[rules-dev] Classpath Resources and Classloaders using cache

[Mauricio Salatino]

Can you? or Can't you?


On Sat, Apr 16, 2011 at 4:24 PM, Pablo Nussembaum <baunax at gmail.com> wrote:

>  Esteban,
> You can assume that a resource that was obtained from the classpath exists
> in your filesystem, for instance it can be a file inside a jar or war that
> are not exploded. In other words you can't always convert an URL to
> "file://".
>
> --
> Bauna
>
>
> On 04/15/2011 08:52 AM, Esteban Aliverti wrote:
>
> 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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>


-- 
 - CTO @ http://www.plugtree.com
 - MyJourney @ http://salaboy.wordpress.com
 - Co-Founder @ http://www.jbug.com.ar

 - Salatino "Salaboy" Mauricio -
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