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From "Richard S. Hall" <he...@ungoverned.org>
Subject Re: Enhanced Apache Felix / JRebel integration
Date Fri, 30 Mar 2012 18:49:07 GMT
On 3/30/12 9:14, Robert Munteanu wrote:
>
>> -----Original Message-----
>> From: Richard S. Hall [mailto:heavy@ungoverned.org]
>> Sent: Thursday, March 29, 2012 6:53 PM
>> To: dev@felix.apache.org
>> Subject: Re: Enhanced Apache Felix / JRebel integration
>>
>> On 3/29/12 6:45, Robert Munteanu wrote:
>>> Hi,
>>>
>>> I am looking into enhancing the integration between Apache Felix and
>> the JRebel [1] JVM agent. If you're not familiar with JRebel, it's a
>> java agent which allows classes loaded into the JVM to be redefined as
>> soon as they are recompiled e.g. in an IDE. For more information about
>> its features, see [2].
>>> JRebel already has basic support for Apache Felix by allowing classes
>> for a deployed bundle to be reloaded without requiring a bundle
>> repackage and reinstall.
>>> What I want to contribute is support for components managed using
>> declarative services. The basic principle is that given a bundle which
>> contains service components descriptors, watch for changes to those
>> component descriptors and refresh the components when the respective
>> descriptors have changed.
>>> I have taken a quick look at both JRebel and Felix to see how an
>> implementation should look like. JRebel offers support for detecting
>> class changes, but not generic filesystem changes. Therefore I'd have
>> to rely on Felix code ( in org.apache.felix.scr ? ) to detect
>>> - when a bundle with component descriptors is deployed and start
>> watching for changes to the descriptors
>>> - when changes to the mentioned descriptors are detected refresh the
>> components
>>
>> Is there some reason you need to refresh at the class level as opposed
>> to the bundle level? If the bundle level was sufficient then it seems
>> like this scenario becomes much simpler.
> My intention is to first refresh the whole bundle when a component descriptor is changed.
If needed I will look into fine-grained refreshes but that's not on my immediate plan.

Refreshing the bundle would definitely be easier.

>
>> Also, what do you mean by "refresh the components" above? Do you
>> specifically mean re-load the class bytes? If so, how do you know that
>> changes to the descriptor require re-loading the class bytes or are you
>> just assuming it does?
> By refresh I mean a deactivate/activate operation taking into account whether a component
is marked as immediate or not. Fortunately refreshing the actual class bytes is handled by
the 'core' JRebel functionality when the class file changes.
>
>> Additionally, it seems you might be confused about the responsibility
>> of
>> some tasks. For example, SCR doesn't look for changes in component
>> descriptors at all, it simply listens for bundles to be activated.
>> Listening to changes in the component descriptor would have to hook
>> into
>> the build process somehow. By and large, none of the Felix subprojects
>> are involved in the build process other than the Maven Bundle plugin.
> I have considered hooking into the maven-bundle-plugin as well, but I have no idea on
how to notify Felix that a bundle needs to be refreshed or even send a custom event which
I can handle myself.

If you use something like File Install and generate bundle's into a 
directory managed by File Install, it will automatically update and 
refresh the bundle when its JAR file changes, which would cause SCR to 
stop, then restart managing it. It's a little more coarse grained that 
what you want, but it should work out of the box with a lot less effort.

-> richard

>
> Thanks,
>
> Robert
>
>> ->  richard
>>
>>> I would like to validate that this is at all possible within Apache
>> Felix and to ask which are the best places to start looking for adding
>> the JRebel functionality. Any thoughts/pointers on how to best start
>> developing this would be greatly appreciated.
>>
>>> If this is feasible, I intend to develop this as a separate JRebel
>> plugin and contribute it to the Apache Felix project.
>>> Thanks,
>>>
>>> Robert
>>>
>>> [1]: http://zeroturnaround.com/jrebel/
>>> [2]: http://zeroturnaround.com/jrebel/features/
>>>

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