Base64是一种将二进制数据编码为ASCII字符串的方法。它通过将3个字节的二进制数据转换为4个可打印字符的ASCII字符，从而将二进制数据转换为可传输的文本格式。Base64编码常用于传输图片或音频文件。Base64编码可以保证数据在传输过程中不丢失，同时可以避免某些系统不支持二进制数据的问题。

        但是Base64转换成图片之前，如何压缩目标图片的大小呢？本文提供两种方式。

方法一：按尺寸压缩（不保证图片质量）

/*** 去掉Base64图片数据的前缀** @param base64Str 含前缀的Base64字符串* @return 不含前缀的Base64字符串*/private static String removeBase64Prefix(String base64Str) {// 使用正则表达式去掉"data:image/\w+;base64,"前缀Pattern pattern = Pattern.compile("^data:image/\\w+;base64,");Matcher matcher = pattern.matcher(base64Str);return matcher.replaceFirst("");}/*** 添加Base64图片数据的前缀** @param base64Str 不含或含其他前缀的Base64字符串* @param prefix 想要添加的Base64前缀，默认为"data:image/png;base64,"* @return 含指定前缀的Base64字符串*/public static String addBase64Prefix(String base64Str, String prefix) {// 检查是否已存在指定的前缀，如果有，则直接返回if (base64Str.startsWith(prefix)) {return base64Str;}return prefix + base64Str;}/*** 压缩base64编码至200K以内** @param base64Img* @return*/public static String resizeImageTo200K(String base64Img) {try {byte[] bytes1 =  Base64.getDecoder().decode(removeBase64Prefix(base64Img));System.out.println( bytes1.length);InputStream stream = new ByteArrayInputStream(bytes1);BufferedImage src = ImageIO.read(stream);// 压缩的尺寸BufferedImage output = output=Thumbnails.of(src).size(640, 480).asBufferedImage();String base64 = imageToBase64(output);double minScalingFactor = 0.7; // 设置最小缩放因子if (base64.length() - base64.length() / 8 * 2 > 200000) {double scalingFactor = Math.max(minScalingFactor, 1 - (base64.length() / 200000));output = Thumbnails.of(output).scale(scalingFactor).asBufferedImage();base64 = imageToBase64(output);}return addBase64Prefix(base64, "data:image/png;base64,");} catch (Exception e) {return addBase64Prefix(base64Img, "data:image/png;base64,");}}// BufferedImage转换成base64，在这里需要设置图片格式，如下是png格式图片：public static String imageToBase64(BufferedImage bufferedImage) {ByteArrayOutputStream baos = new ByteArrayOutputStream();try {ImageIO.write(bufferedImage, "png", baos);} catch (IOException e) {}String str = new String(Base64.getEncoder().encode(baos.toByteArray()));return str;}}

方法二：等质量压缩

 /*** 压缩base64编码图片至目标大小附近，尽量保持图片质量** @param base64Img base64编码的图片字符串* @param targetSize 目标大小（例如：200KB）* @return 调整大小后的base64编码图片字符串*/public static String resizeImageToTargetSize(String base64Img, int targetSize) {try {String s = removeBase64Prefix(base64Img);byte[] imageBytes = Base64.getDecoder().decode(s);ByteArrayInputStream inputStream = new ByteArrayInputStream(imageBytes);BufferedImage src = ImageIO.read(inputStream);// 初始化压缩质量为最高，根据需要逐步降低float quality = 1.0f;float step = 0.1f;boolean compressMore = true;byte[] compressedBytes = null;while (compressMore) {ByteArrayOutputStream outputStream = new ByteArrayOutputStream();Iterator<ImageWriter> writers = ImageIO.getImageWritersByFormatName("jpeg");if (!writers.hasNext()) throw new IllegalStateException("No writers found");ImageWriter writer = writers.next();ImageWriteParam param = writer.getDefaultWriteParam();// 设置压缩质量param.setCompressionMode(ImageWriteParam.MODE_EXPLICIT);param.setCompressionQuality(quality);MemoryCacheImageOutputStream memStream = new MemoryCacheImageOutputStream(outputStream);writer.setOutput(memStream);IIOImage outputImage = new IIOImage(src, null, null);writer.write(null, outputImage, param);writer.dispose();compressedBytes = outputStream.toByteArray();int currentSize = compressedBytes.length;System.out.println("尝试质量: " + quality + ", 大小: " + currentSize);// 如果压缩后大小仍超过目标大小，降低质量继续尝试；否则停止循环if (currentSize > targetSize) {quality -= step;if (quality < 0.1f) { // 防止质量降得过低quality = 0.1f;}} else {compressMore = false;}}String base64Encoded = Base64.getEncoder().encodeToString(compressedBytes);return addBase64Prefix(base64Encoded, "data:image/png;base64,");} catch (Exception e) {return base64Img; // 如果压缩失败，返回原图}}