JAVA Xml Unmarshaller Null 指針異常（+ Z82A734E2CE92B074382591 與我的問題有關，但我別想它）

Question

感謝您的寶貴時間，我已經在 java 編程了一段時間，但我不學習計算機科學，所以我不知道很多東西，我正在學習。

Context : I'm following a tutorial (mainly about javafx: https://code.makery.ch/it/library/javafx-tutorial/part5/ ) to learn how to save and load data to/from an xml file. 我正在編寫一個類似的應用程序，但與教程不同，所以如果我有問題，我不能只看有什么不同，但是代碼仍然非常相似，我可以比較它們，在這種情況下，這樣做我在我的“算法”和本教程使用的算法之間找不到太大的區別

問題：當從 xml 加載數據時，我得到一個 Null 指針異常（見下文“loadPersonDataFromFile”方法），但我不知道是什么原因造成的

在這里，我報告了一些我認為與問題相關的方法和類，如果您需要更多信息，請告訴我，再次感謝您。

“savePersonDataToFile”方法工作正常，這里是：

public void savePersonDataToFile(File file) {
    try {
        JAXBContext context = JAXBContext
                .newInstance(ItemListWrapper.class);
        Marshaller m = context.createMarshaller();
        m.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);

        // Wrapping our person data.
        ItemListWrapper wrapper = new ItemListWrapper();
        wrapper.setPersons(itemData);

        // Marshalling and saving XML to the file.
        m.marshal(wrapper, file);

        // Save the file path to the registry.
        setPersonFilePath(file);
    } catch (Exception e) { // catches ANY exception
        Alert alert = new Alert(AlertType.ERROR);
        alert.setTitle("Error");
        alert.setHeaderText("Could not save data");
        alert.setContentText("Could not save data to file:\n" + file.getPath());

        alert.showAndWait();
    }
}

並創建一個正確的 xml（與教程應用程序創建的相比）

但是“loadPersonDataFromFile”方法沒有，在這里你可以看到它：

public void loadPersonDataFromFile(File file) {
    try {
        System.out.print("1\n");
        JAXBContext context = JAXBContext
                .newInstance(ItemListWrapper.class);
        Unmarshaller um = context.createUnmarshaller();
        
        System.out.print("2\n");
        System.out.print(String.valueOf(file.getPath()) + "\n");

        ItemListWrapper wrapper = (ItemListWrapper) um.unmarshal(file);
        
        System.out.print("3\n");
        
        List<Item> dataDue = wrapper.getItems();
        
        System.out.print("3.5\n");
        System.out.print(String.valueOf(dataDue) + "\n");
        
        String data = dataDue.get(0).getThing();
        System.out.print(data);

        itemData.clear();
        itemData.addAll(wrapper.getItems());
        
        System.out.print("4\n");

        // Save the file path to the registry.
        setPersonFilePath(file);
        
        System.out.print("5\n");


    } catch (Exception e) { // catches ANY exception
        System.out.print(e + "\n");
        Alert alert = new Alert(AlertType.ERROR);
        alert.setTitle("Error");
        alert.setHeaderText("Could not load data");
        alert.setContentText("Could not load data from file:\n" + file.getPath());

        alert.showAndWait();
    }
    
}

這個命令給出了異常（我添加了一些控制台 output 來找到它）：

ItemListWrapper wrapper = (ItemListWrapper) um.unmarshal(file);

文件不是 null 並且它的路徑是正確的。 為 Unmarshaller 導入的庫是正確的（與教程中的相同）。

ItemListWrapper 是一個 class 需要將數據保存到 xml 中：

import java.util.List;

import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement(name = "items")
public class ItemListWrapper {

private List<Item> items;

@XmlElement(name = "item")
public List<Item> getItems() {
    return items;
}

public void setPersons(List<Item> items) {
    this.items = items;
}
}

而項目是 object 類型，我試圖從 xml 獲得。

項目 Class：

import javafx.beans.property.SimpleStringProperty;
import javafx.beans.property.StringProperty;

public class Item 
{

private final StringProperty status;
private final StringProperty thing;

public Item() {
    this(null,null);
}

public Item(String status, String thing) {
    this.status = new SimpleStringProperty(status);
    this.thing = new SimpleStringProperty(thing);
}


public String getStatus()
{
    return status.get();
}

public void setStatus(String status) {
    this.status.set(status);
}

public StringProperty statusProperty()
{
    return status;
}


public String getThing()
{
    return thing.get();
}

public void setThing(String thing) {
    this.thing.set(thing);
}

public StringProperty thingProperty() {
    return thing;
}

}

1 Edit_我將“異常”更改為“JAXBException”，因為我需要處理它，它說NPE是由這個命令給出的： String data = dataDue.get(0).getThing(); 加上我的控制台日志顯示 wrapper = null 這是不正確的（它應該包含我的 Item 對象）。 所以我真正的問題是我不知道為什么 wrapper 是空的

