2，ImageServlet获取图片，代码见本文档开始处；

package servlet; import java.awt.Color; import java.awt.Font; import java.awt.Graphics2D; import java.awt.image.BufferedImage; import java.io.IOException; import java.util.Random; import javax.servlet.ServletException; import javax.servlet.ServletOutputStream; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import com.sun.image.codec.jpeg.JPEGCodec; import com.sun.image.codec.jpeg.JPEGImageEncoder; /** * Servlet implementation class ImageServlet */ public class ImageServlet extends HttpServlet { // 定义可选择的字符 private static final String CHARS = "23456789abcdefghjkmnopqrstuvwxyzABCDEFGHJKLMNOPQRSTUVWXYZ"; static Random random = new Random(); // 得到一个随机的字符串 private String getRandomString() { StringBuffer buffer = new StringBuffer(); for (int i = 0; i < 4; i++) { // 生成六个字符 buffer.append(CHARS.charAt(random.nextInt(CHARS.length()))); } return buffer.toString(); } // 随机背景颜色 public static Color getRandomColor() { // 得到随机颜色 return new Color(random.nextInt(128) + 127, random.nextInt(128) + 127, random.nextInt(128) + 127); } public static Color getReverseColor(Color c) { // 得到颜色的反色 return new Color(255 - c.getRed(), 255 - c.getGreen(), 255 - c.getBlue()); } public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("image/jpeg"); // 设置输出类型 // 得到随机字符串 String randomString = getRandomString(); // 将getSession（）设置为true，当会话不存在是返回null request.getSession(true).setAttribute("randomString", randomString); // 设置图片的宽、高 int width = 80; int height = 30; Color bcolor = getRandomColor(); // 设置背景色 Color fcolor = getReverseColor(bcolor); // 前景色 // 创建一个彩色图片 BufferedImage bimage = new BufferedImage(width, height, BufferedImage.TYPE_INT_BGR); // 创建绘图对象,从bimage上得到的绘制对象，会把数据绘制到bimage上 Graphics2D g = bimage.createGraphics(); // 字体样式为宋体,加粗，20磅 g.setFont(new Font("Vijaya", Font.BOLD, 20)); // 先画出背景色 g.setColor(bcolor); g.fillRect(0, 0, width, height); // 再画出前景色 g.setColor(fcolor); for (int i = 0; i < randomString.length(); i++) { // 绘制随机字符 g.drawString(randomString.charAt(i) + "", 10 + i * 12, 22); } // 画出干扰点 for (int i = 0, n = random.nextInt(100); i < n; i++) { g.fillRect(random.nextInt(width), random.nextInt(height), 1, 1); } // 将图像输出到浏览器 ServletOutputStream outstream = response.getOutputStream(); JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(outstream); encoder.encode(bimage); outstream.flush(); outstream.close(); } }

3，服务器对于Ajax请求是如何处理的呢即validateServlet

package servlet; import java.io.IOException; import java.io.PrintWriter; import javax.servlet.ServletException; import javax.servlet.ServletOutputStream; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; public class validateServlet extends HttpServlet { private static final long serialVersionUID = 1L; protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { String validate= request.getParameter("validate"); String randomString=(String)request.getSession(true).getAttribute("randomString"); PrintWriter out=response.getWriter(); if(validate.equalsIgnoreCase(randomString)){ out.write("true"); }else{ out.write("false"); } } }