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Best approach for dynamic class loading in java webapp?

I´m working in the design of a java web application capable of executing custom code or precompiled classes uploaded by the users, focused mostly in simple validations of datasets.
The custom class must be constrained to a predefined interface and only some libraries and classes must be available to the custom class.
My first solution is to use a custom Classloader capable of loading .jar files from a defined directory in the file system. This approach seems to work but i´m concerned about the security and compatibility of this solution.
Is possible to limit the classes that can be imported by the custom class and run the code in a sandbox in order to avoid some actions like opening files or sockets?
When the loaded class will be unloaded?
application Servers like Weblogic have some restriction about using custom classloaders?
I have evaluated another solutions like OSGi Bundles, but it looks really complex and the support is limited in some applications server also i´m not really sure if OSGi is the right technology for this particular usage. Embedded Scripting Languages like Groovy o Javascript are discarded because the project owner wants the custom code precompiled and written in Java.
What would you recommend for this problem?

OSGi is a good fit for an application that wishes to accept external code (plugins). All the requirements you mention (predefined interfaces, loading jars, mutiple classloaders) are all covered by OSGi services and bundle management. Bundles can be installed, started, stopped, uninstalled, etc, including in runtime.
OSGi support in web application servers is not really that limited. You could even considered embedding an OSGi framework.
Security-wise you will need a solution around security managers.

turn api with hibernate objects into service for multiple applications

i have an API that is being written for a large group of 40 or so applications to share.
my problem is currently they plan on having the API as a simple library included in each war file for each program. the problem thats going to occur is when two apps are running on the same instance with different versions of the api library. ive had a lot of problems in the past with this.
i seem to remember a while ago something where i can wrap my library into an ear file or something and deploy it to tomcat to make it global. simply including it in the lib folder won't work because it will include hibernate systems that have to be deployed to allow the api methods to access the database. then in each application i would have an interface i can implement that allows me to call those api methods. very similar to local EJB3 but not as complex and didn't require an enterprise level server to implement.
anyone else remember something like this or was it a bad dream on my part?

You will have problems if you use a single jar shared by all the webapps, since it will then be impossible for two apps to use a different version of a library. But if each webapp has its own version of the library in its WEB-INF/lib, the container shouldn't have any problem: each webapp has its own classloader, which doesn't see the libraries of other webapps.

How to modularize a JSF/Facelets/Spring application with OSGi?

I'm working with very large JSF/Facelets applications which use Spring for DI/bean management.
My applications have modular structure and I'm currently looking for approaches to standardize the modularization.
My goal is to compose a web application from a number of modules (possibly depending on each other). Each module may contain the following:
Classes;
Static resources (images, CSS, scripts);
Facelet templates;
Managed beans - Spring application contexts, with request, session and application-scoped beans (alternative is JSF managed beans);
Servlet API stuff - servlets, filters, listeners (this is optional).
What I'd like to avoid (almost at all costs) is the need to copy or extract module resources (like Facelets templates) to the WAR or to extend the web.xml for module's servlets, filters, etc. It must be enough to add the module (JAR, bundle, artifact, ...) to the web application (WEB-INF/lib, bundles, plugins, ...) to extend the web application with this module.
Currently I solve this task with a custom modularization solution which is heavily based on using classpath resources:
Special resources servlet serves static resources from classpath resources (JARs).
Special Facelets resource resolver allows loading Facelet templates from classpath resources.
Spring loads application contexts via the pattern classpath*:com/acme/foo/module/applicationContext.xml - this loads application contexts defined in module JARs.
Finally, a pair of delegating servlets and filters delegate request processing to the servlets and filters configured in Spring application contexts from modules.
Last days I read a lot about OSGi and I was considering, how (and if) I could use OSGi as a standardized modularization approach. I was thinking about how individual tasks could be solved with OSGi:
Static resources - OSGi bundles which want to export static resources register a ResourceLoader instances with the bundle context. A central ResourceServlet uses these resource loaders to load resources from bundles.
Facelet templates - similar to above, a central ResourceResolver uses services registered by bundles.
Managed beans - I have no idea how to use an expression like #{myBean.property} if myBean is defined in one of the bundles.
Servlet API stuff - use something like WebExtender/Pax Web to register servlets, filters and so on.
My questions are:
Am I inventing a bicycle here? Are there standard solutions for that? I've found a mentioning of Spring Slices but could not find much documentation about it.
Do you think OSGi is the right technology for the described task?
Is my sketch of OSGI application more or less correct?
How should managed beans (especially request/session scope) be handled?
I'd be generally grateful for your comments.

