Can't readObject() from an external file when running my JAR

I have my program in a JAR file, and some files in a folder in the same directory. After hours of searching I found some code that allowed me to list the files inside that folder (since File.listFiles() didn't work), but now readObject() gives me IOException. It works fine when I run it from the IDE or with cmd. It only throws the exception when running the JAR via double click.

Here's what I'm doing:

public static Template[] loadTemplates() throws IOException, ClassNotFoundException {
        Path dirtemp = Paths.get(path.toString(), TEMPLATE_PATH);
        DirectoryStream<Path> dirStream = Files.newDirectoryStream(dirtemp);
        
        ArrayList<File> files = new ArrayList<>();

        for (Path entry: dirStream) {
            files.add(entry.toFile());
        }

        if (files.size() != 0) {
            ArrayList<Template> templates = new ArrayList<>();

            for (File file: files) {
                FileInputStream fis = new FileInputStream(file);
                ObjectInputStream ois = new ObjectInputStream(fis);

                Object object = ois.readObject();

                if (object instanceof Template) {
                    templates.add((Template) object);
                }
                
                ois.close();
                fis.close();
            }

            Collections.sort(templates);

            return templates.toArray(new Template[0]);
        } else return null;
    }

TEMPLATE_PATH is just a string that contains the name of the folder with the files.

path is a static variable inside my class that contains the current path to the JAR. I initialize it from the main method, since the location is not intended to change. Here's the code that I used to get it, if relevant:

public static void findJARPath() {
        final Class<?> referenceClass = Main.class;
        final URL url =
                referenceClass.getProtectionDomain().getCodeSource().getLocation();

        try {
            final File jarPath = new File(url.toURI()).getParentFile();
            path = Paths.get(jarPath.toURI());
        } catch(final URISyntaxException ignored){}
    }

Does anyone know why this happens?

I'm running on Windows but I want this to run on different OS as well. I'd appreciate if you pointed out if any of this code is not portable, too.

Edit:
The stack trace of the exception:

javax.swing.ImageIcon local class incompatible: stream class desc serialVersionUID = -962022720109015502, local class serialVersionUID = 532615968316031794
java.base/java.io.ObjectStreamClass.initNonProxy(ObjectStreamClass.java:689)
java.base/java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:1903)
java.base/java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1772)
java.base/java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:2060)
java.base/java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1594)
java.base/java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:2355)
java.base/java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:2249)
java.base/java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:2087)
java.base/java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1594)
java.base/java.io.ObjectInputStream.readObject(ObjectInputStream.java:430)
Data.FileManager.loadTemplates(FileManager.java:108)

The code that writes the files:

public static void saveTemplate(Template template) {
        new File(TEMPLATE_PATH).mkdir();
        String filename = StringUtils.toFilename(template.getName(), "templ");

        try {
            FileOutputStream fos = new FileOutputStream(TEMPLATE_PATH + filename);
            ObjectOutputStream oos = new ObjectOutputStream(fos);

            oos.writeObject(template);

            oos.close();
            fos.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

Edit: Figured it out. Apparently, for whatever reason, the ImageIcon UID from the image in my template class wouldn't match when running the jar via double click. Wrapping the image into another class with a fixed UID solved it.

Your plan defeats itself, of course: "The same directory as my jar file" would obviously contain..... your jar file.

Which will crash if you attempt to read it with an OIS. Apply some filtering as you dirstream through this directory. Does it have the right extension? Is it an actual file (vs. a directory)?

Also, your code has a memory leak; dirstreams are resources, as is a FileInputStream. You must close these. The only safe way to do so is with try/finally or try-with-resources. Search the web for "Java try with resources" if you're unfamiliar with these techniques.

You've saved a JDK11 ImageIcon, and are trying to deserialize it back on a VM that doesn't have that, because it has a JDK14 ImageIcon or whatnot.

More generally, you can't serialize anything if the intent is to allow it to deserialize on another system unless it is very carefully managed. ImageIcon by intent is not managed like that, therefore, you can't save ImageIcons with OIS and OOS. It looks like you can, but the caveat is: Breaks on different VMs.

You gotta toss all this code out and think of another way to serialize ImageIcon objects. Perhaps save the raw pixels, save it as a PNG, etc.