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import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.util.Random;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import com.sun.image.codec.jpeg.JPEGCodec;
import com.sun.image.codec.jpeg.JPEGImageEncoder;
/**
* @(#)VerifyCodeServlet.java Dec 9, 2007 8:14:14 PM
*
* @author Yuan
* 驗證碼生成器，使用此類需要將表單里的驗證碼輸入框的name屬性設為"verifycode"
*/
public class VerifyCodeGenerator {

private static final VerifyCodeGenerator generator = new VerifyCodeGenerator();

private final String ATTRIBUTE_NAME = "verifycode";
//圖片的寬度
private final int WIDTH = 15;
//圖片的高度
private final int HEIGHT = 22;
//字符串長度
private final int CODE_LENGTH = 4;
//隨機字符串范圍
private final String RAND_RANGE = "abcdefghijklmnopqrstuvwxyz"
+ "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+ "1234567890"
+ "@#quot;;

private final char[] CHARS = RAND_RANGE.toCharArray();

private Random random = new Random();

private VerifyCodeGenerator(){
//
}

public static VerifyCodeGenerator getInstance(){
return generator;
}

/**
* 生成隨機字符串
* @return 隨機字符串
*/
private String getRandString(){
StringBuilder sb = new StringBuilder();
for (int i = 0; i < CODE_LENGTH; i++)
sb.append(CHARS[random.nextInt(CHARS.length)]);
return sb.toString();
}

/**
* 生成隨機顏色
* @param ll 產生顏色值下限(lower limit)
* @param ul 產生顏色值上限(upper limit)
* @return 生成的隨機顏色對象
*/
private Color getRandColor(int ll, int ul){
if (ll > 255) ll = 255;
if (ll < 1) ll = 1;
if (ul > 255) ul = 255;
if (ul < 1) ul = 1;
if (ul == ll) ul = ll + 1;
int r = random.nextInt(ul - ll) + ll;
int g = random.nextInt(ul - ll) + ll;
int b = random.nextInt(ul - ll) + ll;
Color color = new Color(r,g,b);
return color;
}

/**
* 生成指定字符串的圖像數據
* @param verifyCode 即將被打印的隨機字符串
* @return 生成的圖像數據
* */
private BufferedImage getImage(String verifyCode){

BufferedImage image = new BufferedImage(WIDTH * CODE_LENGTH, HEIGHT, BufferedImage.TYPE_INT_RGB);

//獲取圖形上下文
Graphics graphics = image.getGraphics();

//設置背景色
graphics.setColor(getRandColor(1,50));
//填充背景色
graphics.fillRect(0, 0, WIDTH * 4, HEIGHT);

//設置邊框顏色
graphics.setColor(new Color(0,255,0));
//畫邊框
for (int i=0; i<2; i++)
graphics.drawRect(i, i, WIDTH * CODE_LENGTH - i * 2 - 1, HEIGHT - i * 2 - 1);

//設置隨機干擾線條顏色
graphics.setColor(getRandColor(50,100));
//產生50條干擾線條
for (int i=0; i<50; i++){
int x1 = random.nextInt(WIDTH * CODE_LENGTH - 4) + 2;
int y1 = random.nextInt(HEIGHT - 4) + 2;
int x2 = random.nextInt(WIDTH * CODE_LENGTH - 2 - x1) + x1;
int y2 = y1;
graphics.drawLine(x1, y1, x2, y2);
}

//設置字體
graphics.setFont(new Font("Times New Roman", Font.PLAIN, 18));
//畫字符串
for (int i=0; i<this.CODE_LENGTH; i++){
String temp = verifyCode.substring(i, i+1);
graphics.setColor(getRandColor(100,255));
graphics.drawString(temp, 13 * i + 6, 16);
}

//圖像生效
graphics.dispose();

return image;
}

/**
* 將驗證碼的圖像輸出
* @param request 用戶的請求對象
* @param response 用戶的響應對象
* */
public void printImage(HttpServletRequest request,
HttpServletResponse response){
//將ContentType設為"image/jpeg"，讓瀏覽器識別圖像格式。
response.setContentType("image/jpeg");
//設置頁面不緩存
response.setHeader("Pragma", "No-cache");
response.setHeader("Cache-Control", "no-cache");
response.setDateHeader("Expires", 2000);

//獲得隨機驗證碼
String verifyCode = this.getRandString();
String str = "ssss";
for(int i=0; i<10; i++)
str = str + str;
//獲得驗證碼的圖像數據
BufferedImage bi = this.getImage(verifyCode);
//把驗證碼存入session
request.getSession().setAttribute(ATTRIBUTE_NAME, verifyCode);
try{
//獲得Servlet輸出流
ServletOutputStream outStream = response.getOutputStream();
//創建可用來將圖像數據編碼為JPEG數據流的編碼器
JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(outStream);
//將圖像數據進行編碼
encoder.encode(bi);
//強行將緩沖區的內容輸入到頁面
outStream.flush();
//關閉輸出流
outStream.close();
}catch(IOException ex){
ex.printStackTrace();
}
}

/**
* 檢查輸入的驗證碼是否正確，若用戶輸入的驗證碼與生成的驗證碼相符則返回true，否則返回false。
* @param request 用戶的請求對象
* @return 驗證結果
* */
public boolean check(HttpServletRequest request){
if (((String)request.getParameter(ATTRIBUTE_NAME))
.equalsIgnoreCase((String)request.getSession().getAttribute(ATTRIBUTE_NAME))){
request.getSession().removeAttribute(ATTRIBUTE_NAME);
return true;
}
return false;
}
}<wbr></wbr> <wbr></wbr>
/**此代碼為深秋小雨所編寫，來自J道**/

==================================================================
摘自【http://www.blogjava.net/wuxingjia/archive/2008/04/05/190893.html】

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純JAVA技術驗證碼生成器（服務器端servlet實現）