如何从 byte[] (Blob) 获取文件类型扩展名

Question

How to get file type extension from byte[] (Blob). I'm reading files from DB to byte[] but i don't know how to automatically detect file extension.

Blob blob = rs.getBlob(1);
byte[] bdata = blob.getBytes(1, (int) blob.length());

Answer 1

You mean you want to get the extension of the file for which the blob store the content? So if the BLOB stores the content of a jpeg-file, you want "jpg" ?

That's generally speaking not possible. You can make a fairly good guess by using some heuristic such as Apache Tikas content detection .

A better solution however, would be to store the mime type (or original file extension) in a separate column, such as a VARCHAR .

Answer 2

它并不完美，但Java Mime Magic 库可能能够推断出文件扩展名：

Magic.getMagicMatch(bdata).getExtension();

Answer 3

if(currentImageType ==null){
                ByteArrayInputStream is = new ByteArrayInputStream(image);
                String mimeType = URLConnection.guessContentTypeFromStream(is);
                if(mimeType == null){
                    AutoDetectParser parser = new AutoDetectParser();
                    Detector detector = parser.getDetector();
                    Metadata md = new Metadata();
                    mimeType = detector.detect(is,md).toString();

                    if (mimeType.contains("pdf")){
                        mimeType ="pdf";
                    }
                    else if(mimeType.contains("tif")||mimeType.contains("tiff")){
                        mimeType = "tif";
                    }
                }
                if(mimeType.contains("png")){
                    mimeType ="png";
                }
                else if( mimeType.contains("jpg")||mimeType.contains("jpeg")){
                    mimeType = "jpg";
                }
                else if (mimeType.contains("pdf")){
                    mimeType ="pdf";
                }
                else if(mimeType.contains("tif")||mimeType.contains("tiff")){
                    mimeType = "tif";
                }

                currentImageType = ImageType.fromValue(mimeType);
            }

Answer 4

尝试使用 ByteArrayDataSource (http://download.oracle.com/javaee/5/api/javax/mail/util/ByteArrayDataSource.html) 你会在那里找到 getContentType() 方法，这应该会有所帮助，但我从未亲自尝试过.

Answer 5

An alternative to using a separate column is using Magic Numbers . Here is some pseudo code:

getFileExtn(BLOB)
{
    PNGMagNum[] = {0x89, 0x50, 0x4E, 0x47}
    if(BLOB[0:3] == PNGMagNum)
        return ".png"
    //More checks...
}

You would have to do this for every file type you support. Some obscure file types you might have to find out yourself via a hex editor (the magic number is always the first few bytes of code). The benefit of using the magic number is you get the actual file type, and not what the user just decided to name it.

Answer 6

JDK的URLConnection类中有不错的方法，请参考以下答案： Getting A File's Mime Type In Java

Answer 7

I know this is a 10 year old post but if stumble here like myself and I, truly speaking there is still no method/property that returns a blob file extension directly. However, with modern javascript you can be able to get blob mime type using type property. Then you can split the mime type and get the last array value like this: (blob.type).split('/')[1] . When we use split on a string, you get an array, blob type will always yield an array of 2, that why we are using [1] to grab the last value which is now becomes our file/image extension. Consider:

// Create a jpeg blob if you dont have one yet
const blob = await new Promise(rs => canvas.toBlob(rs, 'image/jpeg', 1,));
// grab the blob type
const blobType = blob.type
// get the image extension as explained above
const blobExtension = blobType.split('/')[1] // returns a jpg
// set image name
const imageName = `your_image_string_here.${blobExtension}`
// Results >> your_image_string_here.jpg