What you're aiming to do sounds doable, with a few caveats:
The View Layer: First, your view layer sounds a little overstuffed. There are other ways to modularize JSF components by using custom components that will avoid the headaches involved with trying to create something as dramatic as late-binding managed beans.
The Modules Themselves: Second, your modules don't seem particularly modular. Your first bullet-list makes it sound as if you're trying to create interoperable web apps, rather than modules per se. My idea of a module is that each component has a well-defined, and more or less discrete, purpose. Like how ex underlies vi. If you're going down the OSGi route, then we should define modular like this: Modular, for the sake of this discussion, means that components are hot-swappable -- that is, they can be added and removed without breaking the app.
Dependencies: I'm a little concerned by your description of the modules as "possibly depending on each other." You probably (I hope) already know this, but your dependencies ought to form a directed acyclic graph. Once you introduce a circular dependency, you're asking for a world of hurt in terms of the app's eventual maintainability. One of the biggest weaknesses of OSGi is that it doesn't prevent circular dependencies, so it's up to you to enforce this. Otherwise your dependencies will grow like kudzu and gradually choke the rest of your system's ecosystem.
Servlets: Fuhgeddaboudit. You can't late-bind servlets into a web app, not until the Servlet 3.0 spec is in production (as Pascal pointed out). To launch a separate utility servlet, you'll need to put it into its own app.
OK, so much for the caveats. Let's think about how this might work:
You've defined your own JSF module to do... what, exactly? Let's give it a defined, fairly trivial purpose: a login screen. So you create your login screen, late-bind it using OSGi into your app and... then what? How does the app know the login functionality is there, if you haven't defined it in your .jspx page? How does the app know to navigate to something it can't know is there?
There are ways to get around this using conditional includes and the like (e.g., <c:if #{loginBean.notEmpty}>), but, like you said, things get a little hairy when your managed loginBean exists in another module that may not have even been introduced to the app yet. In fact, you'll get a servlet exception unless that loginBean exists. So what do you do?
You define an API in one of your modules. All the managed beans that you intend to share between modules must be specified as interfaces in this API layer. And all your modules must have default implementations of any of these interfaces that they intend to use. And this API must be shared between all interoperable modules. Then you can use OSGi and Spring to wire together the specified beans with their implementation.
I need to take a moment to point out that this is not how I would approach this problem. Not at all. Given something like as simple as a login page, or even as complicated as a stock chart, I'd personally prefer to create a custom JSF component. But if the requirement is "I want my managed beans to be modular (i.e., hot-swappable, etc)," this is the only way I know to make it work. And I'm not even entirely sure it will work. This email exchange suggests that it's a problem that JSF developers have only just started to work on.
I normally consider managed beans to be part of the view layer, and as such I use them only for view logic, and delegate everything else to the service layer. Making managed beans late-binding is, to my mind, promoting them out of the view layer and into the business logic. There's a reason why all those tutorials are so focused on services: because most of the time you want to consider what it would take for your app to run "headless," and how easy it would be to "skin" your view if, for instance, you wanted it to run, with all its functionality, on an Android phone.
But it sounds like a lot of what you're working with is itself view logic -- for instance, the need to swap in a different view template. OSGi/Spring should be able to help, but you'll need something in your app to choose between available implementations: pretty much what OSGi's Service Registry was built to do.
That leaves static resources. You can modularize these, but remember, you'll need to define an interface to retrieve these resources, and you'll need to provide a default implementation so your app doesn't choke if they're absent. If i18n is a consideration, this could be a good way to go. If you wanted to be really adventurous, then you could push your static resources into JNDI. That would make them completely hot-swappable, and save you the pain of trying to resolve which implementation to use programmatically, but there are some downsides: any failed lookup will cause your app to throw a NamingException. And it's overkill. JNDI is normally used in web apps for app configuration.
As for your remaining questions:
Am I inventing a bicycle here? Are there standard solutions for that?
You are, a little. I've seen apps that do this kind of thing, but you seem to have stumbled into a fairly unique set of requirements.
Do you think OSGi is the right technology for the described task?
If you need the modules to be hot-swappable, then your choices are OSGi and the lighter-weight ServiceLocator interface.
Is my sketch of OSGI application more or less correct?
I can't really tell without knowing more about where your component boundaries are. At the moment, it sounds like you may be pushing OSGi to do more than it is capable of doing.
But don't take my word for it. I found other reading material in these places.
And since you ask about Spring Slices, this should be enough to get you started. You'll need a Git client, and it looks like you'll be training yourself on the app by looking through the source code. And it's very early prototype code.

