plugins - How to create a pluginable Java program?

Question

I want to create a Java program that can be extended with plugins. How can I do that and where should I look for?

I have a set of interfaces that the plugin must implement, and it should be in a jar. The program should watch for new jars in a relative (to the program) folder and registered them somehow.

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Although I do like Eclipse RCP, I think it's too much for my simple needs.

Same thing goes for Spring, but since I was going to look at it anyway, I might as well try it.

But still, I'd prefer to find a way to create my own plugin "framework" as simple as possible.

Answer 1

I've done this for software I've written in the past, it's very handy. I did it by first creating an Interface that all my 'plugin' classes needed to implement. I then used the Java ClassLoader to load those classes and create instances of them.

One way you can go about it is this:

File dir = new File("put path to classes you want to load here");
URL loadPath = dir.toURI().toURL();
URL[] classUrl = new URL[]{loadPath};

ClassLoader cl = new URLClassLoader(classUrl);

Class loadedClass = cl.loadClass("classname"); // must be in package.class name format

That has loaded the class, now you need to create an instance of it, assuming the interface name is MyModule:

MyModule modInstance = (MyModule)loadedClass.newInstance();