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How to make a basic plugin system / modloader?

So I'm trying to make a plugin system like the "Bukkit" plugin system. See right now on my project I have some classes extending my base class "Plugin" and then I add them to my list. How would I make it so I can make it so it automatically loads jars from a "mods" folder that are extending my "Plugin" class and automatically add them to the arraylist? Thank you VERY much for the help, I'm trying to make a mod loader.

There might be better ways to do this, but this is what I have done in the past:
private static final String JAVA_CLASS_PATH_PROPERTY = "java.class.path";
private static final String CUSTOM_CLASS_PATH_PROPERTY = "custom.class.path";
public static void addPath(String s) throws Exception {
File f = new File(s);
URL u = f.toURI().toURL();
URLClassLoader urlClassLoader = (URLClassLoader) ClassLoader.getSystemClassLoader();
Class<URLClassLoader> urlClass = URLClassLoader.class;
Method method = urlClass.getDeclaredMethod("addURL", new Class[] { URL.class });
method.setAccessible(true);
method.invoke(urlClassLoader, new Object[] { u });
if (System.getProperties().containsKey(CUSTOM_CLASS_PATH_PROPERTY)) {
StringBuilder sb = new StringBuilder();
sb.append(System.getProperty(CUSTOM_CLASS_PATH_PROPERTY));
sb.append(File.pathSeparatorChar);
sb.append(s);
System.setProperty(CUSTOM_CLASS_PATH_PROPERTY, sb.toString());
}
else {
StringBuilder sb = new StringBuilder();
sb.append(System.getProperty(JAVA_CLASS_PATH_PROPERTY));
sb.append(File.pathSeparatorChar);
sb.append(s);
System.setProperty(JAVA_CLASS_PATH_PROPERTY, sb.toString());
}
}
This adds the path to the jar (the s param) to the system class loaders list of URLS, then appends the path to the jar to the end of the custom class path if it exists, or the java class path otherwise. This should allow any other classes whose class loaders can reach the system class loader to use the newly loaded class.

You can load a class from a JAR but using a ClassLoader
URL jarUrl = ...;
URLClassLoader loader = new URLClassLoader(new URL[] { jarUrl });
Class myClass = Class.forName("myjar.mypackage.MyClass", true, loader);
MyPluginInterface myPlugin = myClass.asSubClass(MyPluginInterface.class).newInstance();
The myClass will be a class from the jar. Most likely you will want a JAR of interfaces you share with the plugin. By using these interfaces you can deal with instances which implement those interfaces easily. i.e. MyClass should implement MyPluginInterface which is an interface you provide.
Note: using a ClassLoader for each plugin allows you to unload the ClassLoader/JAR and load a new version of it, should it change.

Dynamic execution is not working when the plugin was installed

I am developing an Eclipse plugin that runs the current active file. I am using this method
public static void runIt(String fileToCompile,String packageName) throws ClassNotFoundException, IllegalArgumentException, IllegalAccessException, InvocationTargetException, InstantiationException, SecurityException, NoSuchMethodException
{
File file = new File(fileToCompile);
try
{
// Convert File to a URL
URL url = file.toURL(); // file:/classes/demo
URL[] urls = new URL[] { url };
// Create a new class loader with the directory
ClassLoader loader = new URLClassLoader(urls);
ClassLoader classLoader = Thread.currentThread()
.getContextClassLoader();
Class<?> thisClass = classLoader.loadClass("NewFile");
Object newClassAInstance = thisClass.newInstance();
Class params[] = new Class[1];
params[0]=String[].class;
Object paramsObj[] = {};
String m=null;
Object instance = thisClass.newInstance();
Method thisMethod = thisClass.getDeclaredMethod("main", params);
String methodParameter = "a quick brown fox";
// run the testAdd() method on the instance:
System.out.println((String)thisMethod.invoke(instance,(Object)m));
}
catch (MalformedURLException e)
{
}
}
But it works when I "Launch Eclipse application" [run the plugin in another Eclipse window] but when I installed the plugin in Eclipse it doesn't work anymore.
The problem is in this line
Class thisClass = classLoader.loadClass("NewFile");
It cannot find the class to be executed

