import java.io.File;

import java.io.IOException;

import static java.lang.System.out;

import javax.imageio.ImageIO;

public class JDemo {

    public static void main(String[] args) throws IOException {

        VerificationCode verificationCode = new VerificationCode(7);

        ImageIO.write(verificationCode.getImage(), "png", new File("C:\\Users\\BuYishi\\Desktop\\New folder\\JDemo.png"));

        out.println(verificationCode.getContent());

    }

}

import java.awt.Color;

import java.awt.Font;

import java.awt.FontMetrics;

import java.awt.Graphics;

import java.awt.image.BufferedImage;

import java.util.Random;

public class VerificationCode {

    private StringBuilder verificationCodeContent;

    public VerificationCode(int length) {

        verificationCodeContent = new StringBuilder();

        Random random = new Random();

        for (int i = 0; i < length; ++i) {

            int charType = random.nextInt(3);  //随机的验证码字符类型，0表示数字，1表示小写字母，2表示大写字母

            char c;

            switch (charType) {

                case 0:

                    c = (char) (random.nextInt(10) + '0');

                    break;

                case 1:

                    c = (char) (random.nextInt(26) + 'a');

                    break;

                default:

                    c = (char) (random.nextInt(26) + 'A');

            }

            verificationCodeContent.append(c);

        }

    }

    public String getContent() {

        return verificationCodeContent.toString();

    }

    public BufferedImage getImage() {

        int width = 200, height = 50;

        BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);

        Graphics graphics = image.getGraphics();

        graphics.setColor(Color.red);

        graphics.fillRect(0, 0, width, height);  //绘制验证码图像背景为红色

        Font currentFont = graphics.getFont();

        Font newFont = new Font(currentFont.getFontName(), currentFont.getStyle(), 30);

        FontMetrics fontMetrics = graphics.getFontMetrics(newFont);

        String string = verificationCodeContent.toString();

        graphics.setColor(Color.green);

        graphics.setFont(newFont);

        //绘制绿色的验证码，并使其居中

        graphics.drawString(string, (width - fontMetrics.stringWidth(string)) / 2, (height - fontMetrics.getHeight()) / 2 + fontMetrics.getAscent());

        graphics.setColor(Color.blue);

        Random random = new Random();

        for (int i = 0; i < 20; ++i) {  //绘制20条干扰线，其颜色为蓝

            int x1 = random.nextInt(width), y1 = random.nextInt(height);

            int x2 = random.nextInt(width), y2 = random.nextInt(height);

            graphics.drawLine(x1, y1, x2, y2);

        }

        return image;

    }

}