修改上面方法，改为可以处理所有格式的图片

要处理所有格式的图片，你可以使用 `javax.imageio.ImageIO` 的 `write` 方法，并将输出流转换为字节数组。下面是修改后的代码：

import java.awt.Image;
import java.awt.image.BufferedImage;
import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.IOException;
import javax.imageio.ImageIO;
import java.util.Base64;

public class ThumbnailConverter {
    public static String convertToThumbnail(String imagePath) {
        try {
            // 读取原始图片
            File imageFile = new File(imagePath);
            BufferedImage originalImage = ImageIO.read(imageFile);

            // 计算缩略图的尺寸
            int targetWidth = 0;
            int targetHeight = 0;
            double aspectRatio = (double) originalImage.getWidth() / originalImage.getHeight();
            if (aspectRatio > 1) {
                targetWidth = 80;
                targetHeight = (int) (targetWidth / aspectRatio);
            } else {
                targetHeight = 80;
                targetWidth = (int) (targetHeight * aspectRatio);
            }

            // 创建缩略图
            Image thumbnailImage = originalImage.getScaledInstance(targetWidth, targetHeight, Image.SCALE_SMOOTH);
            BufferedImage thumbnailBufferedImage = new BufferedImage(targetWidth, targetHeight, BufferedImage.TYPE_INT_RGB);
            thumbnailBufferedImage.getGraphics().drawImage(thumbnailImage, 0, 0, null);

            // 将缩略图转为base64字符串
            ByteArrayOutputStream baos = new ByteArrayOutputStream();
            ImageIO.write(thumbnailBufferedImage, getImageFormat(imagePath), baos);
            baos.flush();
            byte[] thumbnailBytes = baos.toByteArray();
            baos.close();
            String base64Thumbnail = Base64.getEncoder().encodeToString(thumbnailBytes);

            return base64Thumbnail;
        } catch (IOException e) {
            e.printStackTrace();
        }

        return null;
    }

    private static String getImageFormat(String imagePath) {
        int dotIndex = imagePath.lastIndexOf(".");
        if (dotIndex != -1 && dotIndex < imagePath.length() - 1) {
            return imagePath.substring(dotIndex + 1).toLowerCase();
        }
        return "jpg"; // 默认为JPEG格式
    }

    public static void main(String[] args) {
        String imagePath = "path/to/image.jpg";
        String thumbnailBase64 = convertToThumbnail(imagePath);
        System.out.println(thumbnailBase64);
    }
}

这段代码中，我添加了 `getImageFormat` 方法来获取图片的格式。它通过从文件路径中提取最后一个点号之后的字符串来获取格式。如果无法提取格式，则默认为JPEG格式。这样，无论图片是什么格式，都可以正确处理并转换为缩略图的base64字符串。记得将 `"path/to/image.jpg"` 替换为你的实际图片路径。