Loading plugins

Spud <fake@fkfkfkf.com>
Wed, 06 Oct 2010 12:21:10 -0500
My app is extendable via "plugins". A plugin is a .jar file that adds
some functionality to the app. The idea is that the user can copy new
jar files to a subdirectory, restart the app, and the new functionality
is available.

Here's the tricky issue: to get all the jar files on the classpath, we
have to do one of two things:

1. Write an unpleasant startup script that will roll through all the
subdirectories, find the jars, and add them to the -classpath string on
the java command line. I say "unpleasant" because this isn't a pure java
solution and will have to be customized for each platform.

2. Find some way to do this with classloaders after the app has started.

I'd prefer the second approach, but I fear classloader hell and can't be
sure the code would be reliable.

What's the best approach here?

Just for reference, I've pasted some code below that might be a start,
but the whole approach gives me the willies:

public class Start {

    * Runs the server. The method does the same thing as Server.main(),
except that
    * it loads all of the jars files in the /lib dir on the classpath first.
   public static void main(String[] args) {
     Start start = new Start();

   private void load(String [] args) {

     String homeDir = Server.findHomeDir();

     // get the references to the jar files as an array of urls
     File file = new File(homeDir, "lib");
     ArrayList urlList = new ArrayList();
     recurseUrls(file, urlList);
     URL [] urls = new URL[urlList.size()];

     // create a classloader that includes the jars and make it current
     ClassLoader loader = getClassLoader(urls);

     // now load the server and everything else using new class loader
     try {
       Class invokedClass = loader.loadClass("org.mydomain.MyServer");
           Method main = invokedClass.getDeclaredMethod("main",
           main.invoke(null, (Object [])args);
     } catch (Exception e) {

    * Get a classloader that covers the urls
   private ClassLoader getClassLoader(URL[] urls) {
       ClassLoader parent = Thread.currentThread().getContextClassLoader();
       if (parent == null) {
         parent = Start.class.getClassLoader();
       if (parent == null) {
         parent = ClassLoader.getSystemClassLoader();
       URLClassLoader loader = new URLClassLoader(urls, parent);
     return loader;

    * Roll through a directory and add any jar files in it to the ArrayList.
   private void recurseUrls(File file, ArrayList urlList) {
     if (file.isDirectory()) {
       File [] files = file.listFiles();
       for (int i = 0; i < files.length; i++) {
         recurseUrls(files[i], urlList);

     } else {
       String name = file.toString().toLowerCase();
       if (name.endsWith(".jar")) {
         try {
         } catch (MalformedURLException e) {


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