java - JAXB 解码返回 Null

Question

我正在制作这个示例 GUI，它只是将计算机部件从一侧移动到另一侧，并且能够将列表(以 xml 格式)加载和保存到桌面。除了重新加载已保存的 xml 文件外，一切正常。我认为这与 Save.java 中的注释有关。话虽如此，我不确定需要什么，或者这是否是问题所在。任何帮助将不胜感激。

Window.java

import java.awt.EventQueue;

import javax.swing.JFrame;
import javax.swing.JPanel;

import java.awt.BorderLayout;

import javax.swing.JButton;

import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

import javax.swing.DefaultListModel;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.JMenu;
import javax.swing.ListSelectionModel;

import java.awt.Component;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Font;

import javax.swing.BoxLayout;
import javax.swing.JList;

import java.awt.GridBagLayout;
import java.awt.GridBagConstraints;

public class Window {

    private JFrame frame;

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    Window window = new Window();
                    window.frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }

    public Window() {
        initialize();
    }

    public void addTo(JPanel displayPanel, Component contentToAdd)
    {
        displayPanel.add(contentToAdd);
    }

    public void initialize() {

        //setting the dimension for the JList panels
        Dimension sidePanelSize = new Dimension(180, 540);

        frame = new JFrame();
        frame.setBounds(100, 100, 480, 540);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        //Creating the menu bar 
        JMenuBar menuBar = new JMenuBar();
        frame.setJMenuBar(menuBar);

        //adding File option to menu bar
        JMenu mnFile = new JMenu("File");
        menuBar.add(mnFile);

        //adding load option to File
        JMenuItem mntmLoad = new JMenuItem("Load");
        mnFile.add(mntmLoad);

        //adding save option to File
        JMenuItem mntmSave = new JMenuItem("Save");
        mnFile.add(mntmSave);

        //adding exit option to File
        JMenuItem mntmExit = new JMenuItem("Exit");
        mnFile.add(mntmExit);

        //creating Jpanel that will hold JList for computer parts 
        //that you can choose 
        final JPanel itemPanel = new JPanel();
        itemPanel.setPreferredSize(sidePanelSize);
        itemPanel.setBackground(Color.WHITE);
        itemPanel.setLayout(new BorderLayout());

        //Create the model that will hold the computer items
        //For loop to add the strings to the model
        DefaultListModel<String> model = new DefaultListModel<>();
        for (String items : new String [] {"Case", "Motherboard", "CPU", "GPU", "PSU", "RAM", "HDD"})
            model.addElement(items);
        //Create JList(itemList) and set its model to the one 
        //holding the computer parts
        final JList<String> itemList = new JList<>(model);

        //Setting attributes for the JList(itemList) - font, Number of elements you can select at a time
        itemList.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
        itemList.setFont(new Font("SegoeUI", Font.BOLD, 11));
        //adding the JList to the Panel
        itemPanel.add(itemList, BorderLayout.WEST);
        //adding the Panel to the frame
        frame.getContentPane().add(itemPanel, BorderLayout.WEST);

        //creating two panels that are used for centering the JList buttons
        JPanel buttonContainer = new JPanel();
        JPanel buttonList = new JPanel();
        GridBagConstraints c = new GridBagConstraints();

        //setting the layout managers for the panels and 
        //adding color to the background
        buttonList.setLayout(new BoxLayout(buttonList, BoxLayout.Y_AXIS));
        buttonContainer.setLayout(new GridBagLayout());
        buttonContainer.setBackground(new Color(238, 238, 238));

        //adding the button to add content from Jlist on the left(itemList)
        //to the right JList(addToList)
        JButton addButton = new JButton(">>");
        buttonList.add(addButton);

        //adding the button to remove content form the JList(addToList)
        JButton deleteButton = new JButton("<<");
        buttonList.add(deleteButton);

        //setting where to start inputing element into the
        //grid of the ButtonContainer
        c.gridx = 0;
        c.gridy = 0;

        //adding the button panel container and its constraints 
        //to the main container
        //finally adding it all to the main frame
        buttonContainer.add(buttonList, c);
        frame.getContentPane().add(buttonContainer, BorderLayout.CENTER);

        //creating the JList that we will add and remove from
        final JList<String> addToList = new JList<>(new DefaultListModel<String>());

        //creating the panel to hold the JList(addToList)
        //setting its size and layout in the manager
        //finally adding it to the main frame
        final JPanel displayPanel = new JPanel();
        displayPanel.setPreferredSize(sidePanelSize);
        displayPanel.setBackground(Color.WHITE);
        displayPanel.add(addToList, BorderLayout.EAST);
        frame.getContentPane().add(displayPanel, BorderLayout.EAST);    



        //Here is all the action listeners for button click events and menu events
        //contains all the methods for the action events
        final ActionListeners b = new ActionListeners();

        //Listener that adds selected computer parts from left JList(itemList) to the right JList(addToList)
        addButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                b.addContent(itemList, addToList);
            }
        });

        //Listener that removes selected computer part from the JList(addToList) 
        deleteButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                b.removeContent(addToList);
            }
        });

        //Listener that calls the save methods to save JList(addToList) content into xml
        mntmSave.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                b.saveContent((DefaultListModel<String>) addToList.getModel());
            }
        });

        //Listener that call the load methods to load xml into the JList(addToList)
        mntmLoad.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                b.loadContent(addToList);
            }
        });

        //Exits the program entirely 
        mntmExit.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                b.exitProgram();
            }
        });
    }
}

加载.java

import java.io.File;

import javax.swing.DefaultListModel;
import javax.xml.bind.JAXBContext;
import javax.xml.bind.JAXBException;
import javax.xml.bind.Unmarshaller;


public class Load {
    //loads the file into the JList to be displayed in the program
    public DefaultListModel<String> loadXMLFile() {
        //array that holds the content from the xml file

