我正在构建一个小的java应用程序，用于从usb相机加载图像，或者在找不到usb相机时显示错误消息（图像）。应用程序需要自动加载并启动“captureimage（）”，但在“captureimage（）”方法下出现nullpointerexception错误。
如果我通过单击gui上的按钮调用“captureimage（）”（而不是通过启动线程在“initialize（）”中启动它），那么它可以正常工作。任何关于如何正确处理的指导都将不胜感激。

public class WebcamCapture_gui {
    JFrame frmWebcamCapture_gui;
    static Canvas canvas = new Canvas();
    JPanel panel = new JPanel();
    static Image notFound = null;
    static boolean running = true;
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    WebcamCapture_gui window = new WebcamCapture_gui();
                    window.frmWebcamCapture_gui.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }
    public WebcamCapture_gui() throws IOException {
        initialize();
    }
    private void initialize() {
        JButton btnNewButton = new JButton("OK", new ImageIcon(WebcamCapture_gui.class.getResource("/resources/ok-32.png")));
        btnNewButton.setFont(new Font("Lucida Grande", Font.BOLD, 16));
        btnNewButton.setMnemonic('o');
        btnNewButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                CaptureImage();
            }
        });
        btnNewButton.setBounds(918, 573, 156, 83);
        canvas.setBounds(6, 6, 1052, 547);
        frmWebcamCapture_gui = new JFrame();
        frmWebcamCapture_gui.setResizable(false);
        frmWebcamCapture_gui.setUndecorated(true);
        frmWebcamCapture_gui.setTitle("Camera Test");
        frmWebcamCapture_gui.setSize(1080, 662);
        frmWebcamCapture_gui.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frmWebcamCapture_gui.getContentPane().setLayout(null);
        panel.setBorder(new EtchedBorder(EtchedBorder.LOWERED, null, null));
        panel.setBounds(6, 6, 1068, 563);
        panel.setLayout(null);
        panel.add(canvas);
        frmWebcamCapture_gui.getContentPane().add(panel);
        frmWebcamCapture_gui.getContentPane().add(btnNewButton);
        File classPathInput = new File(WebcamCapture_gui.class.getResource("/resources/USB_not_found.png").getFile());
        try {
            notFound = ImageIO.read(classPathInput);
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        /*
        *  If I remove this Thread start and use the button to call 'CaptureImage()' instead, then the images will load and no NPE.
        */
        Thread t1 = new Thread(new thread1());
        t1.start();
        try { 
            t1.join();
        } catch (Exception e1) {
            System.out.println(e1);
        }
    }
    public static class thread1 implements Runnable {
        @Override
        public void run() {
            CaptureImage();
        }
    }
    public static void CaptureImage() {
        Graphics g = canvas.getGraphics();
        Webcam webcam = Webcam.getDefault();
        if (webcam != null) {
            webcam.open();
            g.drawImage(webcam.getImage(), 0, 0, 1060, 554, null);
            webcam.close();
        } else {
            g.drawImage(notFound, 0, 0, 1060, 547, null);
        }
    }
}

堆栈跟踪：

javax.imageio.IIOException: Can't read input file!
    at java.desktop/javax.imageio.ImageIO.read(ImageIO.java:1308)
    at webcamCapture.WebcamCapture_gui.initialize(WebcamCapture_gui.java:80)
    at webcamCapture.WebcamCapture_gui.<init>(WebcamCapture_gui.java:44)
    at webcamCapture.WebcamCapture_gui$1.run(WebcamCapture_gui.java:34)
    at java.desktop/java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:316)
    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$4.run(EventQueue.java:715)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
    at java.base/java.security.ProtectionDomain$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)
[Thread-0] INFO com.github.sarxos.webcam.Webcam - WebcamDefaultDriver capture driver will be used
[Thread-0] WARN com.github.sarxos.webcam.Webcam - No webcam has been detected!
Exception in thread "Thread-0" java.lang.NullPointerException: Cannot invoke "java.awt.Graphics.drawImage(java.awt.Image, int, int, int, int, java.awt.image.ImageObserver)" because "g" is null
    at webcamCapture.WebcamCapture_gui.CaptureImage(WebcamCapture_gui.java:120)
    at webcamCapture.WebcamCapture_gui$thread1.run(WebcamCapture_gui.java:102)
    at java.base/java.lang.Thread.run(Thread.java:832)