JAXB RI ClassFactory 中的空指针异常

Question

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 stackoverflow 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？

Answer 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。