解析JSON字符串返回未定义

Question

This is my java method:

 public String getEvents(String cat, int start, int step) { //JSONArray list = new JSONArray(); List<String> list = new ArrayList(); if (cat.equalsIgnoreCase("D")){ for (int i=0; i<listDog.size(); i++){ JSONObject obj = new JSONObject(); System.out.println("di=" + i + "=" + (String)listDog.get(i)); obj.put("event_id",new String((String)listDog.get(i))); list.add(obj.toJSONString()); } } else { for (int i=0; i<listHorse.size(); i++){ JSONObject obj = new JSONObject(); System.out.println("i=" + i + "=" + (String)listHorse.get(i)); obj.put("event_id",new String((String)listHorse.get(i))); list.add(obj.toJSONString()); } } return list.toString(); } 

and below is how I convert it to json:

 Map map = new HashMap(); Gson gson = new GsonBuilder().disableHtmlEscaping().create() .... String result = service.getEvents(cat, Integer.parseInt(start), Integer.parseInt(step)); String objs = gson.toJson(result); map.put("result", objs); ..... String output = gson.toJson(map); System.out.println("output->" + output); out.println(output); out.flush(); out.close(); 

I'm getting the below JSON string from the server side:

{
  "result": "\"[{\\\"event_id\\\":\\\"2139114\\\"}]\"",
  "eventList": "[{\"eventId\":164151,\"meetingCode\":\"5G8QV\",\"meetingName\":\"Kranji\",\"eventTime\":\"13:20:00\",\"eventCode\":\"07:50:00\",\"category\":\"HR\"},{\"eventId\":163890,\"meetingCode\":\"5G8MW\",\"meetingName\":\"Henlow\",\"eventTime\":\"02:30:00\",\"eventCode\":\"21:00:00\",\"category\":\"DG\"}]"
}

and I convert it like this:

  success: function(data) { console.log("SERVLET DATA: " + data.replace("\\n", "")); if (typeof(data) !== 'undefined' && data !== '' && data !== null) { data = data.replace("\\n", ""); var jsonData = JSON.parse(data); //console.log(jsonData); for (var i in jsonData) { var event = jsonData[i]; var event2 = JSON.stringify(jsonData[i]); var event3 = JSON.parse(event); console.log("init:" + event.event_id); } } 

in all three logs, I'm getting "undefined". Please tell me how can I get the "event_id" value from the below json string?

"result": "\"[{\\\"event_id\\\":\\\"2139114\\\"}]\""

I checked validity of json string online and it says no errors found.

is there any issue here or how to access "event_id" field ?

Answer 1

Firstly, you need to re-factor the getEvents method to return only plain objects (eg List ), then, put the result in to the target map before passing it to the toJSON method

public class Event {
    String event_id;

    Event(String event_id) {
        this.event_id = event_id;
    }
}

public List<Event> getEvents(String cat, int start, int step)  {

    List<Event> list = new ArrayList<Event>();

    if (cat.equalsIgnoreCase("D")){
        for (int i=0; i<listDog.size(); i++){
            System.out.println("di=" + i + "=" + (String)listDog.get(i));

            list.add( new Event( (String)listDog.get(i) ));
        }
    }
    else {
        for (int i=0; i<listHorse.size(); i++){
            System.out.println("i=" + i + "=" + (String)listHorse.get(i));

            list.add( new Event( (String)listHorse.get(i) ));
        }
    }

    return list;

}

Then, convert your result like this:

Map map = new HashMap();
Gson gson = new GsonBuilder().disableHtmlEscaping().create()

....

List<Event> result = service.getEvents(cat, Integer.parseInt(start), Integer.parseInt(step));

map.put("result", result);

String objs = gson.toJson(map);

Answer 2

I changed my java method as below:

 public List < UKDashboardEventDTO > getEvents(String cat, int start, int step) { //JSONArray list = new JSONArray(); List < UKDashboardEventDTO > list = new ArrayList(); UKDashboardEventDTO dto; if (cat.equalsIgnoreCase("D")) { for (int i = start - 1; i < listDog.size(); i = i + step) { //JSONObject obj = new JSONObject(); dto = new UKDashboardEventDTO(); System.out.println("di=" + i + "=" + (String) listDog.get(i)); dto.setEventId(Long.parseLong(listDog.get(i).toString())); list.add(dto); } } else if (cat.equalsIgnoreCase("H")) { for (int i = start - 1; i < listHorse.size(); i = i + step) { dto = new UKDashboardEventDTO(); System.out.println("di=" + i + "=" + (String) listHorse.get(i)); dto.setEventId(Long.parseLong(listHorse.get(i).toString())); list.add(dto); } } else if (cat.equalsIgnoreCase("A")) { for (int i = start - 1; i < listAll.size(); i = i + step) { dto = new UKDashboardEventDTO(); System.out.println("di=" + i + "=" + (String) listAll.get(i)); dto.setEventId(Long.parseLong(listAll.get(i).toString())); list.add(dto); } } return list; } 

and used the dto class as below:

 public class UKDashboardEventDTO implements Serializable{ private Long eventId; ............ } 

and pass the values like this:

 List < UKDashboardEventDTO > result = service.getEvents(cat, Integer.parseInt(start), Integer.parseInt(step)); map.put("result", result); //out.println(objs); ....... try { String objs = gson.toJson(map); System.out.println("objs->" + objs); out.println(objs); out.flush(); out.close(); } catch (Exception ex) { } 

did not want to do any change to the client side logic. this works !