Java -使用Jackson的JsonNode解析嵌套JSON值会导致NullPointerException

6qqygrtg  于 2023-02-06  发布在  Java
关注(0)|答案(2)|浏览(244)

我已经尝试了一段时间让这个解析器正确工作，但是每次都遇到同样的问题，代码非常坚持值是null。
问题是，我调用了Google Books API，它返回了一个相当大的JSON响应，我需要的值是嵌套的，很难得到。
这是一个有问题的JSON的例子。我想得到的是一些书的标识信息，可以在volumeInfo下找到。

{
  "kind": "books#volumes",
  "totalItems": 1,
  "items": [
    {
      "kind": "books#volume",
      "id": "zkqMEAAAQBAJ",
      "etag": "4V9ue/R0aw4",
      "selfLink": "https://www.googleapis.com/books/v1/volumes/zkqMEAAAQBAJ",
      "volumeInfo": {
        "title": "The Song of the Cell",
        "subtitle": "An Exploration of Medicine and the New Human",
        "authors": [
          "Siddhartha Mukherjee"
        ],
        "publisher": "Simon and Schuster",
        "publishedDate": "2022-10-25",
        "description": "Presenting revelatory and exhilarating stories of scientists, doctors, and the patients whose lives may be saved by their work, the author draws on his own experience as a researcher, doctor, and prolific reader to explore how the discovery of cells created a new kind of medicine based on the therapeutic manipulation of cells.",
        "industryIdentifiers": [
          {
            "type": "ISBN_13",
            "identifier": "9781982117351"
          },
          {
            "type": "ISBN_10",
            "identifier": "1982117354"
          }
        ],
        "readingModes": {
          "text": false,
          "image": false
        },
        "pageCount": 496,
        "printType": "BOOK",
        "categories": [
          "History"
        ],
        "averageRating": 5,
        "ratingsCount": 2,
        "maturityRating": "NOT_MATURE",
        "allowAnonLogging": false,
        "contentVersion": "0.6.2.0.preview.0",
        "panelizationSummary": {
          "containsEpubBubbles": false,
          "containsImageBubbles": false
        },
        "imageLinks": {
          "smallThumbnail": "http://books.google.com/books/content?id=zkqMEAAAQBAJ&printsec=frontcover&img=1&zoom=5&edge=curl&source=gbs_api",
          "thumbnail": "http://books.google.com/books/content?id=zkqMEAAAQBAJ&printsec=frontcover&img=1&zoom=1&edge=curl&source=gbs_api"
        },
        "language": "en",
        "previewLink": "http://books.google.com/books?id=zkqMEAAAQBAJ&printsec=frontcover&dq=isbn:9781982117351&hl=&cd=1&source=gbs_api",
        "infoLink": "http://books.google.com/books?id=zkqMEAAAQBAJ&dq=isbn:9781982117351&hl=&source=gbs_api",
        "canonicalVolumeLink": "https://books.google.com/books/about/The_Song_of_the_Cell.html?hl=&id=zkqMEAAAQBAJ"
      },
      "saleInfo": {
        "country": "US",
        "saleability": "NOT_FOR_SALE",
        "isEbook": false
      },
      "accessInfo": {
        "country": "US",
        "viewability": "PARTIAL",
        "embeddable": true,
        "publicDomain": false,
        "textToSpeechPermission": "ALLOWED_FOR_ACCESSIBILITY",
        "epub": {
          "isAvailable": false
        },
        "pdf": {
          "isAvailable": false
        },
        "webReaderLink": "http://play.google.com/books/reader?id=zkqMEAAAQBAJ&hl=&source=gbs_api",
        "accessViewStatus": "SAMPLE",
        "quoteSharingAllowed": false
      },
      "searchInfo": {
        "textSnippet": "Presenting revelatory and exhilarating stories of scientists, doctors, and the patients whose lives may be saved by their work, the author draws on his own experience as a researcher, doctor, and prolific reader to explore how the discovery ..."
      }
    }
  ]
}

我的代码相当简单。我尝试了JsonNode、JsonFactory和JsonParser的几种不同组合，但似乎都不起作用。我在试图修复这个问题时查看了Baeldung上的this article、this tutorial和我无法列出的更多Stack Overflow帖子。API调用本身不是问题。它工作得很好。
这是标准的Java，没有Spring或其他框架。

ObjectMapper mapper = new ObjectMapper();

        URL url= new URL(setURL(bookInfo, infoType));
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        InputStream responseStream = connection.getInputStream();

        JsonNode volumeNode = mapper.readTree(responseStream);
        GoogleVolume testVolume= new GoogleVolume();

        testVolume.setTitle(volumeNode.get("title").textValue());

