How to find format of an image when its byte[] content alone is available?

Question

Hi,
I'm working on code that gets an image content as input in byte[] format. With no other further details provided, how to identify the format of the image contained in the byte array? Specifically, I would like to know if the image contained in it is a PNG, JPG, BMP, etc.
Image or ImageIO API does not seem to provide a means to do this. Is there a way how this can be achieved?
Thanks in advance.

Answer 1

The ImageIO class does provide a way to do what you want... it just takes a couple of steps. Take a look at the determineInputFormat() method below:
import java.io.ByteArrayInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.util.Iterator;
import javax.imageio.ImageIO;
import javax.imageio.ImageReader;
import javax.imageio.stream.ImageInputStream;
public class ImageFormatTest {
   private static final String IMAGE_PATH = "... PATH TO IMAGE HERE ...";
   public static void main( String[] args ) {
      try {
         final byte[] imageBytes = readImageBytes( IMAGE_PATH );
         final String imageFormat = determineImageFormat( imageBytes );
         System.out.println( "Image format is " + imageFormat );
      } catch (Throwable ex) {
         ex.printStackTrace();
   private static byte[] readImageBytes( String imagePath ) throws IOException {
      final File imageFile = new File( imagePath );
      final byte[] buf = new byte[ (int) imageFile.length() ];
      final FileInputStream fStream = new FileInputStream( imageFile );
      fStream.read( buf );
      fStream.close();
      return buf;
   private static String determineImageFormat( byte[] imageBytes ) throws IOException {
      final ByteArrayInputStream bStream = new ByteArrayInputStream( imageBytes );
      final ImageInputStream imgStream = ImageIO.createImageInputStream( bStream );
      final Iterator<ImageReader> iter = ImageIO.getImageReaders( imgStream );
      final ImageReader imgReader = iter.next();
      return imgReader.getFormatName();
}