Java: load User-defined interface implementation (from config file)

The comment is correct; as long as the .jar file is in your classpath, you can load the class.

I have used something like this in the past:

public static MyInterface loadMyInterface( String userClass ) throws Exception
{
    // Load the defined class by the user if it implements our interface
    if ( MyInterface.class.isAssignableFrom( Class.forName( userClass ) ) )
    {
        return (MyInterface) Class.forName( userClass ).newInstance();
    }
    throw new Exception("Class "+userClass+" does not implement "+MyInterface.class.getName() );
}

Where the String userClass was the user-defined classname from a config file.

EDIT

Come to think of it, it is even possible to load the class that the user specifies at runtime (for example, after uploading a new class) using something like this:

public static void addToClassPath(String jarFile) throws IOException 
{
    URLClassLoader classLoader = (URLClassLoader) ClassLoader.getSystemClassLoader();
    Class loaderClass = URLClassLoader.class;

    try {
        Method method = loaderClass.getDeclaredMethod("addURL", new Class[]{URL.class});
        method.setAccessible(true);
        method.invoke(classLoader, new Object[]{ new File(jarFile).toURL() });
    } catch (Throwable t) {
        t.printStackTrace();
        throw new IOException( t );
    }
}

I remember having found the addURL() invocation using reflection somewhere here on SO (of course).