使用 Nimbus 外观时 writeObject() 期间发生 NullPointerException

Question

我有 4 个类：TestGUI、MainMenuBar、MainFrame 和添加项目。 MainFrame 和 ProjectToAdd 实现 Serializable。当我在 MainMenuBar 中单击 New>Project 时，我想创建一个新项目，为它创建一个新目录，并在该目录中编写一个可序列化的对象。写入对象后，我得到 NullPointerException（请参阅末尾的堆栈跟踪）。如果我在 main 方法中删除代码的外观和感觉部分，我不会再收到错误...我正在使用 NetBeans IDE 8.0.2.

测试GUI代码：

package testgui;

import javax.swing.UIManager;
import javax.swing.UIManager.LookAndFeelInfo;

public class RiskManagerGUI {

    public static void main(String[] args) {

        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {

                try {
                    for (LookAndFeelInfo info : UIManager.getInstalledLookAndFeels()) {
                        if ("Nimbus".equals(info.getName())) {
                            UIManager.setLookAndFeel(info.getClassName());
                            break;
                        }
                    }
                } catch (Exception e) {
                    // If Nimbus is not available, fall back to cross-platform
                    try {
                        UIManager.setLookAndFeel(UIManager.getCrossPlatformLookAndFeelClassName());
                    } catch (Exception ex) {
                        // not worth my time
                    }
                }

                new MainFrame().setVisible(true);

            }
        });
    }

}

主菜单条码：

package menu_package;

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;
import testgui.MainFrame;

public class MainMenuBar extends JMenuBar{

    private MainFrame mainFrame;

    public MainMenuBar(MainFrame mainFrame){

        this.mainFrame = mainFrame;

        JMenu fileMenu = new JMenu("File");
        JMenu newMenu = new JMenu("New");

        JMenuItem projectMenuItem = new JMenuItem("Project");  
        projectMenuItem.addActionListener(new AddNewProject());

        newMenu.add(projectMenuItem);

        fileMenu.add(newMenu);

        add(fileMenu);
    }

    class AddNewProject implements ActionListener {
    public void actionPerformed(ActionEvent event) {

            mainFrame.addNewProject();

    }
    }
}

主框架代码：

package testgui;

import java.awt.*;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.ArrayList;
import javax.swing.*;
import javax.swing.border.*;
import left_panel_package.ProjectsJTree;
import menu_package.MainMenuBar;

public class MainFrame extends JFrame implements Serializable{

    private String instalationLocation;
    private ArrayList <ProjectToAdd> listOfProjects;

    public MainFrame(){
        initComponents();
    }

    public void initComponents() {


        setInstalationLocation("C:\Users\Pera\Desktop\RiskManagerGUI");
        listOfProjects = new ArrayList<ProjectToAdd>();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
        setTitle("TEST");
        setName("mainFrame");

        JPanel mainPanel = new JPanel();

        setContentPane(mainPanel);
        setJMenuBar(new MainMenuBar(this));

        GraphicsDevice gd = GraphicsEnvironment.getLocalGraphicsEnvironment().getDefaultScreenDevice();
        int width = gd.getDisplayMode().getWidth();
        int height = gd.getDisplayMode().getHeight();
        setSize(width, height);
        setLocationRelativeTo(null);
    }

    public String getInstalationLocation() {
        return instalationLocation;
    }

    private void setInstalationLocation(String instalationLocation) {
        this.instalationLocation = instalationLocation;
    }

    public void addNewProject(){
        String nameOfProject = JOptionPane.showInputDialog(null, "Enter the name of new project: ", "Add new project",JOptionPane.OK_CANCEL_OPTION);
        if ((nameOfProject != null) && (nameOfProject.length() > 0)) {
            try {
                ProjectToAdd newProject = new ProjectToAdd(this, nameOfProject);
                listOfProjects.add(newProject);

                File theDir = new File(newProject.getProjectPath());
                theDir.mkdirs();

                newProject.writeToFile();

            } catch (IOException ex) {
                System.out.println(ex.getMessage());
            }

