使用GSON解析JSON

Question

This is a continuation of my previous question. After getting the JSON request and turning it into a JSON, I am trying to parse the result into Java Object.

public class NetClientGet {

  @SuppressWarnings("unchecked")
  public static void main(String[] args) {
    String urlYesterday = "http://fids.changiairport.com/webfids/fidsp/get_flightinfo_cache.php?d=-1&type=pa&lang=en";
    String yesterdayJSON = getDataFromWeb(urlYesterday);
    //System.out.println("yesterdayJSON : " + yesterdayJSON);
    //jsonArray.add(yesterdayJSON);
    Gson gson = new Gson();
    List<FlightInfo> flightArray = (List<FlightInfo>) gson.fromJson(yesterdayJSON, FlightInfo.class);
    System.out.println("flightArray : " + flightArray);
  }

  private static String getDataFromWeb(String targetURL) {
    try {
      URL url = new URL(targetURL);
      HttpURLConnection conn = (HttpURLConnection) url.openConnection();
      conn.setRequestMethod("GET");
      conn.setRequestProperty("Accept", "application/json");
      if (conn.getResponseCode() != 200) {
        throw new RuntimeException("Failed : HTTP error code : " + conn.getResponseCode());
      }
      BufferedReader br;
      if ("gzip".equalsIgnoreCase(conn.getContentEncoding())) {
        br = new BufferedReader(new InputStreamReader(
        (new GZIPInputStream(conn.getInputStream()))));
      } else {
        br = new BufferedReader(new InputStreamReader(
        (conn.getInputStream())));
      }
      // Retrieve data from server
      String output = null;
      final StringBuffer buffer = new StringBuffer(16384);
      while ((output = br.readLine()) != null) {
        buffer.append(output);
      }
      conn.disconnect();
      // Extract JSON from the JSONP envelope
      String jsonp = buffer.toString();
      String json = jsonp.substring(jsonp.indexOf("(") + 1, //this is the index of the callback envelope
      jsonp.lastIndexOf(")"));
      //System.out.println("Output from server");
      //System.out.println(json);
      return json;
    } catch (MalformedURLException e) {
      e.printStackTrace();
    } catch (IOException e) {
      e.printStackTrace();
    }
    return null;
  }
}

I tried:

Gson gson = new Gson();
FlightInfo flightArray = gson.fromJson(yesterdayJSON, FlightInfo.class);

but it got me null, I am sure I need to parse it into a List since my data has a lot of entries.

sample of the JSON

"flights": [
  {
    "date": "2013-05-12",
    "scheduled_time": "23:30",
    "estimated_time": "23:09*",
    "airline": "TR",
    "flight_no": "TR2109",
    "airport": "BKK",
    "origin": "Bangkok (Suvarnabhumi)",
    "via": "",
    "terminal": "2",
    "belt": "35",
    "status": "Landed",
    "airline_name": "TIGER AIRWAYS",
    "airline_alias": "",
    "unixtime": "1368372600",
    "master_flight_no": "TR2109",
    "slave_flight_no": []
  }
]

and my FlightInfo.class

import java.sql.Time;
import java.util.Date;

public class FlightInfo {
  private Date date;
  private Time scheduled_time;
  private Time estimated_time;
  private String airline;
  private String flight_no;
  private String airport;
  private String origin;
  private String via;
  private String terminal;
  private String belt;
  private String status;
  private String airline_name;

  public Date getDate() {
    return date;
  }
  public void setDate(Date date) {
    this.date = date;
  }
  public Time getScheduled_time() {
    return scheduled_time;
  }
  public void setScheduled_time(Time scheduled_time) {
    this.scheduled_time = scheduled_time;
  }
  public Time getEstimated_time() {
    return estimated_time;
  }
  public void setEstimated_time(Time estimated_time) {
    this.estimated_time = estimated_time;
  }
  public String getAirline() {
    return airline;
  }
  public void setAirline(String airline) {
    this.airline = airline;
  }
  public String getFlight_no() {
    return flight_no;
  }
  public void setFlight_no(String flight_no) {
    this.flight_no = flight_no;
  }
  public String getAirport() {
    return airport;
  }
  public void setAirport(String airport) {
    this.airport = airport;
  }
  public String getOrigin() {
    return origin;
  }
  public void setOrigin(String origin) {
    this.origin = origin;
  }
  public String getVia() {
    return via;
  }
  public void setVia(String via) {
    this.via = via;
  }
  public String getTerminal() {
    return terminal;
  }
  public void setTerminal(String terminal) {
    this.terminal = terminal;
  }
  public String getBelt() {
    return belt;
  }
  public void setBelt(String belt) {
    this.belt = belt;
  }
  public String getStatus() {
    return status;
  }
  public void setStatus(String status) {
    this.status = status;
  }
  public String getAirline_name() {
    return airline_name;
  }
  public void setAirline_name(String airline_name) {
    this.airline_name = airline_name;
  }
  @Override
  public String toString() {
    return "FlightInfo [date=" + date + ", scheduled_time=" + scheduled_time +
      ", estimated_time=" + estimated_time +
      ", airline=" + airline +
      ", flight_no=" + flight_no +
      ", airport=" + airport +
      ", origin=" + origin +
      ", via=" + via +
      ", terminal=" + terminal +
      ", belt=" + belt +
      ", status=" + status +
      ", airline_name=" + airline_name +
      "]";
  }
}

Answer 1

I assume that your JSON response is surrounded by { } , otherwise it's not valid JSON...

That said, you need another class to parse your JSON response, for example:

public class Response {    
    private List<FlightInfo> flights;        
    //getters & setters
}

Now you can parse your response with:

Gson gson = new Gson();
Response response = gson.fromJson(yesterdayJSON, Response .class);
List<FlightInfo> flightArray = response.getFlights();

---

Note : looking at your class FlightInfo and your JSON response, I think you'll have trouble parsing your Date and Time fields, so I suggest you to parse them as strings and then do the correct transformation... or you'll have to create a custom deserializer .

Answer 2

The above code fails to interpret value as type FlightInfo because Gson invokes list.getClass() to get its class information, but this method returns a raw class, List.class. This means that Gson has no way of knowing that this is an object of type List < FlightInfo>, and not just plain List.

You can solve this problem by specifying the correct parameterized type for your generic type. You can do this by using the TypeToken class.

Type listType = new TypeToken<List<Bar>>() {}.getType();
gson.toJson(list, listType );
gson.fromJson(json, listType );

for more information check gson-user-guide