使用 gson 从 challonge.com json 文件中获取特定值到 ArrayList

Question

我正在尝试从 challonge.com 的 JSON 文件中获取特定值（更多信息请参见 http://api.challonge.com/v1/documents/participants/index）。我试图获得的值是“id”和“name”。

这是我当前的代码。

  HttpClient client = HttpClients.createDefault();

  HttpGet request = new HttpGet(jsonUrl);

    try {
        HttpResponse response = client.execute(request);
        HttpEntity entity = response.getEntity();
        if(entity != null){
            //Allows me to search through the Participant Object using the 
            //Participant/Participants POJO
            Gson gson = new GsonBuilder().
                    registerTypeAdapter(Participant.class, new MyDeserializer<>()).
                    create();
            ContentType contentType = ContentType.getOrDefault(entity);
            Charset charset = contentType.getCharset();
            Reader reader = new InputStreamReader(entity.getContent(), charset);

            /*Goes through seperate JSON files received from the Challonge API.
            */
            ArrayList<Participant> al = new ArrayList<>();


            Type type = new TypeToken<List<Participant>>(){}.getType();
            ArrayList<Participant> list = gson.fromJson(reader, al.getClass());

我也使用了类型变量而不是 al.getClass()，但是当我决定检查 id 和名称时，两者都会给我 java.lang.NullPointerException 的错误：

 System.out.println(list.get(0).getPlayerId() + ": " + list.get(0).getPlayerName());

这也是我的参与者课程

public class Participant {

   private String id;
   private String name;

   public String getPlayerId(){
       return id;
   }

   public String getPlayerName(){
       return name;
   }
}

这里是一些 json 代码的示例

[
  {
    "participant": {
     "active": true,
     "checked_in_at": null,
     "created_at": "2015-01-19T16:54:40-05:00",
     "final_rank": null,
     "group_id": null,
     "icon": null,
     "id": 16543993,
     "invitation_id": null,
     "invite_email": null,
     "misc": null,
     "name": "Participant #1",
     "on_waiting_list": false,
     "seed": 1,
     "tournament_id": 1086875,
     "updated_at": "2015-01-19T16:54:40-05:00",
     "challonge_username": null,
     "challonge_email_address_verified": null,
     "removable": true,
     "participatable_or_invitation_attached": false,
     "confirm_remove": true,
     "invitation_pending": false,
     "display_name_with_invitation_email_address": "Participant #1",
     "email_hash": null,
     "username": null,
     "attached_participatable_portrait_url": null,
     "can_check_in": false,
     "checked_in": false,
     "reactivatable": false
   }
 },
 {
   "participant": {
     "active": true,
     "checked_in_at": null,
     "created_at": "2015-01-19T16:54:43-05:00",
     "final_rank": null,
     "group_id": null,
     "icon": null,
     "id": 16543994,
     "invitation_id": null,
     "invite_email": null,
     "misc": null,
     "name": "Participant #2",
     "on_waiting_list": false,
     "seed": 2,
     "tournament_id": 1086875,
     "updated_at": "2015-01-19T16:54:43-05:00",
     "challonge_username": null,
     "challonge_email_address_verified": null,
     "removable": true,
     "participatable_or_invitation_attached": false,
     "confirm_remove": true,
     "invitation_pending": false,
     "display_name_with_invitation_email_address": "Participant #2",
     "email_hash": null,
     "username": null,
     "attached_participatable_portrait_url": null,
     "can_check_in": false,
     "checked_in": false,
     "reactivatable": false
      }
   }
 ]

提前感谢您的帮助！

Answer 1

我实际上最终找到了一种对我有用的方法。

    HttpClient client = HttpClients.createDefault();
    HttpGet request = new HttpGet(jsonUrl);

    try {
        HttpResponse response = client.execute(request);
        HttpEntity entity = response.getEntity();
        if(entity != null){
            //Allows me to search through the Participant Object using the 
            //Participant/Participants POJO
            Gson gson = new GsonBuilder().
                    registerTypeAdapter(ParticipantWrapper.class, new MyDeserializer<>()).
                    create();
            ContentType contentType = ContentType.getOrDefault(entity);
            Charset charset = contentType.getCharset();
            Reader reader = new InputStreamReader(entity.getContent(), charset);

            /*Goes through seperate JSON files received from the Challonge API.
            */
            ArrayList<Participant> al = new ArrayList<>();

            JsonParser jsonParser = new JsonParser();
            JsonElement jsonElement = jsonParser.parse(reader);


           if(jsonElement.isJsonArray()){
               Iterator itor = jsonElement.getAsJsonArray().iterator();

               while(itor.hasNext()){
                   JsonObject jObject = (JsonObject) itor.next();
                   al.add(gson.fromJson(jObject.get("participant"), Participant.class));
               }
           }
            return al;
        }
    } catch (IOException ex) {
        Logger.getLogger(TournamentInfo.class.getName()).log(Level.SEVERE, null, ex);
        return null;
    }

Answer 2

不幸的是，您的 JSON 比 Java 对象层次结构更深一层。它不包含参与者列表，而是包含参与者的事物列表。

试试这个：

public class ParticipantWrapper {
    public final Participant participant;

    public ParticipantWrapper {
        participant = null;
    }
}

然后：

 ArrayList<ParticipantWrapper> list = gson.fromJson(reader, new TypeToken<List<ParticipantWrapper>>(){}.getType());

最后：

final String id = list.get(0).participant.getPlayerId();

编辑：请查看这个完整且有效的示例：https://gist.github.com/DavidMihola/2ee7740a2c0c22b016b4