Let’s take a look at the effect first:

(Learning video recommendation: java course)

1. Applicable requirements

Generate verification code in the background for login verification.

2. Implementation process

1. View layer idea

(1) input is used to enter the verification code, and an img is used to display the verification code

(2) Verify whether the entered verification code is qualified, double-click the img to refresh the verification code, and bind the onblue lost focus event (an event triggered when the mouse loses focus)

(3) Verify in the onblue event,

(4) The src attribute value in img is the method request path for generating verification code in the background (that is, the path of requestMapping). When you click the verification code again, you can dynamically set the src attribute (the original access address is random Timestamp to prevent the browser from not accessing the same path)

Note: The background directly returns the picture, not the characters of the verification code! If characters are returned, the verification code will lose its meaning (the front desk can easily You can obtain the verification code characters and perform multiple malicious visits) (This takes system security into consideration)

2. Back-end ideas
Use the BufferedImage class to create a picture, and then use Graphics2D to process the picture. Just draw (generate random characters and add interference lines). Note: The generated verification code string must be placed in the session for verification of the subsequent login verification code (of course also in the background).

The front-end code is as follows:

            <td class="tds">驗證碼：</td>
            <td>
                <input type="text" name="valistr" onblur="checkNull(&#39;valistr&#39;,&#39;驗證碼不能為空！&#39;)">
                <img  src="/static/imghw/default1.png"  data-src="${pageContext.request.contextPath}/servlet/ValiImgServlet"  class="lazy"  id="yzm_img"    style="max-width:90%" onclick="changeYZM(this)"/ alt="Use java to generate background verification code" >
                <span id="valistr_msg"></span>
            </td>
               /**
         * 校驗字段是否為空
         */
        function checkNull(name,msg){
            setMsg(name,"")
            var v = document.getElementsByName(name)[0].value;
            if(v == ""){
                setMsg(name,msg)
                return false;
            }
            return true;
        }     
     /**
         * 為輸入項設置提示消息
         */
        function setMsg(name,msg){
            var span = document.getElementById(name+"_msg");
            span.innerHTML="<font color=&#39;red&#39;>"+msg+"</font>";
        }
 /**
         * 點擊更換驗證碼
         */
        function changeYZM(imgObj){
            imgObj.src = "${pageContext.request.contextPath}/servlet/ValiImgServlet?time="+new Date().getTime();
        }

The back-end code is as follows:

package cn.tedu.web;

import cn.tedu.util.VerifyCode;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.io.IOException;

/**
 * 獲取驗證碼
 */
public class ValiImgServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //禁止瀏覽器緩存驗證碼
        response.setDateHeader("Expires",-1);
        response.setHeader("Cache-Control","no-cache");
        response.setHeader("Pragma","no-cache");
        //生成驗證碼
        VerifyCode vc = new VerifyCode();
        //輸出驗證碼
        vc.drawImage(response.getOutputStream());
        //獲取驗證碼的值，存儲到session中
        String valistr = vc.getCode();
        HttpSession session = request.getSession();
        session.setAttribute("valistr",valistr);
        //打印到控制臺
        System.out.println(valistr);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request, response);
    }
}

package cn.tedu.util;

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics2D;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.OutputStream;
import java.util.Random;
import javax.imageio.ImageIO;
/**
 * 動態(tài)生成圖片
 */
public class VerifyCode {
	// {"宋體", "華文楷體", "黑體", "華文新魏", "華文隸書", "微軟雅黑", "楷體_GB2312"}
	private static String[] fontNames = { "宋體", "華文楷體", "黑體", "微軟雅黑",  "楷體_GB2312" };
	// 可選字符
	//"23456789abcdefghjkmnopqrstuvwxyzABCDEFGHJKMNPQRSTUVWXYZ";
	private static String codes = "23456789abcdefghjkmnopqrstuvwxyzABCDEFGHJKMNPQRSTUVWXYZ";
	// 背景色
	private Color bgColor = new Color(255, 255, 255);
	// 基數(一個文字所占的空間大小)
	private int base = 30;
	// 圖像寬度
	private int width = base * 4;
	// 圖像高度
	private int height = base;
	// 文字個數
	private int len = 4;
	// 設置字體大小
	private int fontSize = 22;
	// 驗證碼上的文本
	private String text;

	private BufferedImage img = null;
	private Graphics2D g2 = null;

