JAXB RI ClassFactory 中的空指针异常
Posted
技术标签:
【中文标题】JAXB RI ClassFactory 中的空指针异常【英文标题】:Null Pointer Exception in JAXB RI ClassFactory 【发布时间】:2015-05-09 21:00:10 【问题描述】:简介
我和我的朋友正在开发一个 JavaFX 应用程序,作为我们学校的规划师。我们有任务(课堂作业)、活动、课程和学生信息。为了将数据持久存储在用户的硬盘上,我们使用了 JAXB。
我们已经注释了我们的类,并且可以成功地将 Task 类编组到一个包装器中。问题是从tasks.xml
文件中解组。
以下是相关代码行:
Task.java
@XmlRootElement
public class Task
//constructors
//complete constructor
public Task(String className, String assignment, String description, LocalDate dueDate)
this.className = new SimpleStringProperty(className);
this.assignment = new SimpleStringProperty(assignment);
this.description = new SimpleStringProperty(description);
this.dueDate = new SimpleObjectProperty<LocalDate>(dueDate);
/**
* Sets a model data into the task, sets the
* due date to be tomorrow.
*/
public Task()
this("", "", "", LocalDate.now().plusDays(1));
setClassName("English");
setAssignment("Read");
setDescription("1984");
//setDueDate(LocalDate.now());
//Instance variables
private final SimpleStringProperty className;
private final SimpleStringProperty assignment;
private final SimpleStringProperty description;
private final ObjectProperty<LocalDate> dueDate;
// //Getters and setters
//... Other getters and setters
@XmlJavaTypeAdapter(LocalDateAdapter.class)
public final java.time.LocalDate getDueDate()
return this.dueDateProperty().get();
public final void setDueDate(final java.time.LocalDate dueDate)
this.dueDateProperty().set(dueDate);
TaskListWrapper.java:
//used in saving the objects to XML
@XmlRootElement(name="tasks")
public class TaskListWrapper
private ObservableList<Task> task;
@XmlElement(name="task")
public ObservableList<Task> getTasks()
return task;
public void setTasks(ObservableList<Task> tasks)
this.task = tasks;
AppData.java 中的方法
它处理文件的保存和解组。
/**
* Save to XML using JAXB
* @throws JAXBException
* @throws FileNotFoundException
*/
public static void save() throws JAXBException, FileNotFoundException
//saving other objects
//...
TaskListWrapper tl = new TaskListWrapper();
//MasterTaskList is the entire list of tasks written to memory
tl.setTasks(AppData.getMasterTaskList());
saveObject(tl, new File(System.getProperty("user.dir") + "/resources/xml/tasks.xml"));
saveObject(masterStudentInfo, new File(System.getProperty("user.dir") + "/resources/xml/student_info.xml"));
同一个类中的saveObject()方法:
/**
* Saves a specific Object @code obj to an xml file @code xml using JAXB.
* @param obj
* @param xml
* @throws FileNotFoundException
* @throws JAXBException
*/
private static void saveObject(Object obj, File xml) throws FileNotFoundException, JAXBException
//context is used to determine what kind of class is going to be marshalled or unmarshalled
JAXBContext context = JAXBContext.newInstance(obj.getClass());
//loads to the XML file
Marshaller m = context.createMarshaller();
m.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, Boolean.TRUE);
//loads the current list of courses to the courses.xml file
m.marshal(obj, new FileOutputStream(xml));
App.java 中的 InitFiles()
注意指出空指针异常的注释
/**
* Initial setup for all the files for the program. Contains all the
* persistent data for the planner, such as courses, tasks, and events.
* <p>
* All data is saved in @code [place of installment]/resources/xml/....
