Java SE (Java Platform, Standard Edition)

I've a server running Ubuntu server edition. Therefore it is not running any X11 server. My webapp performs some image rescaling of images that a user uploads. For testing purpose i have a test that automatically checks if the images are transformed correctly. But i get the following error message:

java.awt.HeadlessException: 
No X11 DISPLAY variable was set, but this program performed an operation which requires it.
	at sun.java2d.HeadlessGraphicsEnvironment.getDefaultScreenDevice(HeadlessGraphicsEnvironment.java:82)
	at parken.server.util.ImageUtil.getDefaultConfiguration(ImageUtil.java:11)

After some googling i added the following to my .bashrc

DISPLAY=:0.0
export DISPLAY

Now i get the following exception:

java.lang.InternalError: Can't connect to X11 window server using ':0.0' as the value of the DISPLAY variable.
	sun.awt.X11GraphicsEnvironment.initDisplay(Native Method)
	sun.awt.X11GraphicsEnvironment.access$100(X11GraphicsEnvironment.java:62)
	sun.awt.X11GraphicsEnvironment$1.run(X11GraphicsEnvironment.java:166)
	java.security.AccessController.doPrivileged(Native Method)
	sun.awt.X11GraphicsEnvironment.<clinit>(X11GraphicsEnvironment.java:142)
	java.lang.Class.forName0(Native Method)
	java.lang.Class.forName(Class.java:186)
	java.awt.GraphicsEnvironment.getLocalGraphicsEnvironment(GraphicsEnvironment.java:82)
	parken.server.util.ImageUtil.getDefaultConfiguration(ImageUtil.java:10)

The class in wich the error occurs looks like this:

package parken.server.util;

import java.awt.*;
import java.awt.image.*;
import java.awt.geom.*;

public class ImageUtil {

	private static GraphicsConfiguration getDefaultConfiguration() {
	    GraphicsEnvironment ge = GraphicsEnvironment.getLocalGraphicsEnvironment(); //<-this ist line 10
	    GraphicsDevice gd = ge.getDefaultScreenDevice(); //<-this ist line 11
	    return gd.getDefaultConfiguration();
	}

	private static BufferedImage copy(BufferedImage source, BufferedImage target) {
	    Graphics2D g2 = target.createGraphics();
	    g2.setRenderingHint(RenderingHints.KEY_INTERPOLATION, RenderingHints.VALUE_INTERPOLATION_BILINEAR );
	    double scalex = (double) target.getWidth()/ source.getWidth();
	    double scaley = (double) target.getHeight()/ source.getHeight();
	    AffineTransform at = AffineTransform.getScaleInstance(scalex, scaley);
	    g2.drawRenderedImage(source, at);
	    g2.dispose();
	    return target;
	}
	 
	private static BufferedImage getScaledInstance(BufferedImage image, int width, int height, GraphicsConfiguration gc) {
	    if (gc == null)
	        gc = getDefaultConfiguration();
	    int transparency = image.getColorModel().getTransparency();
	    return copy(image, gc.createCompatibleImage(width, height, transparency));
	}
	
	public static BufferedImage rescaleImage( BufferedImage img, int width , int height ) {
		return getScaledInstance( img , width, height, ImageUtil.getDefaultConfiguration() );
	}
}

Acutally i don't have any idea how to repair this.