html中的null例外Java applet

Question

hi下面是我的java applet代码。当作为applet运行时，它作为独立小程序正常工作。但是，当我将其嵌入HTML页面时，我总是会出现NULL异常错误。我意识到，当我取出图像时，在html中显示没有问题。

为什么图像引起了零异常错误？

public class SymmetryLine extends JApplet {
DrawingCanvas canvas = new DrawingCanvas();
JSlider slider1, slider2, slider3, slider4, slider5, slider6, slider7, slider8;
JPanel outerBig, top, down;
JLabel moveHorizontal, moveVertical, moveRotate;
  double transX = 0.0;
  double transY = 0.0;
  double rotateTheta = 0.0;
  double rotateX = 150.0;
  double rotateY = 150.0;
  double scaleX = 1.0;
  double scaleY = 1.0;
  float width = 1.0f;
  Image image;
  public SymmetryLine()
  {
      try {
         image = ImageIO.read(new File(getClass().getResource("symmetry.jpg").toURI()));
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (URISyntaxException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
  }
public void init()
{
    outerBig=new JPanel();
    outerBig.setLayout(new GridLayout(2,1));

    top = new JPanel();
    top.setLayout(new GridLayout(3,2));
    moveHorizontal = new JLabel("Move Right/Left:");
    moveVertical = new JLabel("Move Up/Down:");
    moveRotate = new JLabel("Rotate Line:");
    top.add(moveHorizontal);        
    slider1 = createSlider(top, JSlider.HORIZONTAL, 0, 300, 150, 100, 50);
    top.add(moveVertical);      
    slider2 = createSlider(top, JSlider.HORIZONTAL, 0, 300, 150, 100, 50);
    top.add(moveRotate);    
    slider3 = createSlider(top, JSlider.HORIZONTAL, 0, 360, 0, 90, 45);

    outerBig.add(top);

    outerBig.add(canvas);
    getContentPane().add(outerBig);

}
public JSlider createSlider(JPanel panel, int orientation, int minimumValue, int maximumValue,
          int initValue, int majorTickSpacing, int minorTickSpacing) {
        JSlider slider = new JSlider(orientation, minimumValue, maximumValue, initValue);
        slider.setPaintTicks(true);
        slider.setMajorTickSpacing(majorTickSpacing);
        slider.setMinorTickSpacing(minorTickSpacing);
        slider.setPaintLabels(true);
        slider.addChangeListener(new SliderListener());
        panel.add(slider);
        return slider;
      }
class SliderListener implements ChangeListener {
    public void stateChanged(ChangeEvent e) {
      JSlider tempSlider = (JSlider) e.getSource();
      if (tempSlider.equals(slider1)) {
        transX = slider1.getValue() - 150.0;
        System.out.println("transX:" +slider1.getValue());
        canvas.repaint();
      } else if (tempSlider.equals(slider2)) {
        transY = slider2.getValue() - 150.0;
        System.out.println("transY:" + slider2.getValue());
        canvas.repaint();
      } else if (tempSlider.equals(slider3)) {
        rotateTheta = slider3.getValue() * Math.PI / 180;
        canvas.repaint();
      } 
    }
  }
class DrawingCanvas extends Canvas {
    public DrawingCanvas() {
      setSize(300, 300);
    }
    public void paint(Graphics g) {
    super.paint(g); 

      g.drawImage(image, 100, 100, this);

      Graphics2D g2D = (Graphics2D) g;
      g2D.translate(transX, transY);
      g2D.rotate(rotateTheta, rotateX, rotateY);
      g2D.scale(scaleX, scaleY);
      BasicStroke stroke = new BasicStroke(width);
      g2D.setStroke(stroke);
      Line2D line1 = new Line2D.Float(0f, 200f, 800f, 200f);
      g2D.draw(line1);

    }
  }
public Image getImage() throws IOException, URISyntaxException
{
    BufferedImage image = ImageIO.read(new File(getClass().getResource("symmetry.jpg").toURI()));
    return image;
}
}

Answer 1

image = ImageIO.read(new File(getClass().getResource("symmetry.jpg").toURI())); 

应该是

image = ImageIO.read(getClass().getResource("symmetry.jpg"));

ImageIO.read(..)被重载以接受ImageInputStream，InputStream，File或URL。由于某些URL不能形成有效的File，因此请坚持使用URL。