I am facing the same problems in my current project. In my opinion, OSGi is the best and cleanest solution in terms of standards and future support, but currently you may hit some problems if you try using it in a web application:
there is no well integrated solution between a Web Container and the OSGi platform yet.
OSGi may be too much for a custom build web application that is just searching for a simple modularized architecture. I would consider OSGi if my project needs to support third party extensions that are not 100% under our control, if the project needs hot redeployments, strict access rules between plugins, etc.
A custom solution based on class loaders and resource filters seems very appropriate for me.
As an example you can study the Hudson source code or Java Plug-in Framework (JPF) Project(http://jpf.sourceforge.net/).
As about extending the web.xml, we may be lucky with the Servlet 3.0 specification(http://today.java.net/pub/a/today/2008/10/14/introduction-to-servlet-3.html#pluggability-and-extensibility).

The "web module deployment descriptor fragment" (aka web-fragment.xml) introduced by the Servlet 3.0 specification would be nice here. The specification defines it as:
A web fragment is a logical
partitioning of the web app in such a
way that the frameworks being used
within the web app can define all the
artifacts without asking devlopers to
edit or add information in the
web.xml.
Java EE 6 is maybe not an option for you right now though. Still, it would to be the standardized solution.

Enterprise OSGi is a fairly new domain so dont think you will get a solution that directly satisfies your need. That said one of the things I found missing from Equinox (osgi engine behind eclipse and hence one with largest user base!) is a consistent configuration / DI service. In my project recently we had some similar needs and ended building a simple configuration osgi service.
One of the problems which will be inherent to modular applications would be around DI, as the module visibility could prevent class access in some cases. We got around this using a registered-buddy policy, which is not too ideal but works.
Other than configuration, you can take a look at the recently released Equinox book for guidance on using OSGi as base for creating modular applications. The examples may be specific to Equinox, but the principles would work with any OSGi framework. Link - http://equinoxosgi.org/

You should look into Spring DM Server (it's being transitioned to Eclipse Virgo but that's not been released yet). There's a lot of good things in the recent OSGi enterprise spec which has also just been released.
Some of the Spring DM tutorials will help, I'd imagine. But yes, it's possible to have both resources and classes loaded from outside the web bundle using standard modularity. In that, it's a good fit.
As for the session context - it gets handled as you would expect in a session. However, you might run into problems with sharing that session between web bundles to the extent that in not sure if it's even possible.
You could also look to have a single web bundle and then use e.g. the Eclipse extension registry to extend the capabilities of you web app.

Using Guice with OSGi

I have a project that I am trying to convert to OSGi. However, this project has been built with Guice as its dependency injection manager. It's a large project with Guice interwoven all throughout. Guice has been giving me many fits with the conversion process. Specifically, it has been given me a NoClassDefFoundError for com.google.inject.internal.cglib.reflect.FastClass. I have made sure that nothing is package-private and I also downloaded and used the guice-2.0-customloader.jar. Still, to no avail.
Is there any advice from anyone that has gone down this path? It is worth mentioning that I use peaberry to get dynamic services from the OSGi environment (for a plugin-type architecture) that I then use in the program later on.
Thanks a lot,
Steve

