二维码生成需要借助第三方jar包。我这里使用的是QRcode。给大家一个连接，自行下载：http://download.csdn.net/detail/xiaokui_wingfly/7957815

生成二维码代码：

    public void encoderQRCoder(String content, HttpServletResponse response) {
        try {
            Qrcode handler = new Qrcode();
            handler.setQrcodeErrorCorrect('M');
            handler.setQrcodeEncodeMode('B');
            handler.setQrcodeVersion(7);
            byte[] contentBytes = content.getBytes("UTF-8");

            BufferedImage bufImg = new BufferedImage(140, 140, BufferedImage.TYPE_INT_RGB);
            Graphics2D gs = bufImg.createGraphics();

            gs.setBackground(Color.WHITE);
            gs.clearRect(0, 0, 140, 140);

            //设定图像颜色：BLACK
            gs.setColor(Color.BLACK);

            //设置偏移量  不设置肯能导致解析出错
            int pixoff = 2;
            //输出内容：二维码
            if (contentBytes.length > 0 && contentBytes.length < 124) {
                boolean[][] codeOut = handler.calQrcode(contentBytes);
                for (int i = 0; i < codeOut.length; i++) {
                    for (int j = 0; j < codeOut.length; j++) {
                        if (codeOut[j][i]) {
                            gs.fillRect(j * 3 + pixoff, i * 3 + pixoff, 3, 3);
                        }
                    }
                }
            } else {
                System.err.println("QRCode content bytes length = " + contentBytes.length + " not in [ 0,120 ]. ");
            }
            gs.dispose();
            bufImg.flush();
            OutputStream outputStream = response.getOutputStream();
            ImageIO.write(bufImg, "PNG", outputStream);
            outputStream.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

第一个参数是链接的内容，比如：https://www.baidu.com
第二个是HttpServletResponse，为什么要传这个参数呢，我是让二维码直接显示到jsp中，在jsp页面，定义:

<i mg  src="code">

code是我servlet,让src指向这个servlet。好吧，可能我表达的不够清除，我把完整代码贴一下：

public class CodeServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
       
    public CodeServlet() {
        super();
    }
    @Override
    protected void service(HttpServletRequest arg0, HttpServletResponse arg1) throws ServletException, IOException {
        // TODO Auto-generated method stub
        super.service(arg0, arg1);
        String content = "http://www.baidu.com";  
        encoderQRCoder(content, arg1);  
    }
    
    public void encoderQRCoder(String content, HttpServletResponse response) {  
        try {  
            
            Qrcode handler = new Qrcode();  
            handler.setQrcodeErrorCorrect('M');  
            handler.setQrcodeEncodeMode('B');  
            handler.setQrcodeVersion(-1);  
              
            System.out.println(content);  
            byte[] contentBytes = content.getBytes("UTF-8");  
              
            BufferedImage bufImg = new BufferedImage(80, 80, BufferedImage.TYPE_INT_RGB);  
              
            Graphics2D gs = bufImg.createGraphics();  
              
            gs.setBackground(Color.WHITE);  
            gs.clearRect(0, 0, 140, 140);  
              
            //设定图像颜色：BLACK  
            gs.setColor(Color.BLACK);  
              
            //设置偏移量  不设置肯能导致解析出错  
            int pixoff = 2;  
            //输出内容：二维码  
            if(contentBytes.length > 0 && contentBytes.length < 124) {  
                boolean[][] codeOut = handler.calQrcode(contentBytes);  
                for(int i = 0; i < codeOut.length; i++) {  
                    for(int j = 0; j < codeOut.length; j++) {  
                        if(codeOut[j][i]) {  
                            gs.fillRect(j * 3 + pixoff, i * 3 + pixoff,3, 3);  
                        }  
                    }  
                }  
            } else {  
                System.err.println("QRCode content bytes length = " + contentBytes.length + " not in [ 0,120 ]. ");  
            }  
              
            gs.dispose();  
            bufImg.flush();  
            response.reset();
            //生成二维码QRCode图片  
            OutputStream outputStream = response.getOutputStream();
            ImageIO.write(bufImg, "png", outputStream);
            outputStream.close();
        } catch (Exception e) {  
            e.printStackTrace();  
        }  
    }  

}

 <servlet>  
    <servlet-name>CodeService</servlet-name>  
    <servlet-class>com.webgo.servlets.CodeServlet</servlet-class>  
  </servlet>  
  <servlet-mapping>  
    <servlet-name>CodeService</servlet-name>  
    <url-pattern>/code</url-pattern>  
  </servlet-mapping>  

注意：当二维码出现显示不全的情况，
BufferedImage bufImg = new BufferedImage(140, 140, BufferedImage.TYPE_INT_RGB);
修改BufferedImage的两个140,140参数，通常和gs.clearRect(0, 0, 140, 140);一致就ok。

笔者能力有限，不足之处欢迎指出！