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首先是用一个servlet ：ImageServlet.java产生验证码并将验证字符串存到session里。

package servlet;import java.awt.Color;import java.awt.Graphics;import java.awt.image.BufferedImage;import java.io.IOException;import java.util.Random;import javax.imageio.ImageIO;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;public class ImageServlet extends HttpServlet {	public void doGet(HttpServletRequest request, HttpServletResponse response)			throws ServletException, IOException {		// 设置页面不缓存		response.setHeader("Pragma", "No-cache");		response.setHeader("Cache", "no-cache");		response.setDateHeader("Expires", 0);		// 在内存中创建图像		int width = 60;		int height = 20;		BufferedImage image = new BufferedImage(width, height,				BufferedImage.TYPE_INT_RGB);		Graphics g = image.getGraphics();		Random random = new Random();		g.setColor(new Color(0, 0, 100));		g.fillRect(0, 0, width, height);		String sRand = "";		for (int i = 0; i < 4; i++) {			String rand = String.valueOf(random.nextInt(10));			sRand += rand;			g.setColor(new Color(30 + random.nextInt(160), 40 + random					.nextInt(170), 40 + random.nextInt(180)));			g.drawString(rand, 13 * i + 6, 16); // 绘制		}		request.getSession().setAttribute("yzm", sRand); // 验证码存入session		g.dispose();		// 输出图像		ImageIO.write(image, "JPEG", response.getOutputStream());	}	public void doPost(HttpServletRequest request, HttpServletResponse response)			throws ServletException, IOException {		this.doGet(request, response);	}}

然后在web.xml里把这个servlet 的URL设置成getImage 然后是JSP

<%@ page language="java" import="java.util.*" pageEncoding="gbk"%><%	String path = request.getContextPath();	String basePath = request.getScheme() + "://"			+ request.getServerName() + ":" + request.getServerPort()			+ path + "/";%>							
   		
   		
   		
   		
   		
   				验证码ImageServlet测试		
		
   
    			用户名:
    
    
			密 码:
    
    
			验证码
    
    
    
 
    			
    		
   
	

acton= main中的main是一个随便写的servlet ，由于要用filter验证验证码，所以如果能访问到这个servlet 则证明验证码正确。

yanzhengmaFilter.java

package Filter;import java.io.FileWriter;import java.io.IOException;import java.io.PrintWriter;import javax.servlet.Filter;import javax.servlet.FilterChain;import javax.servlet.FilterConfig;import javax.servlet.ServletException;import javax.servlet.ServletRequest;import javax.servlet.ServletResponse;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;public class yanzhengmaFilter implements Filter {	@Override	public void destroy() {		// TODO Auto-generated method stub	}	@Override	public void doFilter(ServletRequest request, ServletResponse response,			FilterChain chain) throws IOException, ServletException {		// TODO Auto-generated method stub		HttpServletRequest req = (HttpServletRequest) request;// 转换之后才可以获取sesson		HttpServletResponse res = (HttpServletResponse)response;		String useryzm = request.getParameter("yzm");		String sysyzm = null;		try {			sysyzm = req.getSession().getAttribute("yzm").toString();		} catch (Exception e) {			response.setCharacterEncoding("gbk");			PrintWriter out = response.getWriter();			out.println("");			res.sendRedirect("login.jsp");			return;		}		if (sysyzm.equals(useryzm) && sysyzm != null) {			chain.doFilter(request, response);		} else {			response.setCharacterEncoding("gbk");			PrintWriter out = response.getWriter();			out.println("");						res.sendRedirect("login.jsp");			return;		}	}	@Override	public void init(FilterConfig filterConfig) throws ServletException {		// TODO Auto-generated method stub	}}

在web.xml里把这个过滤器加进去 然后设置当访问main这个servlet时进行验证就行了。

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