我之前遇到過這個問題（使用我的日志找到它）但后來我又遇到了另一個問題，因為控制台日志我放在ItemListWrapper wrapper = (ItemListWrapper) um.unmarshal(file);之后不再出現。

第二次編輯_注意：這是堆棧跟蹤，上面寫着ItemListWrapper wrapper = (ItemListWrapper) um.unmarshal(file); 負責 NPE

 Exception in Application start method java.lang.reflect.InvocationTargetException at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(LauncherImpl.java:389) at com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:328) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at sun.launcher.LauncherHelper$FXHelper.main(LauncherHelper.java:873) Caused by: java.lang.RuntimeException: Exception in Application start method at com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:917) at com.sun.javafx.application.LauncherImpl.lambda$launchApplication$1(LauncherImpl.java:182) at java.lang.Thread.run(Thread.java:748) Caused by: java.lang.NullPointerException at com.sun.xml.internal.bind.v2.runtime.reflect.Lister$CollectionLister.addToPack(Lister.java:289) at com.sun.xml.internal.bind.v2.runtime.reflect.Lister$CollectionLister.addToPack(Lister.java:253) at com.sun.xml.internal.bind.v2.runtime.unmarshaller.Scope.add(Scope.java:106) at com.sun.xml.internal.bind.v2.runtime.property.ArrayERProperty$ReceiverImpl.receive(ArrayERProperty.java:198) at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallingContext.endElement(UnmarshallingContext.java:597) at com.sun.xml.internal.bind.v2.runtime.unmarshaller.SAXConnector.endElement(SAXConnector.java:165) at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.endElement(AbstractSAXParser.java:610) at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanEndElement(XMLDocumentFragmentScannerImpl.java:1784) at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl$FragmentContentDriver.next(XMLDocumentFragmentScannerImpl.java:2969) at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.next(XMLDocumentScannerImpl.java:605) at com.sun.org.apache.xerces.internal.impl.XMLNSDocumentScannerImpl.next(XMLNSDocumentScannerImpl.java:113) at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument(XMLDocumentFragmentScannerImpl.java:507) at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:867) at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:796) at com.sun.org.apache.xerces.internal.parsers.XMLParser.parse(XMLParser.java:142) at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse(AbstractSAXParser.java:1216) at com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl$JAXPSAXParser.parse(SAXParserImpl.java:644) at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallerImpl.unmarshal0(UnmarshallerImpl.java:243) at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallerImpl.unmarshal(UnmarshallerImpl.java:214) at javax.xml.bind.helpers.AbstractUnmarshallerImpl.unmarshal(AbstractUnmarshallerImpl.java:157) at javax.xml.bind.helpers.AbstractUnmarshallerImpl.unmarshal(AbstractUnmarshallerImpl.java:162) at javax.xml.bind.helpers.AbstractUnmarshallerImpl.unmarshal(AbstractUnmarshallerImpl.java:171) at javax.xml.bind.helpers.AbstractUnmarshallerImpl.unmarshal(AbstractUnmarshallerImpl.java:189) at application.Main.loadPersonDataFromFile(Main.java:196) at application.Main.initRoot(Main.java:83) at application.Main.start(Main.java:56) at com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$8(LauncherImpl.java:863) at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$7(PlatformImpl.java:326) at com.sun.javafx.application.PlatformImpl.lambda$null$5(PlatformImpl.java:295) at java.security.AccessController.doPrivileged(Native Method) at com.sun.javafx.application.PlatformImpl.lambda$runLater$6(PlatformImpl.java:294) at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95) at com.sun.glass.ui.win.WinApplication._runLoop(Native Method) at com.sun.glass.ui.win.WinApplication.lambda$null$4(WinApplication.java:187)... 1 more Exception running application application.Main

Answer 1

你可以試試這個

 unmarshaller.setClassesToBeBound(ItemListWrapper.class);