JAXB RI ClassFactory 中的空指针异常

2024-01-28

Intro

我和我的朋友正在开发一个 JavaFX 应用程序，该应用程序充当我们学校的规划器。我们有任务（课程作业）、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 类编组到包装器中。问题是从tasks.xml file.

以下是相关代码行：

任务.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

它涉及保存到文件和从文件中解组。

/**
     * 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)

空指针位于//TODO中所述initFiles() method:

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：

搞乱名称和注释。看来命名不是问题。
系统输出文件位置以确保其正确。
Sysouting 的类JAXBContext知道。它同时识别Task and TaskListWrapper类。
系统路由um.toString()。它显示内存中的有效地址，因此um对象本身并不是引发空指针异常的原因。
改变位置TaskListWrapper.java到同一个包Task.java.
尝试通过将 XML 文件更改为只有一个来解组单个任务<task>因为当我改变时根元素起作用
```
TaskListWrapper taskList = (TaskListWrapper) um.unmarshal(tasks);
```
to
```
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/ http://examples.javacodegeeks.com/core-java/xml/bind/jaxb-unmarshal-example/
许多 stackoverflow 问题都与 bug 有关@XMLAttribute注解。因为我们不使用那些错误不相关的

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") );

Question

Why is TaskListWrapper taskList = (TaskListWrapper) um.unmarshal(tasks);抛出空指针异常？

JAXB 与包装器中的 ObservableList 等 FXCollections 不兼容。您必须编写一个 XmlAdapter 将其解开为普通的 List。因此，编组将起作用，但解组则不起作用，正如您在堆栈跟踪行中看到的那样：

at com.sun.xml.internal.bind.v2.runtime.reflect.Lister$CollectionLister.startPacking(Unknown Source)

有的是列表器$CollectionLister不知道如何处理未知来源。所以适配器应该使用这样的ListWrapper：

public class TaskList {

  @XmlElement(name = "task")
  List<Task> entries = new ArrayList<>();

  public List<Task> getEntries() {
    return entries;
  }
}

对应的Adapter如下所示：

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 TaskList Wrapper 最终应该看起来像这样：

//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，您使用了很多 FX 属性，因此也许您最好用以下方式注释您的类任务@XmlAccessorType(XmlAccessType.PROPERTY)并确保每个要设置的字段都存在 getter/setter。就像 FXProperties 公约所说：

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;
}

注释@XmlAccessType(XmlAccessorType.PROPERTY)详细描述如下：JAXB Java文档 https://jaxb.java.net/nonav/2.2.11/docs/api/javax/xml/bind/annotation/XmlAccessType.html。如果在包或类上没有任何注释，则默认为@XmlAccessorType(XmlAccessType.PUBLIC_MEMBER)，其中 JavaDoc 说：

每个公共 getter/setter 对和每个公共字段都将是自动绑定到 XML，除非由 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该被整理的公共领域怎么办？然后我建议这样做@XmlAccessorType(XmlAccessType.PROPERTY)注解。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。

观察一个词的细微差别public，所以如果注释为@XmlAccessorType(XmlAccessType.PROPERTY)甚至私人 getter/setter 也会被考虑在内。

但我想大多数人都用@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，除非由 XmlTransient 注释。

在具有 FX 属性的 FX 类中，这可能有点棘手。我不会向你推荐它。

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