I would assume that the context classloader is different when running as a plugin. Look at the classloader hierarchy obtained in your line:
ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
See if it differs in the two runtime contexts. If you can step-debug the plugin, you should be able to explore around the available objects and find a classloader among them that works for your dynamic class.
If you want to use a URLClassLoader (which appears to have been abandoned in your code), you need to give it a parent, like:
new URLClassLoader(urls, this.getClass().getClassLoader())

Load a class to Java classpath dynamically using Eclipse and OSGi

Combining the information from many posts on this site and many others, I got the following code to dynamically add (at run time) a directory containing classes to the classpath and load a class within that directory.
I'm using OSGi bundles and running from eclipse an "Eclipse Application" (a kind of Run Configuration).
This is the code I'm using:
CASE 1: (both cases are different things I've tried to do the same thing.)
File file = new File("/Users/alek/fastFIX/myJPass/");
URL url = file.toURI().toURL();
URL[] urls = new URL[]{url};
ClassLoader cl = new URLClassLoader(urls);
Class cls = cl.loadClass("GuiLauncher"); //the file GuiLauncher.class is in the /Users/alek/fastFIX/myJPass/ directory
Class[] argTypes = new Class[] { String[].class };
Method main = cls.getDeclaredMethod("main", argTypes); //trying to run the main class
main.invoke(null, (Object) args);
I don't get any error, and nothing happens.
I've also tryied the following, as I actually need the loaded class to interact with other (already loaded) classes.
CASE 2:
ClassLoader currentThreadClassLoader = Thread.currentThread().getContextClassLoader();
URLClassLoader urlClassLoader = new URLClassLoader(new URL[] { new File("/Users/alek/fastFIX/myJPass/").toURL() }, currentThreadClassLoader);
Thread.currentThread().setContextClassLoader(urlClassLoader);
then i load like this:
Class<?> c = Class.forName("GuiLauncher");
or like this:
Class<?> c = Thread.currentThread().getContextClassLoader().loadClass("GuiLauncher");
and try to invoke the main function like this:
Class[] argTypes = new Class[] { String[].class };
Method main = cls.getDeclaredMethod("main", argTypes); //trying to run the main class
main.invoke(null, (Object) args);
here also nothing happens.
Any clue of what could be happening? I've read all related posts here and many places else with no luck.

In OSGI framework it is necessary to add the OSGI class loader as a parent, something like this:
...
ClassLoader cl = new URLClassLoader(new URL[]{file.toURI().toURL()}, this.getClass().getClassLoader());
...

In case 1, I suspect that the GuiLauncher class is already on the classpath, so may get loaded by the default classloader. Try doing Class.forName() before setting up the dynamic classloader, to confirm that there's no class available. If you are in Eclipse, you need to be careful that the class is not included on the Eclipse classpath, which is what would normally happen. You might need to compile it once then move the .java and .class files elsewhere to hide them from Eclipse!
In case 2:
Class.forName("GuiLauncher");
will not work as you expect, because this will use the system classloader. This should fail, hence my suspicion above. You need use the other version of this method that specifies your dynamic classloader:
Class.forName("GuiLauncher", true, urlClassLoader)
The following code works for me.
import java.net.*;
import java.lang.reflect.*;
import java.io.File;
public class Main{
public static void main(String[] args)
{
try{
Class cls = Class.forName("Plugin");
}
catch(Exception e){
System.out.println("Nothing there!");
}
try{
File file = new File("plugin");
ClassLoader cl = new URLClassLoader(new URL[]{file.toURI().toURL()});
Class cls = Class.forName("Plugin", true, cl);
Method main = cls.getDeclaredMethod("main", new Class[] { String[].class });
main.invoke(null, (Object) args);
}
catch(Exception e){
e.printStackTrace();
}
}
}
The Plugin class is compiled in the plugin subfolder, so it's not on the classpath used to run Main, as shown by the first Class.forName().
public class Plugin{
public static void main(String[] args)
{
System.out.println("Plugin was invoked!");
}
}
and prints out:
Nothing there!
Plugin was invoked!

java.lang.ClassNotFoundException while attempting to load dynamically a class in glassfish