        }   
    }
}

ProjectToAdd 代码：

package testgui;


import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.io.Serializable;

public class ProjectToAdd implements Serializable{

    private String nameOfProject;
    private MainFrame mainFrame;
    private String projectPath;

    public ProjectToAdd (MainFrame mainFrame, String nameOfProject) {

            this.mainFrame = mainFrame;
            setNameOfProject(nameOfProject);
            setProjectPath(mainFrame.getInstalationLocation()+"\Projects\"+nameOfProject);

    }

    public String getNameOfProject() {
        return nameOfProject;
    }

    public void setNameOfProject(String nameOfProject) {
        this.nameOfProject = nameOfProject;
    }

    public void writeToFile() throws IOException{
        ObjectOutputStream objectOutputStream = 
            new ObjectOutputStream(new FileOutputStream(projectPath+
                    "\"+nameOfProject+".bin"));
        objectOutputStream.writeObject(this);
    }
    public void readFromFile(){

    }

    public String getProjectPath() {
        return projectPath;
    }

    public void setProjectPath(String projectPath) {
        this.projectPath = projectPath;
    }
}

堆栈跟踪：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at javax.swing.plaf.synth.SynthLookAndFeel.paintRegion(SynthLookAndFeel.java:371)
    at javax.swing.plaf.synth.SynthLookAndFeel.update(SynthLookAndFeel.java:335)
    at javax.swing.plaf.synth.SynthRootPaneUI.update(SynthRootPaneUI.java:119)
    at javax.swing.JComponent.paintComponent(JComponent.java:780)
    at javax.swing.JComponent.paint(JComponent.java:1056)
    at javax.swing.JComponent.paintToOffscreen(JComponent.java:5210)
    at javax.swing.RepaintManager$PaintManager.paintDoubleBuffered(RepaintManager.java:1579)
    at javax.swing.RepaintManager$PaintManager.paint(RepaintManager.java:1502)
    at javax.swing.RepaintManager.paint(RepaintManager.java:1272)
    at javax.swing.JComponent._paintImmediately(JComponent.java:5158)
    at javax.swing.JComponent.paintImmediately(JComponent.java:4969)
    at javax.swing.RepaintManager.run(RepaintManager.java:831)
    at javax.swing.RepaintManager.run(RepaintManager.java:814)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
    at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:814)
    at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:789)
    at javax.swing.RepaintManager.prePaintDirtyRegions(RepaintManager.java:738)
    at javax.swing.RepaintManager.access00(RepaintManager.java:64)
    at javax.swing.RepaintManager$ProcessingRunnable.run(RepaintManager.java:1732)
    at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:311)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:756)
    at java.awt.EventQueue.access0(EventQueue.java:97)
    at java.awt.EventQueue.run(EventQueue.java:709)
    at java.awt.EventQueue.run(EventQueue.java:703)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:726)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

Answer 1

抛出 NPE 是因为您正在尝试序列化 MainFrame，这是一个包含大量 GUI 碎屑的对象，包括 Nimbus L&F 信息，以及不应序列化且看起来不可序列化的内容。我怎么知道？将该字段标记为瞬态，看看会发生什么：

class ProjectToAdd implements Serializable {
    private static final long serialVersionUID = 1L;
    private String nameOfProject;
    private transient MainFrame mainFrame;  // **********
    private String projectPath;

如果您需要序列化 MainFrame 中保存的信息，则创建一个包含来自 MainFrame 的关键业务数据的非 GUI 模型 class 并序列化它，而不是视图 -- 而不是 GUI 本身.无论如何，通常您应该几乎永远不要序列化视图，从视图代码中重构您的程序逻辑始终是个好主意。

你是对的，这个：

objectOutputStream.writeObject(this);

与NPE有关。向你道歉。

使用 Nimbus 外观时 writeObject() 期间发生 NullPointerException

NullPointerException during writeObject() while using Nimbus Look and Feel

java

swing

serializable

nullpointerexception

nimbus