实现jsp验证码的简单小例子

<%@ page language="java" pageencoding="gbk"%>
<%@ page contenttype="image/jpeg" import="java.awt.*,
java.awt.image.*,java.util.*,javax.imageio.*" %>
<%!
color getrandcolor(int fc,int bc){//给定范围获得随机颜色
        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);
        }
%>
<%
//设置页面不缓存
response.setheader("pragma","no-cache");
response.setheader("cache-control","no-cache");
response.setdateheader("expires", 0);

// 在内存中创建图象
int width=60, height=20;
bufferedimage image = new bufferedimage(width, height, bufferedimage.type_int_rgb);

// 获取图形上下文
graphics g = image.getgraphics();

//生成随机类
random random = new random();

// 设定背景色
g.setcolor(getrandcolor(200,250));
g.fillrect(0, 0, width, height);

//设定字体
g.setfont(new font("times new roman",font.plain,18));

//画边框
//g.setcolor(new color());
//g.drawrect(0,0,width-1,height-1);

// 随机产生155条干扰线，使图象中的认证码不易被其它程序探测到
g.setcolor(getrandcolor(160,200));
for (int i=0;i<155;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);
}

// 取随机产生的认证码(4位数字)
string srand="";
for (int i=0;i<4;i++){
    string rand=string.valueof(random.nextint(10));
    srand+=rand;
    // 将认证码显示到图象中
    g.setcolor(new color(20+random.nextint(110),20+random.nextint(110),20+random.nextint(110)));
//调用函数出来的颜色相同，可能是因为种子太接近，所以只能直接生成
    g.drawstring(rand,13*i+6,16);
}

// 将认证码存入session
session.setattribute("rand",srand);

// 图象生效
g.dispose();

// 输出图象到页面
imageio.write(image, "jpeg", response.getoutputstream());
 out.clear();

%>

页面引用：

<td width="62" valign="bottom"><div align="left"><img name="img" id="img" border=0 src="index/image.jsp" onclick="this.src='index/image.jsp?'+math.random();" /></div></td>

js：

this.src='index/image.jsp?'+math.random();

登陆时候获取验证码：

        string randb = (string) request.getsession().getattribute("rand");