        //model that will have xml file's content added to it 
        //from the array
        DefaultListModel<String> modelToReturn = new DefaultListModel<>();
        String[] partsList = null;

        try {
            String homeDir = System.getProperty("user.home");
            File file = new File(homeDir + "/Desktop/xml.xml");

            JAXBContext jaxbContext = JAXBContext.newInstance(Save.class);
            Unmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller();

            Save load = (Save) jaxbUnmarshaller.unmarshal(file);
            partsList = new String [load.getPartsList().length];

          } catch (JAXBException e) {
            e.printStackTrace();
          }

        //adds the strings in the arrayToReturn
        //to the model that will be returned
        for(int i = 0; i < partsList.length; i++)
        {
            modelToReturn.addElement(partsList[i]);
        }

        return modelToReturn;
    }
}

ActionListeners.java

import java.io.File;
import java.util.List;

import javax.swing.DefaultListModel;
import javax.swing.JList;
import javax.xml.bind.JAXBContext;
import javax.xml.bind.JAXBException;
import javax.xml.bind.Marshaller;

public class ActionListeners {

    public void addContent(JList<String> itemList, JList<String> addToList)
    {
        //gets the value selected to be added to other other JList
        List<String> selected = itemList.getSelectedValuesList();
        //gets the model of the List to be added too
        DefaultListModel<String> displayModel = (DefaultListModel<String>) addToList.getModel();

        //adds the elements to the JList
        for (String item: selected) 
        {
            displayModel.addElement(item);
        }
    }

    public void removeContent(JList<String> addToList)
    {
        //gets the element selected to be removed
        List<String> selected = addToList.getSelectedValuesList();
        //gets the model of the JList where content will be removed
        DefaultListModel<String> displayModel = (DefaultListModel<String>) addToList.getModel();

        //removes the selected element
        for (String item: selected) {
            displayModel.removeElement(item);
        }
    }

    public void saveContent(DefaultListModel<String> addToList)
    {
        Save saveFile = new Save();
        //adds the content in the JList to be saved
        //to the object
        saveFile.setPartsList(addToList);

        try {
            JAXBContext jaxbContext = 
                    JAXBContext.newInstance(Save.class);
            Marshaller jaxbMarshaller = 
                    jaxbContext.createMarshaller();
            jaxbMarshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);

            String homeDir = System.getProperty("user.home");

            jaxbMarshaller.marshal(saveFile, new File(homeDir + "/Desktop", "xml.xml"));

            jaxbMarshaller.marshal(saveFile, System.out);
        } catch (JAXBException e) {
            e.printStackTrace();
        }

        //saves the content
        //saveFile.saveFileXml();
    }

    public void loadContent(JList<String> addToList)
    {
        Load loadFile = new Load();
        //gets the model of the JList that loaded content will be added too
        DefaultListModel<String> newModel= (DefaultListModel<String>) addToList.getModel();
        //makes sure the model is clear
        newModel.removeAllElements();
        //makes model that the loaded content will be set too
        DefaultListModel<String> loadedModel = loadFile.loadXMLFile();

        //adds the loaded elements from the file to JList's model
        for(int i = 0; i < loadedModel.getSize(); i++)
        {
            newModel.addElement(loadedModel.get(i));
        }
    }

    public void exitProgram()
    {
        //closes the entire program
        System.exit(0);
    }
}

保存.java

import javax.swing.DefaultListModel;
import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlRootElement;


@XmlRootElement (name = "lists")
public class Save {
    //array that will hold the content to be saved
    String[] partsListSave;

    //method to set the array partsListSave
    //with the content that will be saved
     public void setPartsList(DefaultListModel<String> model) {
         //Initialize the array with the length of the content
         //to be added
         partsListSave = new String[model.getSize()];

         //adds the content to the array
         for (int i = 0; i < model.getSize(); i++)
         {
             partsListSave[i] = model.getElementAt(i);
         }
     } 

    @XmlElement (name = "parts")
    public String[] getPartsList() {
        return partsListSave;
    }
}

xml.xml

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<lists>
    <parts>Motherboard</parts>
    <parts>Motherboard</parts>
    <parts>Motherboard</parts>
    <parts>Motherboard</parts>
</lists>

Answer 1

你的 Save.java类需要一个合适的 setter 方法:

public void setPartsList(String[] partsListSave) {
    this.partsListSave = partsListSave;
}

为了验证这一点，我创建了文件 xml.xml :

<lists>
    <parts>Part 1</parts>
    <parts>Part 2</parts>
    <parts>Part 3</parts>
</lists>

还有一个测试类:

public static void main(String[] args) throws Exception {
    File file = new File("xml.xml");

    JAXBContext jaxbContext = JAXBContext.newInstance(Save.class);
    Unmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller();
    Save load = (Save) jaxbUnmarshaller.unmarshal(file);

    for (String parts : load.getPartsList())
        System.out.println(parts);
}

用你的Save.java ，它因 NPE 而失败。添加二传手成功了。

最小的、完整的和可验证的。

---

注意

你的问题部分是基于误解。

JAXB 解码器(对 jaxbUnmarshaller.unmarshal(file); 的调用)不返回 null - 它返回 Save 的一个实例， 正如它应该。 unmarshal()本身永远不会返回 null (参见 API docs of Unmarshaller :“解码方法从不返回 null。”)——但是它返回的实例中的字段可能是 null .

在这种情况下，字段 Save.partsListSave是null ，因为 JAXB 无法设置它，因为没有合适的 setter ，如上所述。

您看到的 NullPointerException 是由于尝试使用 getPartsList() 返回的值引起的，即 null .