背景傳回base64編碼圖檔，解決前端 img 标簽 302報錯 問題

前端有時候因為代理重定向等問題img标簽會報302錯誤，常見的是使用驗證碼的時候，此時後端直接傳回base64編碼可以輕松解決此問題。

依賴：

<!--驗證碼 -->
            <dependency>
                <groupId>com.github.penggle</groupId>
                <artifactId>kaptcha</artifactId>
                <version>2.3.2</version>
            </dependency>
           

生成驗證碼的接口，一個直接輸出流，一個輸出base64：

import com.google.code.kaptcha.Constants;
import com.google.code.kaptcha.Producer;
import com.ruoyi.common.core.controller.BaseController;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.ResponseBody;
import org.springframework.web.servlet.ModelAndView;
import sun.misc.BASE64Encoder;

import javax.annotation.Resource;
import javax.imageio.ImageIO;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.awt.image.BufferedImage;
import java.io.ByteArrayOutputStream;
import java.io.IOException;

/**
 * 圖檔驗證碼（支援算術形式）
 *
 * @author ruoyi
 */
@Controller
@RequestMapping("/captcha")
public class SysCaptchaController extends BaseController {
    @Resource(name = "captchaProducer")
    private Producer captchaProducer;

    @Resource(name = "captchaProducerMath")
    private Producer captchaProducerMath;

    /**
     * 驗證碼生成
     */
    @GetMapping(value = "/captchaImage")
    public ModelAndView getKaptchaImage(HttpServletRequest request, HttpServletResponse response) {
        ServletOutputStream out = null;
        try {
            HttpSession session = request.getSession();
            response.setDateHeader("Expires", 0);
            response.setHeader("Cache-Control", "no-store, no-cache, must-revalidate");
            response.addHeader("Cache-Control", "post-check=0, pre-check=0");
            response.setHeader("Pragma", "no-cache");
            response.setContentType("image/jpeg");

            String type = request.getParameter("type");
            String capStr = null;
            String code = null;
            BufferedImage bi = null;
            if ("math".equals(type)) {
                String capText = captchaProducerMath.createText();
                capStr = capText.substring(0, capText.lastIndexOf("@"));
                code = capText.substring(capText.lastIndexOf("@") + 1);
                bi = captchaProducerMath.createImage(capStr);
            } else if ("char".equals(type)) {
                capStr = code = captchaProducer.createText();
                bi = captchaProducer.createImage(capStr);
            }
            session.setAttribute(Constants.KAPTCHA_SESSION_KEY, code);
            out = response.getOutputStream();
            ImageIO.write(bi, "jpg", out);
            out.flush();

        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                if (out != null) {
                    out.close();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return null;
    }

    /**
     * 驗證碼生成
     */
    @GetMapping(value = "/captchaImage_base64")
    @ResponseBody
    public String getKaptchaImageByBase64(HttpServletRequest request, HttpServletResponse response) {
        ServletOutputStream out = null;
        try {
            HttpSession session = request.getSession();
            response.setDateHeader("Expires", 0);
            response.setHeader("Cache-Control", "no-store, no-cache, must-revalidate");
            response.addHeader("Cache-Control", "post-check=0, pre-check=0");
            response.setHeader("Pragma", "no-cache");
            response.setContentType("image/jpeg");

            String type = request.getParameter("type");
            String capStr = null;
            String code = null;
            BufferedImage bi = null;
            if ("math".equals(type)) {
                String capText = captchaProducerMath.createText();
                capStr = capText.substring(0, capText.lastIndexOf("@"));
                code = capText.substring(capText.lastIndexOf("@") + 1);
                bi = captchaProducerMath.createImage(capStr);
            } else if ("char".equals(type)) {
                capStr = code = captchaProducer.createText();
                bi = captchaProducer.createImage(capStr);
            }
            session.setAttribute(Constants.KAPTCHA_SESSION_KEY, code);

            ByteArrayOutputStream baos = new ByteArrayOutputStream();//io流
            ImageIO.write(bi, "png", baos);//寫入流中
            byte[] bytes = baos.toByteArray();//轉換成位元組
            BASE64Encoder encoder = new BASE64Encoder();
            String png_base64 = encoder.encodeBuffer(bytes).trim();//轉換成base64串
            png_base64 = png_base64.replaceAll("\n", "").replaceAll("\r", "");//删除 \r\n
            String imgPrefix = "data:image/jpg;base64,";
            String imgBase64 = imgPrefix + png_base64;
            System.out.println("值為：" + imgBase64);
            return imgBase64;
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                if (out != null) {
                    out.close();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return null;
    }
}