下面是stackTrace：

Exception in thread "JavaFX Application Thread" java.lang.RuntimeException: java.lang.reflect.InvocationTargetException
    at javafx.fxml@19/javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1857)
    at javafx.fxml@19/javafx.fxml.FXMLLoader$ControllerMethodEventHandler.handle(FXMLLoader.java:1724)
    at javafx.base@19/com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
    at javafx.base@19/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:234)
    at javafx.base@19/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
    at javafx.base@19/com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
    at javafx.base@19/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
    at javafx.base@19/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at javafx.base@19/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at javafx.base@19/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at javafx.base@19/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at javafx.base@19/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at javafx.base@19/com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
    at javafx.base@19/com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
    at javafx.base@19/javafx.event.Event.fireEvent(Event.java:198)
    at javafx.graphics@19/javafx.scene.Node.fireEvent(Node.java:8923)
    at javafx.controls@19/javafx.scene.control.Button.fire(Button.java:203)
    at javafx.controls@19/com.sun.javafx.scene.control.behavior.ButtonBehavior.mouseReleased(ButtonBehavior.java:207)
    at javafx.controls@19/com.sun.javafx.scene.control.inputmap.InputMap.handle(InputMap.java:274)
    at javafx.base@19/com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:247)
    at javafx.base@19/com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
    at javafx.base@19/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:234)
    at javafx.base@19/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
    at javafx.base@19/com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
    at javafx.base@19/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
    at javafx.base@19/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at javafx.base@19/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at javafx.base@19/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at javafx.base@19/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at javafx.base@19/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at javafx.base@19/com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
    at javafx.base@19/com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
    at javafx.base@19/javafx.event.Event.fireEvent(Event.java:198)
    at javafx.graphics@19/javafx.scene.Scene$MouseHandler.process(Scene.java:3894)
    at javafx.graphics@19/javafx.scene.Scene.processMouseEvent(Scene.java:1887)
    at javafx.graphics@19/javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2620)
    at javafx.graphics@19/com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:411)
    at javafx.graphics@19/com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:301)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:399)
    at javafx.graphics@19/com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$2(GlassViewEventHandler.java:450)
    at javafx.graphics@19/com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:424)
    at javafx.graphics@19/com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:449)
    at javafx.graphics@19/com.sun.glass.ui.View.handleMouseEvent(View.java:551)
    at javafx.graphics@19/com.sun.glass.ui.View.notifyMouse(View.java:937)
    at javafx.graphics@19/com.sun.glass.ui.gtk.GtkApplication._runLoop(Native Method)
    at javafx.graphics@19/com.sun.glass.ui.gtk.GtkApplication.lambda$runLoop$11(GtkApplication.java:316)
    at java.base/java.lang.Thread.run(Thread.java:833)
Caused by: java.lang.reflect.InvocationTargetException
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:568)
    at com.sun.javafx.reflect.Trampoline.invoke(MethodUtil.java:77)
    at jdk.internal.reflect.GeneratedMethodAccessor2.invoke(Unknown Source)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:568)
    at javafx.base@19/com.sun.javafx.reflect.MethodUtil.invoke(MethodUtil.java:275)
    at javafx.fxml@19/com.sun.javafx.fxml.MethodHelper.invoke(MethodHelper.java:84)
    at javafx.fxml@19/javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1854)
    ... 46 more
Caused by: java.lang.NullPointerException: Cannot invoke "com.fasterxml.jackson.databind.JsonNode.textValue()" because the return value of "com.fasterxml.jackson.databind.JsonNode.get(String)" is null
    at com.libary@0.0.1-SNAPSHOT/com.library.models.GoogleBooksService.getBookData(GoogleBooksService.java:49)
    at com.libary@0.0.1-SNAPSHOT/com.library.controllers.LoginController.attemptLogin(LoginController.java:33)

我很抱歉，如果细节是有点精简，但如果任何信息是需要的，那么我会很乐意提供它。任何输入的原因，这是发生将不胜感激！

JSON

2条答案

按热度按时间
w8f9ii69

w8f9ii691#

根据JSON结构，必须获取第一个项目，并且由于它是一个列表，所以必须基于索引获取第一个项目。

String title = volumeNode.get("items").get(0).get("volumeInfo").get("title").textValue();
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rseugnpd

rseugnpd2#

“title”不是JSON的顶层属性，所以当它搜索这样的属性时，什么也找不到并返回null。而且在我看来，你的代码可能有点太复杂了。你没有提供GoogleVolume类。但是，从结构上看，它应该与JSON的结构相同。我建议你使用ObjectMapper方法：公共T读取值（字符串内容，类值类型）抛出JsonProcessingException，JsonMappingException，其中第二个参数是GoogleVolume.class。或者，您可以尝试使用Map.class将JSON字符串解析为Map<String, Object>的示例。此外，如果我可以建议进一步简化代码，我编写了自己的开放源代码库，称为MgntUtils，它提供（在其他实用程序中）一个非常简单的Http客户端和JsonUtil，用于序列化/解析JSON的简单情况。使用我的库的代码可能如下所示：

HttpClient httpClient = new HttpClient();
String jsonResponse = httpClient.sendHttpRequest(urlStr, HttpClient.HttpMethod.GET);
GoogleVolume volume = JsonUtils.readObjectFromJsonString(jsonResponse, GoogleVolume.class);

为了简单起见，我省略了异常处理。下面是Javadoc for MgntUtils library。该库可以作为Maven artifact from Maven Central或从Github（包括Javadoc和源代码）获得。

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