* @throws IOException
*/
public void initFiles() throws IOException
//... other files for other objects
File tasks = new File(System.getProperty("user.dir") + "/resources/xml/tasks.xml");
//check if each file exists, if so unmarshall
if(tasks.exists())
try
JAXBContext context = JAXBContext.newInstance(TaskListWrapper.class);
//the file location is correct
System.out.println(tasks.toString());
//The context knows that both the Task and TaskListWrapper classes exist
System.out.println(context.toString());
Unmarshaller um = context.createUnmarshaller();
//TODO: null pointer exception
TaskListWrapper taskList = (TaskListWrapper) um.unmarshal(tasks);
//System.out.println(umObject.getClass());
catch (JAXBException e)
e.printStackTrace();
else
tasks.createNewFile();
//... other checks for files
来自编组的格式良好的 XML 文档:
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<tasks>
<task>
<assignment>Book</assignment>
<className>Math</className>
<description>problems</description>
<dueDate>2015-01-17</dueDate>
</task>
<task>
<assignment>Textbook</assignment>
<className>Religion</className>
<description>problems</description>
<dueDate>2015-01-17</dueDate>
</task>
<task>
<assignment>Read</assignment>
<className>English</className>
<description>1984</description>
<dueDate>2015-03-05</dueDate>
</task>
</tasks>
例外情况:
java.lang.NullPointerException
at com.sun.xml.internal.bind.v2.ClassFactory.create0(Unknown Source)
at com.sun.xml.internal.bind.v2.ClassFactory.create(Unknown Source)
at com.sun.xml.internal.bind.v2.runtime.reflect.Lister$CollectionLister.startPacking(Unknown Source)
at com.sun.xml.internal.bind.v2.runtime.reflect.Lister$CollectionLister.startPacking(Unknown Source)
at com.sun.xml.internal.bind.v2.runtime.unmarshaller.Scope.add(Unknown Source)
at com.sun.xml.internal.bind.v2.runtime.property.ArrayERProperty$ReceiverImpl.receive(Unknown Source)
at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallingContext.endElement(Unknown Source)
at com.sun.xml.internal.bind.v2.runtime.unmarshaller.SAXConnector.endElement(Unknown Source)
at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.endElement(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanEndElement(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl$FragmentContentDriver.next(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.next(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLNSDocumentScannerImpl.next(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument(Unknown Source)
at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(Unknown Source)
at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(Unknown Source)
at com.sun.org.apache.xerces.internal.parsers.XMLParser.parse(Unknown Source)
at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse(Unknown Source)
at com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl$JAXPSAXParser.parse(Unknown Source)
at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallerImpl.unmarshal0(Unknown Source)
at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallerImpl.unmarshal(Unknown Source)
at javax.xml.bind.helpers.AbstractUnmarshallerImpl.unmarshal(Unknown Source)
at javax.xml.bind.helpers.AbstractUnmarshallerImpl.unmarshal(Unknown Source)
at javax.xml.bind.helpers.AbstractUnmarshallerImpl.unmarshal(Unknown Source)
at javax.xml.bind.helpers.AbstractUnmarshallerImpl.unmarshal(Unknown Source)
at org.sjcadets.planner.App.initFiles(App.java:136)
at org.sjcadets.planner.App.start(App.java:68)
at com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$153(Unknown Source)
at com.sun.javafx.application.LauncherImpl$$Lambda$51/1390460753.run(Unknown Source)
at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$166(Unknown Source)
at com.sun.javafx.application.PlatformImpl$$Lambda$45/1051754451.run(Unknown Source)
at com.sun.javafx.application.PlatformImpl.lambda$null$164(Unknown Source)
at com.sun.javafx.application.PlatformImpl$$Lambda$47/231444107.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.application.PlatformImpl.lambda$runLater$165(Unknown Source)
at com.sun.javafx.application.PlatformImpl$$Lambda$46/1775282465.run(Unknown Source)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(Unknown Source)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.lambda$null$141(Unknown Source)
at com.sun.glass.ui.win.WinApplication$$Lambda$37/1109371569.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
空指针在initFiles()
方法中声明的//TODO
处:
JAXBContext context = JAXBContext.newInstance(TaskListWrapper.class);
//the file location is correct
System.out.println(tasks.toString());
//The context knows that both the Task and TaskListWrapper classes exist
System.out.println(context.toString());
Unmarshaller um = context.createUnmarshaller();
//TODO: null pointer exception
TaskListWrapper taskList = (TaskListWrapper) um.unmarshal(tasks);
我们尝试过的事情:
混淆了名称和注释。命名似乎不是问题。 系统输出文件位置以确保其正确。 系统化JAXBContext
知道的类。它可以识别 Task
和 TaskListWrapper
类。
Sysouting um.toString()
。它显示了内存中的有效地址,因此um
对象本身并不是引发空指针异常的原因。
将TaskListWrapper.java
的位置更改为与Task.java
相同的包。
尝试通过将 XML 文件更改为只有一个 <task>
作为根元素来解组单个任务,当我更改时
TaskListWrapper taskList = (TaskListWrapper) um.unmarshal(tasks);
到
Task taskList = (Task) um.unmarshal(tasks);
我们寻找答案的地方:
http://examples.javacodegeeks.com/core-java/xml/bind/jaxb-unmarshal-example/ 大量 *** 问题与@XMLAttribute
注释的错误有关。由于我们不使用那些与错误无关的内容
学习 Java:第 4 版,作者 Patrick Niemeyer 和 Daniel Leuck。我们复制了他们设置解组器的确切方法。他们有一个简单的方法:
JAXBContext context = JAXBContext.newInstance(Inventory.class);
Unmarshaller unmarshaller = context.createUnmarshaller();
Inventory inventory = (Inventory) unmarshaller.unmarshall(
new File("zooinventory.xml") );
问题
为什么TaskListWrapper taskList = (TaskListWrapper) um.unmarshal(tasks);
会抛出空指针异常?
【问题讨论】:
我唯一能想到的可能是解组器本身是空的。您可以继续发布堆栈的其余部分吗?NullPointerException
对应你代码的哪一行? um
变量是否为空?
