Applet loads jars from different domains; has problem loading resouces (e.g. images) from jars.

Question

Hi,

I have a sandboxed Applet, but that should not make a difference.

The Applet consists of 2 jars. If I host both files on the same domain1 all is fine. Due to certain special circumstances I need to host jar2 on a different domain2.

In the applet tag I list both files archive="jar1.jar, http://domain2.com/jar2.jar".

First it seems to be working great. Both jars are loaded and everything works fine. Still, as soon as I try to access a resource (like an image) from INSIDE one of the jars there is a problem.

How do I get resources from my jars? I get resources by first getting an URL for the location of the resource:

public static URL getURL(String name) {return ResUtils.class.getResource(path);}

If I host both jars on domain1 this method returns an URL. When hosted on two domains getURL(...) returns NULL which then results in an Exception when fetching the resource:

Caused by: java.lang.IllegalArgumentException: input == null!
    at javax.imageio.ImageIO.read(Unknown Source)

I guess ResUtils.class.getResource uses its Classloader to return an URL. For some reason when hosting both jars on two domains add calls to getRessource() return NULL.

So my question is:

• Why does getURL return NULL?

Is the Java plugin designed to disallow hosting those jars on different domains? How do I have to load my resources when I host the jars on different domains?

I am stuck for a week now - thanks for your help,

Answer 1

Try something like this:

      ClassLoader cl = Thread.currentThread().getContextClassLoader();
      URL url = cl.getResource("imagename.gif");
      System.out.println("URL is: " + url);
      ImageIcon icon = new ImageIcon(url);

Using the context classloader is generally the best way to load images from jar files.

Edit1: BTW, I am surprised that the first suggestion did not work. If memory serves me right, the Thread.currentThread().getContextClassLoader() suggestion should have given you a reference to a classloader that could have located resources from either jar file.

Edit2: none of the above works in this case. Curiosity got the best of me so I had to try it. The plugin classloader just seems too confused to load resources from jar files hosted on another server, even though it has no problems loading classes. What I found is that a manual method does work to load resources like this:

  String manualString = "jar:http://spider.grunt.webhop.net/clubjar.jar!/club.gif";
  URL manualURL = new URL(manualString);
  BufferedImage buf = ImageIO.read(manualURL);
  System.out.println("manual buf: " +buf);

To me, this looks like a bug with the plugin classloader. We should be able to use the classloader getResource() or getResourceAsStream() methods to locate a valid resource. Not so in this case. The ImageIO class can read our manual URL, but the classloader cannot give us one.

Answer 2

Thread.currentThread().getContextClassLoader(); doesn't solve my problem.

All my resources are in the same 'res' folder/package - same as the Class to fetch them: ResUtils (see source below).

If a class in jar1 from domain1 calls ResUtils.getImageIcon("trash.gif"); getURL(...) returns an URL: url -> jar:http://domain1/bun.jar!/res/trash.gif

If a class in jar2 from domain2 calls ResUtils.getImageIcon("trash.gif"); getURL(...) returns an NULL: url -> null

package res;

public class ResUtils {
       private static final String PATH =  "/" + (ResUtils.class.getPackage().getName().replace(".", "/")) + "/";
         public static URL getURL(String name)  {return ResUtils.class.getResource(PATH + name);}
          public static ImageIcon getImageIcon(String name) {
           return new ImageIcon(getBufferedImage(name));
       }

       public static BufferedImage getBufferedImage(String name) {
           try {
               URL url = getURL(name);
               return ImageIO.read(url);
           } catch (Exception e) {
               e.printStackTrace();
               return new BufferedImage(10,10, BufferedImage.TYPE_INT_RGB);
           }
       }
}