how to extract image from PDF in java

how to extract image from PDF in java:

	public static void readImageFromPDF(String filePath, String imagePath) {
		try {
			// 加载PDF文件
			PDDocument document = PDDocument.load(filePath);
			// 将PDF根据每页读入到list中
			List<?> pages = document.getDocumentCatalog().getAllPages();
			Iterator<?> iter = pages.iterator();
			while (iter.hasNext()) {
				PDPage page = (PDPage) iter.next();
				PDResources resources = page.getResources();
				// 取出图片
				Map<?, ?> images = resources.getImages();
				if (images != null) {
					Iterator<?> imageIter = images.keySet().iterator();
					while (imageIter.hasNext()) {
						String key = (String) imageIter.next();
						PDXObjectImage image = (PDXObjectImage) images.get(key);
						File file = getUniqueFileName(key, image.getSuffix(), imagePath);
						System.out.println("file name: " + file.getName());
						// 将图片写入到已经定义好的文件中
						image.write2file(file);
					}
				}
			}
		} catch (Exception e) {
			e.printStackTrace();
		}
	}

	/**
	 * 生成唯一的图片名称
	 * @param prefix 图片的前缀
	 * @param suffix 图片的后缀，也即图片的格式
	 * @param imagePath 图片的存储路径
	 * @return 返回结果为创建好的文件
	 */
	private static File getUniqueFileName(String prefix, String suffix, String imagePath) {
		String uniqueName = null;
		File f = null;
		while (f == null || f.exists()) {
			uniqueName = prefix + "-" + imageCounter;
			f = new File(imagePath + uniqueName + "." + suffix);
			imageCounter++;
		}
		return f;
	}