添加 PropertyChangeListener 会导致 java.lang.NullPointerException

Question

我是使用 Netbeans 和 PropertyChangeListeners 的新手，在 Java bean 中实现 PropertyChangeSupport 时，我遇到了同样的（对我而言）奇怪的行为。

所以我有一个名为 TTTCell 的 bean，我在其中初始化了一个 PropertyChangeSupport 变量。然后我实现了维护属性更改侦听器列表的函数。

package tttboard;

import java.awt.Color;
import java.beans.PropertyChangeEvent;
import java.beans.PropertyChangeListener;
import java.beans.PropertyChangeSupport;
import java.beans.PropertyVetoException;
import java.beans.VetoableChangeListener;
import java.beans.VetoableChangeSupport;

public class TTTCell extends javax.swing.JPanel {

    public static enum State
    {
        INITIAL,
        PLAYER_X,
        PLAYER_O,
        WON,
        DISABLE
    }
    
    // Variables
    private State state;
    private PropertyChangeSupport propChange = new PropertyChangeSupport(this);
    
    public TTTCell() {
        
        initComponents();
        state = State.INITIAL;
    }
    
    @Override
    public void addPropertyChangeListener(PropertyChangeListener listener) {
        propChange.addPropertyChangeListener(listener); // it depends on that line whether the erroroccurs or not
    }
    @Override
    public void removePropertyChangeListener(PropertyChangeListener listener) {
        propChange.removePropertyChangeListener(listener);
    }
    

我现在把这个 bean 放在另一个名为 TTTBoard 的 bean 中，它给了我以下错误：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    
at tttboard.TTTCell.addPropertyChangeListener(TTTCell.java:55)
    at java.desktop/javax.swing.plaf.synth.SynthPanelUI.installListeners(SynthPanelUI.java:83)
    at java.desktop/javax.swing.plaf.synth.SynthPanelUI.installUI(SynthPanelUI.java:63)
    at java.desktop/javax.swing.JComponent.setUI(JComponent.java:685)
    at java.desktop/javax.swing.JPanel.setUI(JPanel.java:150)
    at java.desktop/javax.swing.JPanel.updateUI(JPanel.java:126)
    at java.desktop/javax.swing.JPanel.<init>(JPanel.java:86)
    at java.desktop/javax.swing.JPanel.<init>(JPanel.java:109)
    at java.desktop/javax.swing.JPanel.<init>(JPanel.java:117)
    at tttboard.TTTCell.<init>(TTTCell.java:38)
    at tttboard.TTTBoard.initComponents(TTTBoard.java:89)
    at tttboard.TTTBoard.<init>(TTTBoard.java:27)
    at tttboard.TTTBoard$2.run(TTTBoard.java:203)
    at java.desktop/java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:313)
    at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:770)
    at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:721)
    at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:715)
    at java.base/java.security.AccessController.doPrivileged(Native Method)
    at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85)
    at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:740)
    at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:203)
    at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:124)
    at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:113)
    at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:109)
    at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
    at java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:90)

对我来说这没有任何意义 - PropertyChangeSupport 变量被实例化，为什么它应该是一个空指针？

仅供参考，我使用的是 Netbeans IDE 11.1 和 openjdk 11.0.11

Answer 1

一个类的非静态字段在该类进入其初始化/构造之前不会被初始化。超类的构造总是首先发生。

如果查看堆栈跟踪，可以看到异常是超类构造的结果：

at java.desktop/javax.swing.JPanel.<init>(JPanel.java:117)
at tttboard.TTTCell.<init>(TTTCell.java:38)

在 TTTCell 字段初始化和构造函数可以运行之前，超类构造函数（即 JPanel 构造函数）必须完成。

JPanel 构造函数调用其 updateUI 方法，该方法最终调用 addPropertyChangeListener：

at tttboard.TTTCell.addPropertyChangeListener(TTTCell.java:55)
at java.desktop/javax.swing.plaf.synth.SynthPanelUI.installListeners(SynthPanelUI.java:83)
at java.desktop/javax.swing.plaf.synth.SynthPanelUI.installUI(SynthPanelUI.java:63)
at java.desktop/javax.swing.JComponent.setUI(JComponent.java:685)
at java.desktop/javax.swing.JPanel.setUI(JPanel.java:150)
at java.desktop/javax.swing.JPanel.updateUI(JPanel.java:126)
at java.desktop/javax.swing.JPanel.<init>(JPanel.java:86)

但是 TTTCell 还没有机会运行它的初始化程序，其中不仅包括 TTTCell() 构造函数，还包括字段的初始化，包括这个：

private PropertyChangeSupport propChange = new PropertyChangeSupport(this);

由于在调用 addPropertyChangeListener 覆盖时该行尚未运行（因为 JPanel 构造函数尚未完成），propChange 仍然为空。

解决方案很简单：删除您对 addPropertyChangeListener 和 removePropertyChangeListener 的覆盖。他们没有提供任何好处。 Component 类已经提供了每个方法的工作实现。没有什么能阻止你使用它们。