	/**
	 * 生成驗證碼圖片
	 */
	public void drawImage(OutputStream outputStream) {
		// 1.創(chuàng)建圖片緩沖區(qū)對象, 并設置寬高和圖像類型
		img = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
		// 2.得到繪制環(huán)境
		g2 = (Graphics2D) img.getGraphics();
		// 3.開始畫圖
		// 設置背景色
		g2.setColor(bgColor);
		g2.fillRect(0, 0, width, height);

		StringBuffer sb = new StringBuffer();// 用來裝載驗證碼上的文本

		for (int i = 0; i < len; i++) {
			// 設置畫筆顏色 -- 隨機
			// g2.setColor(new Color(255, 0, 0));
			g2.setColor(new Color(getRandom(0, 150), getRandom(0, 150),getRandom(0, 150)));

			// 設置字體
			g2.setFont(new Font(fontNames[getRandom(0, fontNames.length)], Font.BOLD, fontSize));

			// 旋轉文字(-45~+45)
			int theta = getRandom(-45, 45);
			g2.rotate(theta * Math.PI / 180, 7 + i * base, height - 8);

			// 寫字
			String code = codes.charAt(getRandom(0, codes.length())) + "";
			g2.drawString(code, 7 + i * base, height - 8);
			sb.append(code);
			g2.rotate(-theta * Math.PI / 180, 7 + i * base, height - 8);
		}

		this.text = sb.toString();

		// 畫干擾線
		for (int i = 0; i < len + 2; i++) {
			// 設置畫筆顏色 -- 隨機
			// g2.setColor(new Color(255, 0, 0));
			g2.setColor(new Color(getRandom(0, 150), getRandom(0, 150),
					getRandom(0, 150)));
			g2.drawLine(getRandom(0, 120), getRandom(0, 30), getRandom(0, 120),
					getRandom(0, 30));
		}
		//TODO:
		g2.setColor(Color.GRAY);
		g2.drawRect(0, 0, this.width-1, this.height-1);
		// 4.保存圖片到指定的輸出流
		try {
			ImageIO.write(this.img, "JPEG", outputStream);
		} catch (Exception e) {
			e.printStackTrace();
			throw new RuntimeException(e);
		}finally{
			// 5.釋放資源
			g2.dispose();
		}
	}

	/**
	 * 獲取驗證碼字符串
	 * @return
	 */
	public String getCode() {
		return this.text;
	}

	/*
	 * 生成隨機數的方法
	 */
	private static int getRandom(int start, int end) {
		Random random = new Random();
		return random.nextInt(end - start) + start;
	}

	/*public static void main(String[] args) throws Exception {
		VerifyCode vc = new VerifyCode();
		vc.drawImage(new FileOutputStream("f:/vc.jpg"));
		System.out.println("執(zhí)行成功~!");
	}*/
}

Summary:

Introduction: It is "Completely Automated Public Turing test to tell Computers and "Humans Apart" (the abbreviation of "Fully Automated Turing Test to Distinguish Computers and Humans") is a public, fully automated program that distinguishes whether a user is a computer or a human.

History: It is the abbreviation of "Completely Automated Public Turing test to tell Computers and Humans Apart". It is a public fully automated test to distinguish whether the user is a computer or a human. program.

Function: Prevent malicious cracking of passwords, ticket brushing, forum flooding, and page brushing.

Category: Gif animation verification code, mobile phone SMS verification code, mobile phone voice verification code, video verification code

Common verification codes:

(1) four-digit sum Letters may be letters or numbers, a random 4-digit string, the most primitive verification code, and the verification effect is almost zero. The CSDN website user login uses GIF format, a commonly used random digital picture verification code. The characters on the picture are quite satisfactory, and the verification effect is better than the previous one.

(2) Chinese characters are the latest verification code currently registered, which is randomly generated and difficult to type, such as the QQ complaint page.

(3) MS hotmail application is in BMP format, with random numbers, random uppercase English letters, random interference pixels, and random positions.

(4) Korean or Japanese, now the MS registration on Paopao HF requires Korean, which increases the difficulty.

(5) Google's Gmail registration is in JPG format, with random English letters, random colors, random positions, and random lengths.

(6) Other major forums are in XBM format, and the content is random

(7) Advertising verification code: Just enter part of the content in the advertisement, which is characterized by bringing additional content to the website Income can also refresh users. Advertising verification code

(8) Question verification code: The question verification code is mainly filled in in the form of question and answer. It is easier to identify and enter than the modular verification code. The system can generate questions such as "1 2=?" for users to answer. Of course, such questions are randomly generated. Another type of question verification code is a text-based question verification code, such as generating the question "What is the full name of China?". Of course, some websites also provide prompt answers or direct answers after the question.

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