code: public static void writeJpeg(RenderedImage im, Object targetJpg, float quality) {
          ImageWriter writer = null;
          try {
              writer = ImageIO.getImageWritersBySuffix("jpg").next();
              ImageWriteParam param = writer.getDefaultWriteParam();
              param.setCompressionMode(ImageWriteParam.MODE_EXPLICIT);
              param.setCompressionQuality(quality);
              ImageOutputStream os = ImageIO.createImageOutputStream(targetJpg);
              writer.setOutput(os);
              writer.write((IIOMetadata) null, new IIOImage(im, null, null), param);
              os.flush();
              os.close();
          }
          catch (IOException e) {
              throw Lang.wrapThrow(e);
          }
          finally {
              if (writer != null) {
                  try {
                      writer.dispose();
                  }
                  catch (Throwable e) {}
              }
          }
      }
docs: 写入一个 JPG 图像
  
  @param im
  图像对象
  @param targetJpg
  目标输出 JPG 图像文件
  @param quality
  质量 0.1f ~ 1.0f