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I decided to implement dynamic class loading in my glassfish web application, as a way of trying it out and to support small plugins that could be loaded and executed by the web app at runtime.

I added the following class:

public class PluginManager { private static final String dropBoxDir = "file:///path/to/dropbox/"; private static final URLClassLoader dropBoxClassLoader; static { try { URL dropBoxURL = new URL(dropBoxDir); dropBoxClassLoader = URLClassLoader.newInstance(new URL[]{dropBoxURL}); } catch (MalformedURLException mue) { throw new RuntimeException("MalformedURLException thrown during PluginManager initialization - the hardcoded URL " + dropBoxDir + " must be invalid.", mue); } } //this method is called by a web service public static void runPluginFromDropBox(String fullClassName) { try { //load the plugin class Class<?> pluginClass = dropBoxClassLoader.loadClass(fullClassName); //instantiate it Runnable plugin = (Runnable)pluginClass.newInstance(); //call its run() method plugin.run(); } catch (ClassNotFoundException cnfe) { throw new RuntimeException("The class file for " + fullClassName + " could not be located at the designated directory (" + dropBoxDir + "). Check that the specified class name is correct, and that its file is in the right location.", cnfe); } catch (InstantiationException ie) { throw new RuntimeException("InstantiationException thrown when attempting to instantiate the plugin class " + fullClassName + " - make sure it is an instantiable class with a no-arg constructor.", ie); } catch (IllegalAccessException iae) { throw new RuntimeException("IllegalAccessException thrown when attempting to instantiate the plugin class " + fullClassName + " - make sure the class and its no-arg constructor have public access.", iae); } catch (ClassCastException cce) { throw new RuntimeException("Plugin instance could not be cast to Runnable - plugin classes must implement this interface.", cce); } } }

Then in a separate project, I created a test plugin:

public class TestPlugin implements Runnable { @Override public void run() { System.out.println("plugin code executed"); } }

I deployed the web application, then compiled TestPlugin into a .class file and dropped it into the designated folder. I called a web service that hits runPluginFromDropBox() with the class name and got the expected output.

This all worked as a proof of concept, but my plugin is effectively useless unless it can be made aware of my web application's classes. I've since read that .war is intended only as a standalone application, and not meant to be on other libraries' classpaths, which doesn't bode well for this little side-project.

I had a look at this discussion: Extending Java Web Applications with plugins and get the feeling I'm wading into a swamp of design challenges for no huge reason and should turn around. However that post is kind of old and is Tomcat-specific, so I just thought I'd ask if there's any straightforward way for me to approach this without some elaborate third party framework.

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The classes of a war file are loaded by a specific classloader, to isolate the war from other webapps deployed on the same server, and to be able to undeploy the war.

To be aware of the webapp classes, your plugin classloader should have the webapp classloader as its parent. I'm not aware of all the problems you might have by using an additional classloader, but I suspect you might have memory leaks and other nasty problems (static values kept in memory, etc.) when the container will undeploy and redeploy the webapp. And you might have differences between containers as well.

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When you say "plugin classloader" do you mean the URLClassLoader I'm using to load the plugin? That code is in the web app though. I'm wondering how hard it would be for a plugin to have the war on its classpath somehow. So for example if the web app has a Foo class, my plugin could import mywebapp.Foo; and then interact with Foo in its run() method - or am I naively off track?
Yes, I'm talking about the classloader used to load the plugin class. And my answer is that this classloader should have the war's classloader as its parent classloader. Look at the constructors of URLClassLoader. There is one taking a parent ClassLoader as argument. Note that import is only used at compile time. At compile time, you just have to set the appropriate classpath to have access to the webapp classes. At runtime you need the war's classloader as parent classloader.
Okay, I understand what you're saying about the parent classloader. However I think this is already the case - unless I'm mistaken the call to newInstance() is using the default designated ClassLoader as the parent, which should be the web app's. So I'm not sure why that's relevant unless I'm missing something. I think my question boils down to the plugin library's compile-time awareness of the web app's classes. Am I missing something simple here - can I just put the war on the plugin's classpath?
newInstance uses the default ClassLoader as parent. Not the webapp's classloader. There's an overloaded newInstance method taking a parent classloader as argument. And yes, of course, when compiling your plugin you need the webapp's classes in the plugin's project classpath. Don't put the war in the classpath. Put its WEB-INF/classes directory and the jars in WEB-INF/lib. To compile a class depending on another class, you need this other class in the classpath. Whether this other class is used in a webapp at runtime doesn't matter for the Java compiler.
Okay I didn't realize the distinction between the webapp's classloader and the default. Thanks for being patient with me. As far as putting the directory/jars you mentioned on the classpath, I can't figure out how to do it through Netbeans as it doesn't let me access the contents of the war. Not sure how I'm noobing this up.
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