Loading all files in a directory in a Java applet

Question

How would one go about programatically loading all the resource files in a given directory in a JAR file for an applet? The resources will probably change several times over the lifetime of the program so I don't want to hardcode names in.

Normally I would just traverse the directory structure using File.list(), but I get permission issues when trying to do that within an applet. I also looked at using an enumeration with something line ClassLoader.getResources() but it only finds files of the same name within the JAR file.

Essentially what I want to do is (something like) this:

ClassLoader imagesURL = this.getClass().getClassLoader();
MediaTracker tracker = new MediaTracker(this);
Enumeration<URL> images = imagesURL.getResources("resources/images/image*.gif");
while (images.hasMoreElements()){
    tracker.add(getImage(images.nextElement(), i);
    i++;
}

I know I'm probably missing some obvious function, but I've spent hours searching through tutorials and documentation for a simple way to do this within an unsigned applet.

Answer 1

You can do it in two ways:

1. Rename your images, so you can enumerate them via some algorithm (e.g. image_1, image_2, image_3), then you can collect all resources you need in one go.

2. Otherwise you need to code a lot. The idea is that you have to:

   • determine the path to your JAR file:

     private static final String ANCHOR_NAME = "some resource you know";
     
     
     URL location = getClass().getClassLoader().getResource(ANCHOR_NAME);
     URL jarLocation;
     String protocol = location.getProtocol();
     if (protocol.equalsIgnoreCase("jar"))
     {
         String path = location.getPath();
         int index = path.lastIndexOf("!/" + ANCHOR_NAME);
         if(index != -1)
            jarLocation = new URL(path.substring(0, index));
     }
     if (protocol.equalsIgnoreCase("file"))
     {
         String string = location.toString();
         int index = string.lastIndexOf(ANCHOR_NAME);
         if(index != -1)
             jarLocation = new URL(string.substring(0, index));
     }
     

   • open it as java.util.jar.JarFile

     JarFile jarFile = new JarFile(jarLocation);
     

   • iterate through all entries and match their name with a given mask

     for (Enumeration entries = jarFile.entries(); entries.hasMoreElements();)
     {
          JarEntry entry = (JarEntry) entries.nextElement();
          String entryPath = entry.getName();
          if (entryPath.endsWith(".jpg"))
          {
             // do something with it
          }
     }
     

For additional code support, I will refer you to Spring PathMatchingResourcePatternResolver#doFindPathMatchingJarResources().