public class Test {
    
    private static final int WIDTH = 200;
    private static final int HEIGHT = 300;
    private static final String OUT_PATH = "image.jpg";
    public static void main(String[]args)
    {
        BufferedImage buffImg = new BufferedImage(WIDTH, HEIGHT, BufferedImage.TYPE_INT_RGB);
        Graphics  g = buffImg.getGraphics();
        g.setColor(Color.WHITE);
        g.fillRect(0, 0, WIDTH, HEIGHT);
        g.setColor(Color.BLACK);
        g.fillOval(0, 0, 30, 30);
        g.fillOval(100, 100, 30, 30);
        g.drawLine(15, 15, 115, 115);
        g.dispose();
        ImageIcon icon = new ImageIcon(buffImg);
        File f = new File(OUT_PATH);
        OutputStream out;
        try {
            out = new FileOutputStream(f);
            ImageIO.write(buffImg, "jpg", out);
            out.close();
        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    
        
        
        
        
    }
    
}

------解决方案--------------------
Java code
BufferedImage bi = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
Graphics2D g2d = bi.createGraphics();

Ellipse2D.Double circle = new Ellipse2D.Double(x,y,w,h);
g2d.setPaint(Color.BLUE);
g2d.fill(circle);

Line2D.Double line = new Line2D.Double(x1,y1,x2,y2);
g2d.draw(line);

... // draw/fill other shapes

g2.dispose();

ImageIO.write(bi,"PNG",new File(...)); // write to file

------解决方案--------------------
还应该使用 setRenderingHint(RenderingHints.Key hintKey, Object hintValue) 设置一下抗锯齿。