I'm trying to load dynamically a class contained in a .jar file. I know the whole class name and I know for sure that the class implements the interface AlgorithmClass.
My code looks like this:
addURLToSystemClassLoader(dir.toURI().toURL());
Class cl = Class.forName(algorithm.getClassName());
AlgorithmClass algorithmClass = (AlgorithmClass)cl.newInstance();
Where dir is the File object of the .jar file and addURLToSystemClassLoader(URL) looks like this:
private void addURLToSystemClassLoader(URL url) throws IntrospectionException {
URLClassLoader systemClassLoader = (URLClassLoader) ClassLoader.getSystemClassLoader();
Class<URLClassLoader> classLoaderClass = URLClassLoader.class;
try {
Method method = classLoaderClass.getDeclaredMethod("addURL", new Class[]{URL.class});
method.setAccessible(true);
method.invoke(systemClassLoader, new Object[]{url});
} catch (Throwable t) {
t.printStackTrace();
throw new IntrospectionException("Error when adding url to system ClassLoader ");
}
}
I checked and the URL is being added to the class loader.
When I try to get the Class object I get the error:
SEVERE: javax.servlet.ServletException: java.lang.ClassNotFoundException: id3.Algorithm
(id3.Algorithm is the full name of the class I'm trying to load)
I've tried creating a new ClassLoader like below:
ClassLoader cload = new URLClassLoader(new URL[]{dir.toURI().toURL()}, ClassLoader.getSystemClassLoader());
Class cl = Class.forName(algorithm.getClassName(), false, cload);
AlgorithmClass algorithmClass = (AlgorithmClass)cl.newInstance();
But then I get the error:
java.lang.NoClassDefFoundError: lib/algorithm/AlgorithmClass
I've tried creating a new URLClassLoader with all the URLs that the system class loader has but the effect was the same.
The "worst" part of this is that both ways are working perfectly fine on the jUnit test that I have for testing this part of my code.
I'm using Glassfish 3.1.1 as my app server.

dir shouldn't contain 'lib'.
Try this:
ClassLoader cload = new URLClassLoader(new URL[]{dir.toURI().toURL()}, Thread.currentThread().getContextClassLoader());
Class cl = Class.forName(algorithm.getClassName(), true, cload);
AlgorithmClass algorithmClass = (AlgorithmClass)cl.newInstance();

You have class-loading issue. You shoud be aware that your addURLToSystemClassLoader() is actually the heck...
Put your jar to the classpath. Use Class.forName() idiom. If it fails use version that receives ClassLoader as parammeter namely
public static Class<?> forName(String name, boolean initialize,
ClassLoader loader)
and path Thread.currentThread().getContextClassLoader() as ClassLoader parameter.
See also my another answer below.

java dynamic classloader

how can I dynamically load a class in Java with two parameters which are the absolute filepath of the class file and the name of the method I wish to call?
eg path: c:\foo.class
method: print()
I am just interested in the basics as a simple cmd line tool. A code example would b appreciated.
cheers hoax

Use URLClassLoader. The name of the method is irrelevant. You must pass the root directory of your package to the class loader. Then you can use the fully qualified class name (package + class name) in Class.forName() to get the Class instance. You can use the normal reflection calls to create an instance of this class and call methods on it.
To make your life more simple, have a look at commons-beanutils. It makes invoking methods much more simple.

Check out this example:
// Create a File object on the root of the directory containing the class file
File file = new File("c:\\myclasses\\");
try {
// Convert File to a URL
URL url = file.toURL(); // file:/c:/myclasses/
URL[] urls = new URL[]{url};
// Create a new class loader with the directory
ClassLoader cl = new URLClassLoader(urls);
// Load in the class; MyClass.class should be located in
// the directory file:/c:/myclasses/com/mycompany
Class cls = cl.loadClass("com.mycompany.MyClass");
} catch (MalformedURLException e) {
} catch (ClassNotFoundException e) {
}
After this, you could do something like this to first create a new instace using the default constructor and invoking the method "print" without arguments:
Object object = cls.newInstance();
cls.getMethod("print").invoke(object);