[rules-dev] Classpath Resources and Classloaders using cache

[Esteban Aliverti]

Hi Guys,

I want to discuss a problem I have found when using the combination of
knowledge agent + classpathResources.
I will try to describe what am I doing first to give you some context.
I'm deploying drools-camel-server in a Tomcat 7 container. Inside the
WEB-INF/classes directory I have some DRL files that I want to use.
My knowledge-services.xml file declares the following kagent:

<drools:kagent id="kagent1" kbase="kbase1" new-instance="false">
<drools:resources>
             <drools:resource type="DRL" source="*classpath*:simple.drl"/>
             ...
    </drools:resources>
</drools:kagent>

When spring parses this configuration file it creates a KnowledgeAgent
instance with a ChangeSet containing all the listed resources.
The next step is to start ResourceChangeNotifierService
and ResourceChangeScannerService.
So far so good.

The problem:
The problem I'm having is not directly related to drools, but I think it is
quite easy to provide a solution for the people that is in my same
situation.

ClassPathResource is the class that represents a resource defined as "*
classpath:"*

This class has 2 important methods:

public long getLastModified(){
  return this.classLoader.getResource( this.path
).openConnection().getLastModified();
}

public InputStream getInputStream(){
  return this.classLoader.getResourceAsStream( this.path );
}


The first method is used by ResourceChangeScannerService to check whether
the resource has changed or not. It works fine. When the resource in the
filesystem changes, the scanner detects the change without any problem.
The scanner ends up notifying the kagent about the change, and the kagent
passes the Resource to an instance of KnowledgeBuilder.
An here is when things fail.
The kbuilder uses the second method of ClassPathResource (getInputStream())
to get the content of the resource. In the case of Tomcat (and probably some
other environments), it seems that the classloader (Tomcat's classloader) is
using a cache. So the InputStream returned doesn't reflect the current state
of the resource.
Long story short: the agent is notified about a change in the resource, but
the change is never applied to the kbase because the kbuilder is unable to
get it :P

Solutions:
The first solution is not to use classpath resources :). You can use just
url resources like http:// or file:/. But honestly, when you have your rules
inside your webapp, it is much more comfortable and even manageable to avoid
the use of real paths.

What I was thinking about (I already have a working prototype) is to create
a new Resource type for these cases. This resource type will let you define
your resources present in your classpath as usually but it will translate
them to URL Resource internally.
So, in the example above:

<drools:resource type="DRL" source="*URLClasspath*:simple.drl"/>

is going to be translated (internally and in a transparent way) to something
like: file:/usr/local/apache-tomcat-7/webapps/MyWebapp/WEB-INF/simple.drl.

Opinions?

XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

Esteban Aliverti
- Developer @ http://www.plugtree.com
- Blog @ http://ilesteban.wordpress.com
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