[Solved][Adv. Java] Read external jar with reference to plugin

Yeah, I know. What is this for strangely worded question?! I'm sorry but I could not find a better way to describe the problem I've gotten myself into.

I've done a lot of searching but can't find the solution to my problem. I will try and be as clear as possible and hopefully you'll get what I mean. (For the examples below I've removed simple stuff like finding the files and adding the imports etc.)

I'm trying to make a plugin that reads external jar files to extend the plugin itself.

Code:

                String lcStr = fileName.toLowerCase() + "." + fileName;
                URLClassLoader cl = null;
                try {
                    cl = URLClassLoader.newInstance(new URL[]{file.toURI().toURL()});
                    try {
                        Class loadedClass = cl.loadClass(lcStr);
                        for(Constructor c:this.loadedClass.getConstructors()) {             
                            plugin.log.debug("Constructor: " + c.toGenericString());
                        }
                    } catch (ClassNotFoundException ex) {
                        plugin.log.severe("Error in external jar. Can't find class '" + lcStr + "'", ex);
                    }
                } catch (MalformedURLException ex) {
                    plugin.log.severe("Error in external jar.", ex);
                }

It looks simple enough right? Load the class and check all constructors and print them out.
Here is the constructor of the plugin (found in the directory Bukkit\plugins\PName\extra\ )

Code:

    public Extra_Location() {
        this.type = "Extra Location";
    }

All goes well. I can find the constructor and use it to initialize the new jar.
Here comes the problem!
I want the external jar to reference the plugin itself. I tought it would be as easy as making a new constructor but with a argument to my plugin. So I added a new constructor.

Code:

    public Extra_Location(MyPlugin plugin) {
        this.type = "Extra Location";
        this.plugin = plugin;
    }

This is where the plugin breaks. It can't find the class defenition to MyPlugin anymore. Strange as it is being called from MyPlugin.
Things I've done to solve this:

• Add a class-path to the manifest.mf file of the external file (tried every know path, even directly to MyPlugin.jar itself!)
• Added MyPlugin as a library to the external file
• Added full path to the plugin in manifest.mf. This results in not able to find the class in the external file.

I hope you Bukkit/Java guru's out there could help me! I'm pretty stuck on this problem right now
Many thanks!

Edit: Problem solved! Thanks to nisovin .
This is the only piece that needs to be changed from

Code:

cl = URLClassLoader.newInstance(new URL[]{file.toURI().toURL()});

to

Code:

cl = URLClassLoader.newInstance(new URL[]{file.toURI().toURL()}, this.getClass().getClassLoader());

This piece of code works if your putting this in your main class. Else you have to change this to a reference to your plugin.

 

This is known as a "Module" system btw

Anyhow, maybe you instead of putting an argument in the constructor, you could simply make a method which has the main class as argument.

EDIT by Moderator: merged posts, please use the edit button instead of double posting.

 

If it could find MyPlugin then I could reference it in the external file. But that's the whole problem. If I put any reference to it in the external file it would error in a NoClassDefFoundError for MyPlugin.

Update: I've probably found a way to implement the class-path in the manifest.mf but this resolves into a new error. When I add the full path to my plugin (on windows: D:\\Bukkit\\Plugins\\MyPlugin.jar) in the Class-Path section of the manifest.mf it gives me a "ClassNotFoundException" to the class of the external file where this normally won't. I will update the opening post.

 

you could try using Plugin instead of your plugin class, would that help?

did you look into how bukkit is doing it?

 

When you create a URLClassLoader, you can pass in your current class loader as a second parameter. You need to do this for it all to work properly.

Code:

cl = URLClassLoader.newInstance(new URL[]{file.toURI().toURL()}, getClassLoader());

 

You sir deserves the helpful post of the week award!

I've changed the opening post to reflect the change. (Only one extra variable... sigh!)