Servlet - 4位驗證碼圖片：CheckCodeServlet

package com.itheima.web.servlet;

import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.util.Random;

/**
 * 驗證碼
 */
@WebServlet("/checkCode")
public class CheckCodeServlet extends HttpServlet {
	public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {
		
		//伺服器通知瀏覽器不要快取
		response.setHeader("pragma","no-cache");
		response.setHeader("cache-control","no-cache");
		response.setHeader("expires","0");
		
		//在記憶體中建立一個長80，寬30的圖片，預設黑色背景
		//引數一：長
		//引數二：寬
		//引數三：顏色
		int width = 80;
		int height = 30;
		BufferedImage image = new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB);
		
		//獲取畫筆
		Graphics g = image.getGraphics();
		//設定畫筆顏色為灰色
		g.setColor(Color.GRAY);
		//填充圖片
		g.fillRect(0,0, width,height);
		
		//產生4個隨機驗證碼，12Ey
		String checkCode = getCheckCode();
		//將驗證碼放入HttpSession中
		request.getSession().setAttribute("CHECKCODE_SERVER",checkCode);
		
		//設定畫筆顏色為黃色
		g.setColor(Color.YELLOW);
		//設定字型的小大
		g.setFont(new Font("黑體",Font.BOLD,24));
		//向圖片上寫入驗證碼
		g.drawString(checkCode,15,25);
		
		//將記憶體中的圖片輸出到瀏覽器
		//引數一：圖片物件
		//引數二：圖片的格式，如PNG,JPG,GIF
		//引數三：圖片輸出到哪裡去
		ImageIO.write(image,"PNG",response.getOutputStream());
	}
	/**
	 * 產生4位隨機字串 
	 */
	private String getCheckCode() { 
		String base = "0123456789ABCDEFGabcdefg";
		int size = base.length();
		Random r = new Random();
		StringBuffer sb = new StringBuffer();
		for(int i=1;i<=4;i++){
			//產生0到size-1的隨機值
			int index = r.nextInt(size);
			//在base字串中獲取下標為index的字元
			char c = base.charAt(index);
			//將c放入到StringBuffer中去
			sb.append(c);
		}
		return sb.toString();
	}
	public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		this.doGet(request,response);
	}
}