Question

我在启动Java应用程序时收到此错误消息：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at engine.Display.GamePanel.paintComponent(GamePanel.java:102)
at javax.swing.JComponent.paint(Unknown Source)
at javax.swing.JComponent.paintChildren(Unknown Source)
at javax.swing.JComponent.paint(Unknown Source)
at javax.swing.JLayeredPane.paint(Unknown Source)
at javax.swing.JComponent.paintChildren(Unknown Source)
at javax.swing.JComponent.paintToOffscreen(Unknown Source)
at javax.swing.RepaintManager$PaintManager.paintDoubleBuffered(Unknown Source)
at javax.swing.RepaintManager$PaintManager.paint(Unknown Source)
at javax.swing.RepaintManager.paint(Unknown Source)
at javax.swing.JComponent.paint(Unknown Source)
at java.awt.GraphicsCallback$PaintCallback.run(Unknown Source)
at sun.awt.SunGraphicsCallback.runOneComponent(Unknown Source)
at sun.awt.SunGraphicsCallback.runComponents(Unknown Source)
at java.awt.Container.paint(Unknown Source)
at java.awt.Window.paint(Unknown Source)
at javax.swing.RepaintManager$3.run(Unknown Source)
at javax.swing.RepaintManager$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source)
at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source)
at javax.swing.RepaintManager.prePaintDirtyRegions(Unknown Source)
at javax.swing.RepaintManager.access$1100(Unknown Source)
at javax.swing.RepaintManager$ProcessingRunnable.run(Unknown Source)
at java.awt.event.InvocationEvent.dispatch(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$400(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

应用程序没有关闭，它让我正常运行它没有视觉错误等等，但是当我启动应用程序时，我仍然会收到此错误消息。

我指向的唯一的代码是我的GamePanel类中的第102行，这是我的paintComponent方法：

    @Override
public void paintComponent(Graphics g){
    super.paintComponent(g);
    //g.drawImage(test, picX, picY, picSizeX, picSizeY, null);
    mao.Draw(picX, picY, g);
}

在mao.Draw中我们有这个：

    public void Draw(int x, int y, Graphics g){
    Graphics2D g2d = (Graphics2D) g;
        g2d.drawImage(characterImage, x, y, null);
        g2d.drawRect(x, y, getWidth(), getHeight());
}

我已经解决了这个问题，但我似乎无法找到错误。

Answer 1

错误是在GamePanel.java的第102行引起的，我认为是

  mao.Draw(picX, picY, g);

因为mao尚未初始化，即在调用方法时具有值null。可能您的应用程序尚未完全初始化，但GamePanel已经显示。

您可以执行以下操作来检查是否是这种情况：

if (mao != null) {
    mao.Draw(picX, picY, g);
}

Answer 2

NullPointerException方法中的例外情况为paintComponent()。

所以问题存在，而不是你的Draw()方法。在paintComponent()中，您访问mao标识符，我认为该标识符是GamePanel类的属性。

Swing中的所有绘画都是在EDT主题上完成的，我假设您初始化并设置此mao属性不在EDT主题上=>这是一个同步问题。在EDT线程上分配mao属性，或使其同步，以便正确访问它的多个线程看到它的真实值。

一旦或罕见的情况和您的应用程序正常运行，您会收到此异常，因为只有paintComponent()将被中止，但后续尝试绘制组件可能会成功（如果mao最终可见EDT线程或同时被修改。

“AWT-EventQueue-0”NullpointerException不一致的错误消息

2 个答案: