用Java给您的图片瘦身之Thumbnailator技术（缩略图）

https://blog.csdn.net/yelangkingwuzuhu/article/details/127151913

package com.huaze.form.util;

import lombok.SneakyThrows;
import net.coobird.thumbnailator.Thumbnails;
import org.apache.commons.io.FilenameUtils;
import org.springframework.web.multipart.MultipartFile;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.math.BigDecimal;
import java.util.Base64;

public class Thumbnailator {

    @SneakyThrows
    public static String compression(MultipartFile file) {
        return compression(file.getInputStream(), file.getOriginalFilename());
    }

    @SneakyThrows
    public static String compression(InputStream inputStream, String fileName) {

        // 获取扩展名
        String extName = FilenameUtils.getExtension(fileName);

        if (!FileUtil.isImg(extName)){
            return "";
        }

        long size = inputStream.available();
        // 目标压缩大小 默认60K， base64大概6k
        long target = 60000;

        // 小于目标大小 直接返回base64
        if (size < target) {
            return inputStream2Base64(inputStream);
        }

        // 计算压缩比例
        double scale = new BigDecimal(target).divide(new BigDecimal(size), 2, BigDecimal.ROUND_HALF_UP).doubleValue();
        // 如果比例实在太大，默认给0.01
        if (scale <= 0) {
            scale = 0.01;
        }
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        // 压缩
        Thumbnails.of(inputStream).scale(scale).toOutputStream(baos);
        // 转成base64
        String small = outputStream2Base64(baos);

        return small;
    }


    public static String outputStream2Base64(ByteArrayOutputStream swapStream) throws Exception {
        byte[] data = swapStream.toByteArray();
        return Base64.getEncoder().encodeToString(data);
    }

    public static String inputStream2Base64(InputStream is) throws Exception {
        byte[] data = null;
        try {
            ByteArrayOutputStream swapStream = new ByteArrayOutputStream();
            byte[] buff = new byte[100];
            int rc = 0;
            while ((rc = is.read(buff, 0, 100)) > 0) {
                swapStream.write(buff, 0, rc);
            }
            data = swapStream.toByteArray();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (is != null) {
                try {
                    is.close();
                } catch (IOException e) {
                    throw new Exception("输入流关闭异常");
                }
            }
        }

        return Base64.getEncoder().encodeToString(data);
    }

}

package com.huaze.form.util;

import lombok.extern.slf4j.Slf4j;
import org.springframework.stereotype.Component;
import org.springframework.util.CollectionUtils;

import javax.servlet.http.HttpServletResponse;
import java.io.*;
import java.net.URLEncoder;
import java.nio.charset.StandardCharsets;
import java.util.List;


@Component
@Slf4j
public class FileUtil {

    // 图片 扩展名
    private static final String[] IMG = {"jpg", "jpeg","png", "bmp", "tif", "gif"};

    public static boolean isImg(String extName) {
        List arr = CollectionUtils.arrayToList(IMG);
        // 扩展名统一转小写处理
        extName = extName.toLowerCase();
        return arr.contains(extName);
    }

    /**
     * 下载文件名重新编码
     *
     * @param response 响应对象
     * @param realFileName 真实文件名
     */
    public static void setAttachmentResponseHeader(HttpServletResponse response, String realFileName) throws UnsupportedEncodingException
    {
        String percentEncodedFileName = percentEncode(realFileName);
        StringBuilder contentDispositionValue = new StringBuilder();
        contentDispositionValue.append("attachment; filename=")
                .append(percentEncodedFileName)
                .append(";")
                .append("filename*=")
                .append("utf-8''")
                .append(percentEncodedFileName);

        response.addHeader("Access-Control-Expose-Headers", "Content-Disposition,download-filename");
        response.setHeader("Content-disposition", contentDispositionValue.toString());
        response.setHeader("download-filename", percentEncodedFileName);
    }

    public static void setAttachmentResponseHeaderDecode(HttpServletResponse response, String realFileName)
    {
        StringBuilder contentDispositionValue = new StringBuilder();
        contentDispositionValue.append("attachment; filename=")
                .append(realFileName)
                .append(";")
                .append("filename*=")
                .append("utf-8''")
                .append(realFileName);

        response.addHeader("Access-Control-Expose-Headers", "Content-Disposition,download-filename");
        response.setHeader("Content-disposition", contentDispositionValue.toString());
        response.setHeader("download-filename", realFileName);
    }

    /**
     * 百分号编码工具方法
     *
     * @param s 需要百分号编码的字符串
     * @return 百分号编码后的字符串
     */
    public static String percentEncode(String s) throws UnsupportedEncodingException
    {
        String encode = URLEncoder.encode(s, StandardCharsets.UTF_8.toString());
        return encode.replaceAll("\\+", "%20");
    }


    /**
     * 转输出流
     *
     * @param filePath 文件路径
     * @param os 输出流
     * @return
     */
    public static void writeOut(String filePath, OutputStream os) throws IOException
    {
        File file = new File(filePath);
        if (!file.exists())
        {
            throw new FileNotFoundException(filePath);
        }
        FileInputStream fis = new FileInputStream(file);
        writeOut(fis, os);
    }

    public static void writeOut(InputStream is, OutputStream os) throws IOException
    {
        try
        {
            byte[] b = new byte[1024];
            int length;
            while ((length = is.read(b)) > 0)
            {
                os.write(b, 0, length);
            }
        }
        catch (IOException e)
        {
            throw e;
        }
        finally
        {
            os.close();
            is.close();
        }
    }
}