JsonProperty("units")
        public String units;
        @JsonProperty("value")
        public double value;

        MagneticDataElement(String u, double v) {
            units = u;
            value = v;
        }
    }


    public void deserialize(String str) throws IOException {

        //data.setText(jsonNode.get("total_intensity").textValue());
    }



    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        click = findViewById(R.id.button);
        data = findViewById(R.id.fetchedData);

        click.setOnClickListener(new View.OnClickListener() {
            String dataString = "";
            @Override
            public void onClick(View v) {
                AsyncTask.execute(new Runnable() {
                    @Override
                    public void run() {
                        double altitude = 0;
                        double latitude = 0;
                        double longitude = 0;
                        double year = 2019;
                        URL url = null;
                        try {
                            url = new URL("https://globalmagnet.amentum.space/api/calculate_magnetic_field?altitude="+altitude+"&latitude="+latitude+"&longitude="+longitude+"&year="+year);
                            HttpsURLConnection myConnection = (HttpsURLConnection) url.openConnection();
                            myConnection.setRequestMethod("GET");
                            myConnection.setRequestProperty("Accept", "application/json");
                            if (myConnection.getResponseCode() == 200) {
                                Log.e("TYPE", myConnection.getContentType());
                                Log.e("Content", String.valueOf(myConnection.getInputStream()));
                                InputStream inputStream = myConnection.getInputStream();
                                BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream));
                                String line = "";
                                while (line != null) {
                                    line = bufferedReader.readLine();
                                    dataString = dataString + line;
                                }
                                Log.e("DATA", dataString);
                                MagneticData magneticData = new ObjectMapper().readValue(dataString, MagneticData.class);

                                runOnUiThread(new Runnable() {

                                    @Override
                                    public void run() {

                                    }
                                });

                            } else {
                                Log.e("ERROR", String.valueOf(myConnection.getResponseCode()));
                                Log.e("ERROR", myConnection.getResponseMessage());
                                Log.e("ERROR", String.valueOf(myConnection.getHeaderField("altitude")));

                            }
                        } catch (java.io.IOException e) {
                            e.printStackTrace();
                        }
                    }
                });

            }
        });



    }
}