解组 JAXB 编组列表失败并出现 NullPointerException

Posted 2023-02-19

【中文标题】解组 JAXB 编组列表失败并出现 NullPointerException

【英文标题】：Unmarshall JAXB marshalled List failing with NullPointerExceptin

【发布时间】：2014-12-26 23:23:47

【问题描述】：

我在 ListWrapper 中有一个 Person 对象列表，每个 Person 对象都包含一个电话号码列表。通过 JAXB.marschall(file, PersonList.class) 编组工作正常。问题是解组。我得到一个 NullPointerException，请参阅下面的堆栈跟踪。我怀疑注释丢失或错误，但我自己无法弄清楚......

代码

public class TelefonNr [..]
public class Person 
  [..]
  List<TelefonNr> telefonNrs;
  [..]
  @XmlElement(name = "number")
  @XmlElementWrapper(name = "telefonnumbers")
  public List<TelefonNr> getTelefonNrs()
    return this.telefonNrs;
  
  [..]

@XmlRootElement(name = "persons")
public class PersonList 
  List<Person> persons;
  [..]
  @XmlElement(name = "person")
public List<Person> getPersons() 
    return this.Persons;

  [..]

异常堆栈跟踪

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$ReceiverImpl.receive(ArrayERProperty.java:198)
at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallingContext.endElement(UnmarshallingContext.java:523)
at com.sun.xml.internal.bind.v2.runtime.unmarshaller.SAXConnector.endElement(SAXConnector.java:143)
at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.endElement(AbstractSAXParser.java:609)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanEndElement(XMLDocumentFragmentScannerImpl.java:1782)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl$FragmentContentDriver.next(XMLDocumentFragmentScannerImpl.java:2973)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.next(XMLDocumentScannerImpl.java:606)
at com.sun.org.apache.xerces.internal.impl.XMLNSDocumentScannerImpl.next(XMLNSDocumentScannerImpl.java:117)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument(XMLDocumentFragmentScannerImpl.java:510)
at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:848)
at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:777)
at com.sun.org.apache.xerces.internal.parsers.XMLParser.parse(XMLParser.java:141)
at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse(AbstractSAXParser.java:1213)
at com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl$JAXPSAXParser.parse(SAXParserImpl.java:648)
at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallerImpl.unmarshal0(UnmarshallerImpl.java:203)
at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallerImpl.unmarshal(UnmarshallerImpl.java:175)
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 org.twinhowi.organizer.core.DatastructureDemo.testUnmarshalling(DatastructureDemo.java:131)
at org.twinhowi.organizer.core.DatastructureDemo.main(DatastructureDemo.java:144)

【问题讨论】：

Help troubleshooting JAXB unmarshalling NPE 的可能重复项

这是一个简单的初始化问题，就像下面建议的 lexicore

【参考方案1】：

理论：

您可能必须初始化集合 - 无论是急切的还是懒惰的。

List<TelefonNr> telefonNrs = new LinkedList<TelefonNr>();

或者：

public List<Person> getPersons() 
    if (this.persons == null) 
       this.persons =  new LinkedList<Person>();
    
    return this.persons;

但也请参阅这个问题：

Help troubleshooting JAXB unmarshalling NPE

【讨论】：

+1 - 如果没有相应的setPersons(List<Persion> persons) 方法，List 将需要按照您的说法进行初始化。