Hard to say exactly what the problem is, classloader issues can be very situation specific in OSGi and hard to debug.
If guice really is a core to your application it may make sense to make it a Fragment bundle of the System Bundle. The System Bundle is the root node in the classloader tree for ALL bundles. If classes exist in this bundle, they'll be accessible everywhere. This is a technique I've used successfully to wrap Swing LAF (Substance, Alloy, Jide) as OSGi bundles. They've gotta be at the root level since all of my bundles use Swing (especially the Swing classloader)
You'll want to check the OSGi specs for creating a Fragment bundle and making that fragment bundle a framework extension fragment. I've only been able to do this using Eclipse's Equinox runtime. At the time I wrote most of the supporting classes, Apache Felix did not yet support framework extension fragments. It may have changed now. I have no idea about frameworks (knopplefish, spring-dm)

Just use Peaberry. It has all the support you need for using Guice in OSGi. No need to reinvent everything from scratch!

I'm assuming your comments on package-private are related to this page on Guice and bridging class loaders?
There's a bug report/patch that may be related to your issue: http://code.google.com/p/google-guice/issues/detail?id=343
Have you tried adding Guice and cglib to the standard classpath and doing boot delegation for them? That would prove/disprove your issue is the same one as mentioned on the above bug report.

You could try my latest patch for Issue 343 - just attached a freshly patched build which is the latest Guice trunk plus this one patch, so people can try it and give feedback.
The latest patch avoids a lot of these CGLIB exceptions without forcing you to make classes public, and also provides a better error message in the few remaining places where this is still necessary (basically when you explicitly use method interception).

How to create a pluginable Java program?

I want to create a Java program that can be extended with plugins. How can I do that and where should I look for?
I have a set of interfaces that the plugin must implement, and it should be in a jar. The program should watch for new jars in a relative (to the program) folder and registered them somehow.
Although I do like Eclipse RCP, I think it's too much for my simple needs.
Same thing goes for Spring, but since I was going to look at it anyway, I might as well try it.
But still, I'd prefer to find a way to create my own plugin "framework" as simple as possible.

I've done this for software I've written in the past, it's very handy. I did it by first creating an Interface that all my 'plugin' classes needed to implement. I then used the Java ClassLoader to load those classes and create instances of them.
One way you can go about it is this:
File dir = new File("put path to classes you want to load here");
URL loadPath = dir.toURI().toURL();
URL[] classUrl = new URL[]{loadPath};
ClassLoader cl = new URLClassLoader(classUrl);
Class loadedClass = cl.loadClass("classname"); // must be in package.class name format
That has loaded the class, now you need to create an instance of it, assuming the interface name is MyModule:
MyModule modInstance = (MyModule)loadedClass.newInstance();

Look into OSGi.
On one hand, OSGi provides all sorts of infrastructure for managing, starting, and doing lots of other things with modular software components. On the other hand, it could be too heavy-weight for your needs.
Incidentally, Eclipse uses OSGi to manage its plugins.

I recommend that you take a close look at the Java Service Provider (SPI) API. It provides a simple system for finding all of the classes in all Jars on the classpath that expose themselves as implementing a particular service. I've used it in the past with plugin systems with great success.

Although I'll second the accepted solution, if a basic plugin support is needed (which is the case most of the time), there is also the Java Plugin Framework (JPF) which, though lacking proper documentation, is a very neat plugin framework implementation.
It's easily deployable and - when you get through the classloading idiosynchrasies - very easy to develop with. A comment to the above is to be aware that plugin loadpaths below the plugin directory must be named after the full classpath in addition to having its class files deployed in a normal package path named path. E.g.
plugins
`-com.my.package.plugins
`-com
`-my
`-package
`-plugins
|- Class1.class
`- Class2.class

At the home-grown classloader approach:
While its definitely a good way to learn about classloaders there is something called "classloader hell", mostly known by people who wrestled with it when it comes to use in bigger projects. Conflicting classes are easy to introduce and hard to solve.
And there is a good reason why eclipse made the move to OSGi years ago.
So, if its more then a pet project, take a serious look into OSGi. Its worth looking at.
You'll learn about classloaders PLUS an emerging technolgy standard.

Have you considered building on top of Eclipse's Rich Client Platform, and then exposing the Eclipse extension framework?
Also, depending on your needs, the Spring Framework might help with that and other things you might want to do: http://www.springframework.org/