java中对图片文件的获取

系统中某些图片可能已经被修改了后缀名，所以不能只是对文件名进行判断。下面的示例代码就是如何判断一个图片的具体类型的。
  
   public static void main(String[] args) {
        File f = new File("d://test.jpg");
        if (f.exists()) {
            System.out.println(getFormatInFile(f));
        }
    }

    // Returns the format of the image in the file 'f'.
    // Returns null if the format is not known.
    public static String getFormatInFile(File f) {
        return getFormatName(f);
    }
   
    // Returns the format name of the image in the object 'o'.
    // Returns null if the format is not known.
    private static String getFormatName(Object o) {
        try {
            // Create an image input stream on the image
            ImageInputStream iis = ImageIO.createImageInputStream(o);
   
            // Find all image readers that recognize the image format
            Iterator<ImageReader> iter = ImageIO.getImageReaders(iis);
            if (!iter.hasNext()) {
                // No readers found
                return null;
            }
   
            // Use the first reader
            ImageReader reader = iter.next();
   
            // Close stream
            iis.close();
   
            // Return the format name
            return reader.getFormatName();
        } catch (IOException e) {
            //
        }
       
        // The image could not be read
        return null;
    }