[rules-dev] Classpath Resources and Classloaders using cache

[Geoffrey De Smet]

> The kbuilder uses the second method of ClassPathResource 
> (getInputStream()) to get the content of the resource.

Shouldn't that also supply a ClassLoader or a Class (of which 
Class.getClassLoader() is used)?

> This resource type will let you define your resources present in your 
> classpath as usually but it will translate them to URL Resource 
> internally.
It's probably dangerous to store the native URL the concept of a 
classpath is designed to have a map of files "that are just there" and 
you don't need to worry what's the OS-specific underlying details.
Instead, I believe, the classpath key (for example "simpler.drl") and 
the ClassLoader (for example MyApp.class.getClassLoader()) should be stored.

Op 15-04-11 13:52, Esteban Aliverti schreef:
> 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 <http://www.plugtree.com>
> - Blog @ http://ilesteban.wordpress.com
>
>
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-- 
With kind regards,
Geoffrey De Smet

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