What you can do is load the classes into a classloader at runtime and then pass that classloader to the PackageBuillderConfiguration RuleBaseConfiguration. Any DRLs that it then compiles will compile against the classes in the parent classloader. So while we don't have explicit support for this, you could build your own api, that wraps drools, to do this.

Godmar Back wrote:
On 9/26/07, Edson Tirelli <tirelli@post.com> wrote:
    Drools does not have any special treatment/feature for classloaders.
Default java classloader hirarchy is used. So, set your URLClassLoader as
the context classloader before building the package and it shall work.


I find it unlikely that this works. The compiler probably won't load
classes from the context classloader (after all, the classloader
doesn't have an interface to obtain the bytecode associated with a
name - all it provides is a method to retrieve a java.lang.Class
object, which the compiler doesn't need. The compiler needs to examine
the bytecode of referred classes.)

Even if it did work, it would be insufficient because I do not know
the URL until I've seen the drl file. Otherwise, I would have to keep
that dependency information somewhere else - I'd like to keep it in
the .drl file and let drool do the work.

Let me propose the following enhancement.
Add an optional "from" clause to the "import" statement. For instance,
allow a user to say:

import org.somefeature.somepackage from http://somefeature.org/ourpackages.jar;

This would instruct the compiler to include this .jar in its compile
class path. In addition, it would compile a list of jar URLs upon
which the rules depend when being executed. A method on package
builder could then be added that would retrieve that list of URLs such
that a classloader could be created to load those classes.  Either
"URL [] PackageBuilder.getReferencedURLs()" or possibly better
"ClassLoader PackageBuilder.createLoader()" where a suitably
configured subclass of URLClassLoader is returned.

 - Godmar

2007/9/25, Godmar Back <godmar@gmail.com>:
My question is how to manage a rule base that is composed from sets of
rules drawn from different sources.

Our goal is to supply sets of rules as .jar files - each .jar file
containing a related set of rules. For instance, consider an expert
system where different experts could export their expertise in such a
jar file. Such a .jar file would contain one or more .drl files and a
set of supporting .java classes. We'd like to be able to refer to
these .jar files using a URLClassLoader and/or JarURLConnection so
they can be referred to without a user having to download them prior
to starting their application.

Users should be able to combine those provided rules (that is, the
rules of those experts whose knowledge they wish to exploit) with
their own rules in their own .drl files.

My question is whether drools supports such a setup easily.
Specifically, is there a way for a user to import external .jar files
in their .drl files? A statement such as
use jar:http://expertbase.com/expert1/knowledge5.jar
which would instruct the rule compiler to load the classes from this
jar to its compile class path, and which would instruct the class
loader used to load the user's classes to delegate to the loader used
to load the jar file's classes? [Note that a given expert's knowledge
should only be loaded by one classloader.]

I browsed through the API and discovered that the package builder
configuration allows the specification of a class loader. Does this
mean the rule compiler will attempt to load related classes through
this loader as well? If so, it would need to use reflection to examine
those classes (which I'd find unlikely). If not, is there a way to
control the rule compiler's compiler classpath?

In our current prototype, we need to load everything through the
application classpath to avoid compile errors.

- Godmar
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  Edson Tirelli
  Software Engineer - JBoss Rules Core Developer
  Office: +55 11 3529-6000
  Mobile: +55 11 9287-5646
  JBoss, a division of Red Hat @ www.jboss.com
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