java - Google图书Api-图像链接没有值(value)-Android JSONException

我正在使用Google Books Api显示书籍列表，但是当我尝试从imageLinks JSONObject获取缩略图网址时，JSONException表示imageLinks没有任何值，即使该值存在于其中宾语。

我尝试了JSONObject.isNull()或optString()之类的方法，而不是getString()，但它仍然没有任何价值。

这是我要从中获取数据的URL：https://www.googleapis.com/books/v1/volumes?q=android

这是代码：

JsonObjectRequest jsonObjectRequest = new JsonObjectRequest(Request.Method.GET, url, null, new Response.Listener < JSONObject > () {
 @Override
 public void onResponse(JSONObject response) {
  if (response != null) {
   try {
    JSONArray jsonItemsArray = response.getJSONArray("items");
    for (int i = 0; i < jsonItemsArray.length(); i++) {
     String thumbnailUrl = "";
     String title = "";
     JSONObject item = jsonItemsArray.getJSONObject(i);
     JSONObject volumeInfo = item.getJSONObject("volumeInfo");
     JSONObject thumbnailUrlObject = volumeInfo.getJSONObject("imageLinks");
     if (!thumbnailUrlObject.isNull("thumbnail")) {
      thumbnailUrl = thumbnailUrlObject.getString("thumbnail");
     }
     title = volumeInfo.getString("title");
     bookList.add(new Book(title, thumbnailUrl));
     booksAdapter.notifyDataSetChanged();
    }

   } catch (JSONException e) {
    e.printStackTrace();
   }
  }
 }
}, new Response.ErrorListener() {
 @Override
 public void onErrorResponse(VolleyError error) {
  error.printStackTrace();
 }
});

这是JSON响应的一部分：

"kind": "books#volumes",
 "totalItems": 500,
 "items": [
  {
   "kind": "books#volume",
   "id": "JUVjAgAAQBAJ",
   "etag": "kbnCYPNPKq4",
   "selfLink": "https://www.googleapis.com/books/v1/volumes/JUVjAgAAQBAJ",
   "volumeInfo": {
    "title": "Android. Podstawy tworzenia aplikacji",
    "authors": [
     "Andrzej Stasiewicz"
    ],
    "publisher": "Helion",
    "publishedDate": "2013-11-10",
    "description": "Na szczęście dostępna jest już książka Android.",
    "industryIdentifiers": [
     {
      "type": "ISBN_13",
      "identifier": "9788324688418"
     },
     {
      "type": "ISBN_10",
      "identifier": "8324688412"
     }
    ],
    "readingModes": {
     "text": true,
     "image": true
    },
    "pageCount": 216,
    "printType": "BOOK",
    "categories": [
     "Computers"
    ],
    "averageRating": 4.0,
    "ratingsCount": 1,
    "maturityRating": "NOT_MATURE",
    "allowAnonLogging": true,
    "contentVersion": "1.4.4.0.preview.3",
    "imageLinks": {
     "smallThumbnail": "http://books.google.com/books/content?id=JUVjAgAAQBAJ&printsec=frontcover&img=1&zoom=5&edge=curl&source=gbs_api",
     "thumbnail": "http://books.google.com/books/content?id=JUVjAgAAQBAJ&printsec=frontcover&img=1&zoom=1&edge=curl&source=gbs_api"
    }

我想知道为什么volumeInfo.getJSONObject("imageLinks")给我JSONException没有价值，即使imageLinks有价值。

最佳答案

并非每个volumeInfo节点都有imageLinks节点，因此您需要使用node.has方法或node.opt*方法检查给定节点是否存在，并检查结果是否不是null。在下面，您可以找到获取thumbnail节点的安全方法：

JSONObject thumbnailUrlObject = volumeInfo.optJSONObject("imageLinks");
if (thumbnailUrlObject != null && thumbnailUrlObject.has("thumbnail")) {
    thumbnailUrl = thumbnailUrlObject.getString("thumbnail");
}

简单的控制台应用程序，显示其工作原理：

import org.json.JSONArray;
import org.json.JSONObject;

import java.io.File;
import java.nio.file.Files;

public class OrgJsonApp {

    public static void main(String[] args) throws Exception {
        File jsonFile = new File("./resource/test.json").getAbsoluteFile();
        String json = String.join("", Files.readAllLines(jsonFile.toPath()));

        JSONObject response = new JSONObject(json);

        // get items
        JSONArray jsonItemsArray = response.getJSONArray("items");
        for (int i = 0; i < jsonItemsArray.length(); i++) {
            String thumbnailUrl = "";
            JSONObject item = jsonItemsArray.getJSONObject(i);
            JSONObject volumeInfo = item.getJSONObject("volumeInfo");
            JSONObject thumbnailUrlObject = volumeInfo.optJSONObject("imageLinks");
            if (thumbnailUrlObject != null && thumbnailUrlObject.has("thumbnail")) {
                thumbnailUrl = thumbnailUrlObject.getString("thumbnail");
            }
            String title = volumeInfo.getString("title");
            System.out.println("title => " + title);
            System.out.println("thumbnail => " + thumbnailUrl);
        }
    }
}

上面的JSON有效负载打印代码：

title => Android Aplikacje wielowątkowe techniki przetwarzania
thumbnail =>
title => Android. Receptury
thumbnail => http://books.google.com/books/content?id=pZ5iAgAAQBAJ&printsec=frontcover&img=1&zoom=1&edge=curl&source=gbs_api
title => Android. Podstawy tworzenia aplikacji
thumbnail => http://books.google.com/books/content?id=JUVjAgAAQBAJ&printsec=frontcover&img=1&zoom=1&edge=curl&source=gbs_api
title => Optymalizacja wydajności aplikacji na Android
thumbnail => http://books.google.com/books/content?id=WJ1iAgAAQBAJ&printsec=frontcover&img=1&zoom=1&edge=curl&source=gbs_api
title => Inteligentny dom. Automatyzacja mieszkania za pomocą platformy Arduino, systemu Android i zwykłego komputera
thumbnail => http://books.google.com/books/content?id=koiKAgAAQBAJ&printsec=frontcover&img=1&zoom=1&edge=curl&source=gbs_api
title => Android UI. Podręcznik dla projektantów. Smashing Magazine
thumbnail => http://books.google.com/books/content?id=HEJjAgAAQBAJ&printsec=frontcover&img=1&zoom=1&edge=curl&source=gbs_api
title => Android. Programowanie gier na tablety
thumbnail => http://books.google.com/books/content?id=7J1iAgAAQBAJ&printsec=frontcover&img=1&zoom=1&edge=curl&source=gbs_api
title => Profesjonalne tworzenie gier internetowych dla systemu Android w językach HTML5, CSS3 i JavaScript
thumbnail => http://books.google.com/books/content?id=vlNjAgAAQBAJ&printsec=frontcover&img=1&zoom=1&edge=curl&source=gbs_api
title => Beginning Android 2
thumbnail => http://books.google.com/books/content?id=2XeNswkT_2YC&printsec=frontcover&img=1&zoom=1&edge=curl&source=gbs_api
title => Pro Android 2
thumbnail => http://books.google.com/books/content?id=Bam8K5SIiTkC&printsec=frontcover&img=1&zoom=1&edge=curl&source=gbs_api