@BlaiseDoughan 当我 sysout um
它显示:com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallerImpl@2185e44d 所以我不认为它为空
@fdsa 我 sysout 了 um,它在内存中显示了一个有效的地址,因为我用 BlaiseDoughan 解决了(哈哈,对双关语感到抱歉)我在原始帖子中发布了堆栈的其余部分
【参考方案1】:
JAXB 与包装器中的 ObservableList 等 FXCollections 不兼容。您必须编写一个 XmlAdapter 才能将其与普通列表解开。所以编组会起作用,但解组不会,正如您在堆栈跟踪的行中看到的那样:
at com.sun.xml.internal.bind.v2.runtime.reflect.Lister$CollectionLister.startPacking(Unknown Source)
有 Lister$CollectionLister 不知道如何处理未知来源。因此,Adpater 应该像这样使用 ListWrapper:
public class TaskList
@XmlElement(name = "task")
List<Task> entries = new ArrayList<>();
public List<Task> getEntries()
return entries;
对应的Adapter如下:
public class TaskListAdapter extends XmlAdapter<TaskList, ObservableList<Task>>
@Override
public ObservableList<Task> unmarshal(TaskList v) throws Exception
ObservableList<Task> list = FXCollections.observableArrayList(v.entries);
return list;
@Override
public TaskList marshal(ObservableList<Task> v) throws Exception
TaskList taskList = new TaskList();
v.stream().forEach((item) ->
taskList.entries.add(item);
);
return taskList;
这样你的 TaskListWrapper 最终应该是这样的:
//used in saving the objects to XML
@XmlRootElement(name="tasks")
public class TaskListWrapper
private ObservableList<Task> task;
@XmlJavaTypeAdapter(TaskListAdapter.class)
public ObservableList<Task> getTasks()
return task;
public void setTasks(ObservableList<Task> tasks)
this.task = tasks;
顺便说一句,你使用了很多 FX 属性,所以也许你最好用 @XmlAccessorType(XmlAccessType.PROPERTY)
注释你的类 Task 并确保每个要设置的字段都存在一个 getter/setter。就像 FXProperties 约定所说:
private final StringProperty description = new SimpleStringProperty();
public String getDescription()
return description.get();
public void setDescription(String description)
this.description.set(description);
public StringProperty descriptionProperty()
return description;
注解@XmlAccessType(XmlAccessorType.PROPERTY)
在这里详细描述:JAXB JavaDoc。如果在包或类上没有任何注释,则默认值为@XmlAccessorType(XmlAccessType.PUBLIC_MEMBER)
,JavaDoc 表示:
每个公共 getter/setter 对和每个公共字段都将 自动绑定到 XML,除非被 XmlTransient 注释。
因此,在 FX 类(特殊模型)中,您尝试将使用的属性隐藏在私有字段中。但是,如果您需要一个不应编组的公共字段怎么办?然后我建议做@XmlAccessorType(XmlAccessType.PROPERTY)
注释。它的 JavaDoc 说:
JAXB 绑定类中的每个 getter/setter 对都将自动 绑定到 XML,除非由 XmlTransient 注释。
注意一个词public
的细微差别,所以如果用@XmlAccessorType(XmlAccessType.PROPERTY)
注释,即使是私有的getter/setter也会被考虑在内。
但我认为大多数人使用@XmlAccessorType(XmlAccessType.FIELD)
,JavaDoc 说:
JAXB 绑定类中的每个非静态、非瞬态字段都将是 自动绑定到 XML,除非被 XmlTransient 注释。
这在具有 FX 属性的 FX 类中可能有点棘手。我不会向你推荐它。
【讨论】:
非常感谢!那成功了。您介意解释更多@XmlAccessorType(XmlAccessType.PROPERTY)
吗?此外,XML 文档添加了另一个根 <task>
标记,因此它读取 <tasks> <task> <task> ... <\task> <task>... <\task> <\task> <\tasks>
我将完整的 XML 文档添加到问题中
不客气。双 @XmlElement(name = "tasks")
时,额外的标签只需更改为<tasks>
。我相信它仍在阅读 getter 和 setter 并放置任何名称。例如,getTasks()
将产生 <tasks>
。有没有办法让 JAXB 忽略它?
我很抱歉,我认为 TaskListWrapper
类名存在一些误解。它只是用作TaskList
类。我修复了代码并删除了TaskListWrapper
,因为它不需要。这摆脱了额外的<tasks>
标签。感谢您对其他注释的帮助和解释!以上是关于JAXB RI ClassFactory 中的空指针异常的主要内容,如果未能解决你的问题,请参考以下文章
Android实现滑动效果,但是其中mActionBar.setDisplayHomeAsUpEnabled(false);这句代码报空指,怎么解决
org.xml.sax.SAXNotRecognizedException:无法识别功能“http://javax.xml.XMLConstants/feature/secure-processing