生成的验证码效果如下：

                        

代码如下：

package com.hh.test;
import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.util.Random;

import javax.imageio.ImageIO;
import javax.imageio.stream.ImageOutputStream;
import javax.servlet.http.HttpServletResponse;

import org.apache.struts2.ServletActionContext;

import com.opensymphony.xwork2.ActionContext;
import com.opensymphony.xwork2.ActionSupport;

public class RandomAction extends ActionSupport{
    private ByteArrayInputStream inputStream;
    public String execute() throws Exception{
//         在内存中创建图象
        int width = 65, height = 24;
        BufferedImage image = new BufferedImage(width, height,BufferedImage.TYPE_INT_RGB);
        Graphics g = image.getGraphics();
        Random random = new Random();
        g.setColor(getRandColor(200, 250));
        g.fillRect(0, 0, width, height);
        g.setFont(new Font("Times New Roman", Font.PLAIN, 24));
        g.setColor(getRandColor(160, 200));

        for (int i = 0; i < 150; i++) {
            int x = random.nextInt(width);
            int y = random.nextInt(height);
            int xl = random.nextInt(12);
            int yl = random.nextInt(12);
            g.drawLine(x, y, x + xl, y + yl);
        }

        //画边框
        g.setColor(new Color(50,143,207));
        g.drawRect(0, 0, width-1, height-1);
//         取随机产生的认证码(6位数字)
        String sRand="";
        char[] codeSequence = { 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H','K', 'M', 'N', 'P','R','S','T','W','X', 'Y','3', '4', '5', '6', '7', '8', '9'};
        for (int i = 0; i < 4; i++) {
            String rand = String.valueOf(codeSequence[random.nextInt(codeSequence.length)]);
            sRand += rand;
            g.setColor(new Color(20 + random.nextInt(110), 20 + random.nextInt(110), 20 + random.nextInt(110)));
            g.drawString(rand, 13 * i + 6, 20);
        }
        Object object = ActionContext.getContext().getSession().get("rand");
        if(null!=object){
            ActionContext.getContext().getSession().remove("rand");
        }
//         将认证码存入SESSION
        ActionContext.getContext().getSession().put("rand",sRand);
        System.out.println("action中生成的验证码为："+sRand);
         HttpServletResponse response = ServletActionContext.getResponse();
         response.setHeader("Pragma", "No-cache");
         response.setHeader("Cache-Control", "no-cache");
         response.setDateHeader("Expires", 0);
//         图象生效
        g.dispose();
        ByteArrayOutputStream output = new ByteArrayOutputStream();
        ImageOutputStream imageOut = ImageIO.createImageOutputStream(output);
        ImageIO.write(image, "JPEG", imageOut);
        imageOut.close();
        ByteArrayInputStream input = new ByteArrayInputStream(output.toByteArray());
        this.setInputStream(input);
        return SUCCESS;
    }
    /*
     * 给定范围获得随机颜色
     */
    private Color getRandColor(int fc,int bc){
        Random random = new Random();
        if(fc>255) fc=255;
        if(bc>255) bc=255;
        int r=fc+random.nextInt(bc-fc);
        int g=fc+random.nextInt(bc-fc);
        int b=fc+random.nextInt(bc-fc);
        return new Color(r,g,b);
   }
    public void setInputStream(ByteArrayInputStream inputStream) {
        this.inputStream = inputStream;
    }
    public ByteArrayInputStream getInputStream() {
        return inputStream;
    }

}

具体页面引用方式已经配置方法，见

加法验证码生成