Java - Using Jackson's JsonNode to parse nested JSON values results in NullPointerException-CodePudding

I've been trying for a while to get this parser to work correctly but I keep getting the same issue every time. The code is very insistent that the value is null.

The issue is this, I have this call to the Google Books API which returns a fairly large JSON response. The values I need from this are nested and are proving hard to get at.

This is an example of the JSON in question. What I want to get to are a few of the books' identifying information which can be found under volumeInfo.

{
  "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 ..."
      }
    }
  ]
}

My code is fairly simple. I've tried a few different combinations of JsonNode, JsonFactory and JsonParser but nothing has seemed to work like it should. Some of the resources I looked at while trying to fix this were this article on Baeldung, this tutorial and more Stack Overflow posts than I can list. The API call itself is not the problem. It works just fine.

This is standard Java. No Spring or other frameworks.

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());

Here is the 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 [email protected]/com.library.models.GoogleBooksService.getBookData(GoogleBooksService.java:49)
    at [email protected]/com.library.controllers.LoginController.attemptLogin(LoginController.java:33)

I apologize if the details are a bit lite, but if any information is needed then I'll be happy to provide it. Any input on why this is happening would be greatly appreciated!

CodePudding user response：

According to the JSON structure, the first items must be fetched, and since it is a list, the first items must be fetched based on index.

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

CodePudding user response：

Well, "title" is not a property at the top level of your JSON so when it searches for such property it finds nothing and returns null. Also it seems to me that your code may be a bit too complex. You didn't provide your GoogleVolume class. However by its structure it should be the same as the structure of your JSON. What I would suggest that you use ObjectMapper method: public T readValue(String content, Class valueType) throws JsonProcessingException, JsonMappingException where the second parameter would be your GoogleVolume.class. Or You can try use Map.class to parse your JSON String to instance of Map<String, Object>. Also if I may suggest to simplify your code even further I wrote my own Open Source library called MgntUtils that provides (among other utils) a very simple Http client and JsonUtil for simple cases of serializing/parsing JSON. your code with use of my library may look something like this:

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

For simplicity I omitted exception handling. Here is the Javadoc for MgntUtils library. The library can be obtained as Maven artifact from Maven Central or from Github (including Javadoc and source code)