How could we modify the above bootstrap program to load multiple apps? Should we run each app within a seperate thread?

Great Article btw

Whenever you start the JVM with a “-cp” argument, all those jars and libs on your classpath are on the system classpath. Any code you run in that JVM will have those libs in their classloader.

When you’re writing middleware or some kind of container, you’ll run into a problem where your container might need one version of a library but the application you want to deploy has the same library but a different version. You need bootstrapping to reconcile this. The code example I gave shows how you would set the system classloader (which is empty, because you’re bootstrapping) as the parent classloader to the application you want to deploy. This leaves the application free of your container’s dependencies.

I hope that helps. All this stuff becomes a lot clearer when you write a server that deploys other apps within it. Most people probably never do that, but we did when we wrote our message bus (http://blog.markturansky.com/archives/26)

Mark

I gave it another shot, motivated by this blog entry. It poses a few problems though, the JNLPClassLoader is package private in com.sun.jnlp namespace and hence completely inaccessable. But by casting to an URLClassLoader, the remote CLASSES (in fact, works for non-jws execution too) can be fetched and passed along to the new ClassLoader instance.

URLClassLoader jnlpClassLoader = (URLClassLoader)this.getClass().getClassLoader();
ClassLoader bootstrapClassLoader = new URLClassLoader( jnlpClassLoader.getURLs());

…and that works, but causes issues with the security manager later on. Another approach:

URLClassLoader jnlpClassLoader = (URLClassLoader)this.getClass().getClassLoader();
URLClassLoader classLoader = URLClassLoader.newInstance( jnlpClassLoader .getURLs(), jnlpClassLoader);

…seems to get slightly further, but causing errors on the EDT. I feel I’m getting close, but I am not sure how to attempt this off the EDT or possibly simply empty the EDT first. Now I remember why I am no fan of classloaders!

/Casper

Thanks for the inquiry. Short answer, I don’t know.

I’m unfamiliar with Web Start. I don’t know how to bootstrap in that scenario.

I work mostly with web applications and back-end systems (messaging/integration). I know we can boostrap Jetty as part of our system as well as all the disparate components deployed around the network.

I hope the code I posted can help you figure out how to bootstrap a Web Start app. If you figure it out, please post the solution online! I’d love to see it.

Mark

The above code works just fine except that you *will* want to add this single line before you reflectively invoke the main method:


Thread.currentThread().setContextClassLoader(classloader);


Lots of well-written Java software use the thread’s context classloader to resolve classes and classpath-based resources. This is a very effective way to avoid crossing the streams between classloaders.

Other software packages don’t always play as nicely with classloaders, even when you set the context classloader. Bootstrapping helps in this case, but well-behaved software would be even better.

FYI: Dan is one hell of a smart developer, groks all the bootstrapping and classloading challenges you’re likely to face, and — best of all — works for me on whatever madness our bosses let us pursue