空指針異常，同時將XML解組到具有@XMLElementWrapper的Java對象

Question

我正在使用JAXB將XML文件解組到Java Object。

該XML包含列表。 例如

<items>
<item>
...
</item>
...
</items>

和Java類具有@XMLElementWrapper的項目

@XMLElementWrapper(name="items")
@XMLElement(name="item")
private List<Item> items;

當我嘗試解組時，我得到了空指針異常。

 java.lang.NullPointerException
          at sun.reflect.UnsafeFieldAccessorImpl.ensureObj(UnsafeFieldAccessorImpl.java:54)
          at sun.reflect.UnsafeObjectFieldAccessorImpl.get(UnsafeObjectFieldAccessorImpl.java:36)
          at java.lang.reflect.Field.get(Field.java:379)
          at com.sun.xml.bind.v2.runtime.reflect.Accessor$FieldReflection.get(Accessor.java:185)
          at com.sun.xml.bind.v2.runtime.reflect.Lister$CollectionLister.startPacking(Lister.java:249)
          at com.sun.xml.bind.v2.runtime.reflect.Lister$CollectionLister.startPacking(Lister.java:224)
          at com.sun.xml.bind.v2.runtime.unmarshaller.Scope.add(Scope.java:78)
          at com.sun.xml.bind.v2.runtime.property.ArrayERProperty$ReceiverImpl.receive(ArrayERProperty.java:150)
          at com.sun.xml.bind.v2.runtime.unmarshaller.UnmarshallingContext.endElement(UnmarshallingContext.java:404)
          at com.sun.xml.bind.v2.runtime.unmarshaller.SAXConnector.endElement(SAXConnector.java:125)
          at org.apache.xerces.parsers.AbstractSAXParser.endElement(Unknown Source)
          at org.apache.xerces.impl.XMLNSDocumentScannerImpl.scanEndElement(Unknown Source)
          at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl$FragmentContentDispatcher.dispatch(Unknown Source)
          at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl.scanDocument(Unknown Source)
          at org.apache.xerces.parsers.XML11Configuration.parse(Unknown Source)
          at org.apache.xerces.parsers.XML11Configuration.parse(Unknown Source)
          at org.apache.xerces.parsers.XMLParser.parse(Unknown Source)
          at org.apache.xerces.parsers.AbstractSAXParser.parse(Unknown Source)
          at com.sun.xml.bind.v2.runtime.unmarshaller.UnmarshallerImpl.unmarshal0(UnmarshallerImpl.java:200)
          at com.sun.xml.bind.v2.runtime.unmarshaller.UnmarshallerImpl.unmarshal(UnmarshallerImpl.java:173)
          at javax.xml.bind.helpers.AbstractUnmarshallerImpl.unmarshal(AbstractUnmarshallerImpl.java:157)
          at javax.xml.bind.helpers.AbstractUnmarshallerImpl.unmarshal(AbstractUnmarshallerImpl.java:214)
          at com.amazon.epubs.edition.manifest.serializer.JAXBMarshaller.unmarshall(JAXBMarshaller.java:45)
          at amazon.PeriodicalsManagementService.handler.PreviewPublicationHandler.getFolioRenditions(PreviewPublicationHandler.java:293)
          at amazon.PeriodicalsManagementService.handler.PreviewPublicationHandler.doMainProcessing(PreviewPublicationHandler.java:176)
          ... 19 more

Answer 1

嘗試使用List<>的包裝器類：

@XmlRootElement
class Items {
    @XmlElement
    List<Item> item;
}

然后對Items進行unmarshall