Detect the file type of the input stream prior to reading the image : ImageIO « 2D Graphics « Java Tutorial

1. Java Tutorial
2. 2D Graphics
3. ImageIO

import java.awt.image.BufferedImage;
import java.net.URL;
import java.util.Iterator;

import javax.imageio.ImageIO;
import javax.imageio.ImageReader;
import javax.imageio.stream.ImageInputStream;

public class Main {
  public static void main(String[] argv) throws Exception {
    ImageInputStream imageStream = ImageIO.createImageInputStream(new URL("").openStream());
    Iterator<ImageReader> readers = ImageIO.getImageReaders(imageStream);
    ImageReader reader = null;
    if (!readers.hasNext()) {
      imageStream.close();
      return;
    } else {
      reader = readers.next();
    }
    String formatName = reader.getFormatName();
    if (!formatName.equalsIgnoreCase("jpeg") && !formatName.equalsIgnoreCase("png")
        && !formatName.equalsIgnoreCase("gif")) {
      imageStream.close();
      return;
    }
    reader.setInput(imageStream, true, true);
    BufferedImage theImage = reader.read(0);
    reader.dispose();
    imageStream.close();
  }
}