生成随机数字验证码
例程7-11 check_code.html
<h3>带有验证码的登录页面</h3>
<form action="servlet/LogonFormServlet" method="post">
用户名：<input type="text" name="name"><br>
密 码：<input type="password" name="pass"><br>
验证码：<input type="text" name="check_code">
<img src="servlet/CheckCodeServlet"><br>
<input type="submit" value="登录">
</form>
例程7-12 CheckCodeServlet.java
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.awt.*;
import java.awt.image.*;
import javax.imageio.ImageIO;
public class CheckCodeServlet extends HttpServlet
{
private static int WIDTH = 60;
private static int HEIGHT = 20;
public void doGet(HttpServletRequest request,HttpServletResponse response)
throws ServletException,IOException
{
HttpSession session = request.getSession();
response.setContentType("image/jpeg");
ServletOutputStream sos = response.getOutputStream();
//设置浏览器不要缓存此图片
response.setHeader("Pragma","No-cache");
response.setHeader("Cache-Control","no-cache");
response.setDateHeader("Expires", 0);
//创建内存图象并获得其图形上下文
BufferedImage image =
new BufferedImage(WIDTH, HEIGHT, BufferedImage.TYPE_INT_RGB);
Graphics g = image.getGraphics();
//产生随机的认证码
char [] rands = generateCheckCode();
//产生图像
drawBackground(g);
drawRands(g,rands);
//结束图像的绘制过程，完成图像
g.dispose();
//将图像输出到客户端
ByteArrayOutputStream bos = new ByteArrayOutputStream();
ImageIO.write(image, "JPEG", bos);
byte [] buf = bos.toByteArray();
response.setContentLength(buf.length);
//下面的语句也可写成：bos.writeTo(sos);
sos.write(buf);
bos.close();
sos.close();
//将当前验证码存入到Session中
session.setAttribute("check_code",new String(rands));
//直接使用下面的代码将有问题，Session对象必须在提交响应前获得
//request.getSession().setAttribute("check_code",new String(rands));
}
private char [] generateCheckCode()
{
//定义验证码的字符表
String chars = "0123456789abcdefghijklmnopqrstuvwxyz";
char [] rands = new char[4];
for(int i=0; i<4; i++)
{
int rand = (int)(Math.random() * 36);
rands[i] = chars.charAt(rand);
}
return rands;
}
private void drawRands(Graphics g , char [] rands)
{
g.setColor(Color.BLACK);
g.setFont(new Font(null,Font.ITALIC|Font.BOLD,18));
//在不同的高度上输出验证码的每个字符
g.drawString("" + rands[0],1,17);
g.drawString("" + rands[1],16,15);
g.drawString("" + rands[2],31,18);
g.drawString("" + rands[3],46,16);
System.out.println(rands);
}
private void drawBackground(Graphics g)
{
//画背景
g.setColor(new Color(0xDCDCDC));
g.fillRect(0, 0, WIDTH, HEIGHT);
//随机产生120个干扰点
for(int i=0; i<120; i++)
{
int x = (int)(Math.random() * WIDTH);
int y = (int)(Math.random() * HEIGHT);
int red = (int)(Math.random() * 255);
int green = (int)(Math.random() * 255);
int blue = (int)(Math.random() * 255);
g.setColor(new Color(red,green,blue));
g.drawOval(x,y,1,0);
}
}
}
例程7-13 LogonFormServlet.java
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class LogonFormServlet extends HttpServlet
{
public void service(HttpServletRequest request,
HttpServletResponse response) throws ServletException, IOException
{
response.setContentType("text/html;charset=GB2312");
PrintWriter out = response.getWriter();
HttpSession session = request.getSession(false);
if(session == null)
{
out.println("验证码处理问题!");
return;
}
String savedCode = (String)session.getAttribute("check_code");
if(savedCode == null)
{
out.println("验证码处理问题!");
return;
}
String checkCode = request.getParameter("check_code");
if(!savedCode.equals(checkCode))
{
/*验证码未通过，不从Session中清除原来的验证码，
以便用户可以后退回登录页面继续使用原来的验证码进行登录*/
out.println("验证码无效!");
return;
}
/*验证码检查通过后，从Session中清除原来的验证码，
以防用户后退回登录页面继续使用原来的验证码进行登录*/
session.removeAttribute("check_code");
out.println("验证码通过，服务器正在校验用户名和密码!");
}
}
编译上面的两个Java源文件，确保编译后生成的class文件存放在了<tomcat安装目录>\webapps\it315\WEB-INF\classes目录中。将check_code.html文件保存在<tomcat安装目录>\webapps\it315目录中。
（2）在<tomcat安装目录>\webapps\it315\WEB-INF\web.xml文件中注册有关的Servlet，并设置其映射URL。在web.xml文件中的相应位置处增加如下两段内容：
<servlet>
<servlet-name>CheckCodeServlet</servlet-name>
<servlet-class>CheckCodeServlet</servlet-class>
</servlet>
<servlet>
<servlet-name>LogonFormServlet</servlet-name>
<servlet-class>LogonFormServlet</servlet-class>
</servlet>
……
<servlet-mapping>
<servlet-name>CheckCodeServlet</servlet-name>
<url-pattern>/servlet/CheckCodeServlet</url-pattern>
</servlet-mapping>
<servlet-mapping>
<servlet-name>LogonFormServlet</servlet-name>
<url-pattern>/servlet/LogonFormServlet</url-pattern>
</servlet-mapping>
保存web.xml文件后，重新启动Tomcat。
（3）在浏览器地址栏中输入如下地址：
http://localhost:8080/it315/check_code.html
浏览器中显示出如图7.25所示的效果，然后就可以对验证码的功能进行测试了
