code: public static ImageWriter getWriter(String formatName) {
  		ImageWriter writer = null;
  		Iterator<ImageWriter> iter = ImageIO.getImageWritersByFormatName(formatName);
  		if (iter.hasNext()) {
  			writer = iter.next();
  		}
  		if (null == writer) {
  			// 尝试扩展名获取
  			iter = ImageIO.getImageWritersBySuffix(formatName);
  			if (iter.hasNext()) {
  				writer = iter.next();
  			}
  		}
  		return writer;
  	}
docs: 根据给定的图片格式或者扩展名获取{@link ImageWriter}，如果未找到合适的Writer，返回null
  
  @param formatName 图片格式或扩展名，例如"jpg"、"png"
  @return {@link ImageWriter}
  @since 4.3.2