用Jackson解析JSON返回null

Question

我正在使用API​​在Java上訓練自己。 我的練習主題是詢問Google圖書API，以獲取有關圖書的一些信息。 我使用Jersey作為客戶端，使用Jackson來解析JSON。

我的問題是，當我運行JerseyVolumeGet() ，響應是這樣的：

Volume{title='null', numberOfPages=null, author='null'}

為什么呢 我的錯誤在哪里？ 我懷疑解析JSON時出錯，但是看不到確切位置。

這是我的getClass（搜索網址帶有條形碼，對我來說這不是問題）

public class JerseyVolumeGet {
    public static void main(String[] args) {
        try {

            Client client = Client.create();

            WebResource webResource = client
                    .resource("https://www.googleapis.com/books/v1/volumes?q=1984");

            ClientResponse response = webResource.accept("application/json")
                    .get(ClientResponse.class);

            if (response.getStatus() != 200) {
                throw new RuntimeException("Noob you Failed : HTTP error code : "
                        + response.getStatus());
            }

            String output = response.getEntity(String.class);

            // read from file, convert it to user class
            ObjectMapper mapper = new ObjectMapper();
            Volume volume = mapper.readValue(output, Volume.class);


            // display to console
            System.out.println(volume);


        } catch (Exception e) {

            e.printStackTrace();

        }

    }

}

這是我的查詢的JSON結果：

{
 "kind": "books#volumes",
 "totalItems": 641,
 "items": [
  {
   "kind": "books#volume",
   "id": "RY8yQWeDVFYC",
   "etag": "Lf0P50PVz9c",
   "selfLink": "https://www.googleapis.com/books/v1/volumes/RY8yQWeDVFYC",
   "volumeInfo": {
    "title": "(1984).",
    "subtitle": "",
    "authors": [
     "Guy Serbat",
     "Jean Taillardat",
     "Gilbert Lazard"
    ],
    "publisher": "Peeters Publishers",
    "publishedDate": "1984-01-01",
    "description": "(Peeters 1984)",
    "industryIdentifiers": [
     {
      "type": "ISBN_10",
      "identifier": "2904685030"
     },
     {
      "type": "ISBN_13",
      "identifier": "9782904685033"
     }
    ],
    "readingModes": {
     "text": false,
     "image": true
    },
    "pageCount": 280,
    "printType": "BOOK",
    "categories": [
     "Language Arts & Disciplines"
    ],
    "contentVersion": "1.1.1.0.preview.1",
    "imageLinks": {
     "smallThumbnail": "http://bks9.books.google.fr/books?id=RY8yQWeDVFYC&printsec=frontcover&img=1&zoom=5&edge=curl&source=gbs_api",
     "thumbnail": "http://bks9.books.google.fr/books?id=RY8yQWeDVFYC&printsec=frontcover&img=1&zoom=1&edge=curl&source=gbs_api"
    },
    "language": "fr",
    "previewLink": "http://books.google.fr/books?id=RY8yQWeDVFYC&printsec=frontcover&dq=1984&hl=&cd=1&source=gbs_api",
    "infoLink": "http://books.google.fr/books?id=RY8yQWeDVFYC&dq=1984&hl=&source=gbs_api",
    "canonicalVolumeLink": "http://books.google.fr/books/about/1984.html?hl=&id=RY8yQWeDVFYC"
   },
   "saleInfo": {
    "country": "FR",
    "saleability": "NOT_FOR_SALE",
    "isEbook": false
   },
   "accessInfo": {
    "country": "FR",
    "viewability": "PARTIAL",
    "embeddable": true,
    "publicDomain": false,
    "textToSpeechPermission": "ALLOWED",
    "epub": {
     "isAvailable": false
    },
    "pdf": {
     "isAvailable": false
    },
    "webReaderLink": "http://books.google.fr/books/reader?id=RY8yQWeDVFYC&hl=&printsec=frontcover&output=reader&source=gbs_api",
    "accessViewStatus": "SAMPLE",
    "quoteSharingAllowed": false
   },
   "searchInfo": {
    "textSnippet": "(Peeters 1984)"
   }
  },
more items...
}

然后，我有一個Volume.class像這樣：

@JsonIgnoreProperties(ignoreUnknown=true)
public class Volume {
    @JsonProperty("title")
    String title;
    @JsonProperty("pageCount")
    Integer pageCount;
    @JsonProperty("authors")
    String authors;

getters , setters and toString..

Answer 1

您需要為完整的JSON結構建模，如下所示：

public class BookData {
    String kind;
    Integer totalItems;
    List<Item> items;
}

public class Item {
    String kind;
    String id;
    //...
    Volume volumeInfo;
}

然后，您可以使用ObjectMapper來讀取BookData ：

BookData bookData = new ObjectMapper().readValue(output, BookData.class);

並從BookData每個Item中提取Volume信息。