为什么读取bmp的字节数组并且丢失了宽度和高度

Question

我的服务器通过上传接收到 bmp 的字节数组，我读取了它的宽度和高度，但得到了-1 。 我尝试用下面的代码模拟这个上传案例：

public class ImageReader {

    private static URL imgUrl;

    public static void main(String[] args) throws IOException {
        imgUrl = ImageReader.class.getResource("myimage.jpg");
        printWidthHeight(imgUrl, "jpg");
        imgUrl = ImageReader.class.getResource("flag.bmp");
        printWidthHeight(imgUrl, "bmp");
    }

    private static void printWidthHeight(URL imgUrl, String format) throws IOException {
        BufferedImage image = ImageIO.read(imgUrl);
        System.out.println(image.getType());
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        boolean isAppropriate = ImageIO.write(image, format, baos);
        System.out.println(isAppropriate);
        byte[] bytes = baos.toByteArray();

        ImageIcon imageIcon = new ImageIcon(bytes);
        System.out.format( "%s, %s \n", imageIcon.getIconWidth(), imageIcon.getIconHeight());
    }
}

它打印：

5
true
171, 125 
5
true
-1, -1 

我发现

• jpg 仍然有宽度和高度，但 bmp 没有

有谁知道原因？

Answer 1

来自 Java 文档，ImageIcon 不支持 BMP 格式：

你可以使用这个： Get Image Dimension and Image Size from Binary

Answer 2

感谢您的提示。 我发现如果我用ImageIO读取这些字节，我可以成功获得图像的宽度和高度。

public class ImageReader {

    private static URL imgUrl;
    private static byte[] bytes;

    public static void main(String[] args) throws IOException {
        imgUrl = ImageReader.class.getResource("myimage.jpg");
        bytes = getBytes(imgUrl, "jpg");
        printWidthHeight(bytes);
        imgUrl = ImageReader.class.getResource("flag.bmp");
        bytes = getBytes(imgUrl, "bmp");
        printWidthHeight(bytes);
    }

    private static void printWidthHeight(byte[] bytes) throws IOException {
        ImageIcon imageIcon = new ImageIcon(bytes);
        System.out.format( "%s, %s \n", imageIcon.getIconWidth(), imageIcon.getIconHeight());


        BufferedImage image = ImageIO.read(new ByteArrayInputStream(bytes));
        ImageIcon imageIcon2 = new ImageIcon(image);
        System.out.format( "%s, %s \n", imageIcon2.getIconWidth(), imageIcon2.getIconHeight());
    }

    private static byte[] getBytes(URL imgUrl, String format) throws IOException {
        BufferedImage image = ImageIO.read(imgUrl);
        System.out.println(image.getType());
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        boolean isAppropriate = ImageIO.write(image, format, baos);
        System.out.println(isAppropriate);
        byte[] bytes = baos.toByteArray();
        return bytes;
    }
}

output

5
true
171, 125 
171, 125 
5
true
-1, -1 
124, 124