On Fri, Jul 31, 2009 at 3:10 PM, <mazz@redhat.com> wrote:

(be prepared: this is a long email :)

First of all, the plugin/classloading system is not as robust as the Eclipse plugin system, so please don't assume if you can do it in Eclipse, you can do it in the Jopr plugin system :-)

Second, when you say:

"This package is not declared in Tomcat's <plugin name... package="org.jboss.on.plugins.tomcat" declaration and therefore it won't be loaded by the Axis plugin class loader."

this is an incorrect assumption. The package= attribute has no bearing on the classloading or what classes get shared. This is only used to allow for "shorthand" classnames in the descriptor. Any relative classnames (not fully qualified) specified in the descriptor will be assumed to be in that package. That is all it is used for.

Third, yes there is this concept of transative deps such that if you have plugin A depend on plugin B depend on plugin C then C classes are in A (because C's classloader is the parent of B's classloader which is in turn the parent to A's classloader).

Forth, if you have this:

<depends plugin="A" />
<depends plugin="B" useClasses="true" />

The only dependency on plugin "A" is its metadata - NOT its classes. In the Jopr plugin system, you can only have a "useClasses" dependency on ONLY ONE plugin. So do not assume you can access "A"s classes in your plugin - you can only inherit its metadata and resource types. You do have access to Plugin B's classes in addition to its metadata/resource types.

In the new classloader stuff introduced in trunk (not in any releases yet but it is explained in that wiki page I posted earlier), think of <depends> as a "required" dependency - i.e. your plugin will NOT be able to be deployed UNLESS plugins A and B are deployed. Think of the <runs-inside> as an inferred dependency - called an "optional dependency". If you only have an optional dependency on plugin "A" (i.e. you have a <runs-inside> on a plugin "A" resource type) you don't have to have plugin A deployed - its "optional". The old way (in the current release) you have to have a <depends> for any external resource type you specify in the descriptor. This forces you to ensure that "A" and "B" are deployed along with your plugin (which sometimes you don't want to require, which is one reason for the new way we implemented).

However, the new way still has the restriction of only ONE <depends> can have a "useClasses". That sounds restrictive, and it is, but in our experience it usually is not a blocker because usually, plugins that depend on other plugins can usually rely on stacked/transative dependencies. For example, you have this:

<depends plugin="JMX" />
<depends plugin="Tomcat" useClasses="true" />

You do not need to specify the dependency on JMX. Since Tomcat has a <depends plugin="JMX">, you pick it up by <depend>ing on Tomcat. You don't have to specify it too.

Your component code is probably going to monitor Axis using JMX - so you can use the JMX components and classes to do the work, in the same way Tomcat plugin monitors other Tomcat things. And because you are depending on Tomcat, you can INJECT your resource types directly into the Tomcat type hierarchy. This sounds like the piece you are missing. There are two types of extending the type hierarchy in Jopr - Injection and Extension. You want Injection because you want to INJECT a "Axis" server type inside the Tomcat Server.

For more on this Injection/Extension stuff, see: http://jopr.org/confluence/display/RHQ/AMPS-Plugin+Extensions

So, in your descriptor, I suspect you'll want something like this:

<depends plugin="Tomcat" />

<server name="Axis"
discovery="org.rhq.plugins.jmx.MBeanResourceDiscoveryComponent"
class="AxisComponent"
description="Embedded Axis server running in Tomcat">

<runs-inside>
<parent-resource-type name="Tomcat Server" plugin="Tomcat"/>
</runs-inside>

<plugin-configuration>
<c:simple-property name="objectName" readOnly="true" default="..the Axis MBean ObjectName here..."/>
<c:simple-property name="nameTemplate" default="Axis"/>
<c:simple-property name="descriptionTemplate" default="Embedded Axis server running in Tomcat"/>
</plugin-configuration>

... your metric definitions, operations, etc. that you want to support for managing Axis ...
</server>

If you still can't get it, I'd be interested in helping you get off the ground with this. Feel free to email me your current plugin (zipped up with the source) and I'll take a quick look.

Also, let me know what version of Jopr you are using.

Thanks,
John Mazz

----- Original Message -----
From: "Bruno Wassermann" <bruno.wassermann@googlemail.com>

To: "John Mazzitelli" <mazz@redhat.com>
Cc: "jopr-dev" <jopr-dev@lists.jboss.org>
Sent: Friday, July 31, 2009 6:36:30 AM GMT -05:00 US/Canada Eastern
Subject: Re: [jopr-dev] Plugin Dependencies

Okay, I think I figured it out. The Tomcat plugin class I want to reuse in the Axis plugin is in package org.jboss.on.plugins.tomcat.helper. This package is not declared in Tomcat's <plugin name... package="org.jboss.on.plugins.tomcat" declaration and therefore it won't be loaded by the Axis plugin class loader.

Two observations. One could argue that I shouldn't write Axis code as a separate plugin, but rather include it as a server in the Tomcat plugin. That would solve the problem and seems to follow the approach generally taken in Jopr. In that case, is there a guarantee that all Tomcat instances will have been discovered, before the discovery component for Axis servers gets called? If not, how do you deal with that?

Second, it might be nice to allow plugin developers to specify the packages they want exposed to other plugins (i.e. that will be loaded by A's plugin class loader, if it depends on B). If someone has already implemented some useful feature, it would seem better to reuse it instead of duplicating the code. There is a similar feature for Eclipse plugins. However, with Eclipse you will usually build a reference implementation exercising your plugins' extension points and hence have an opportunity to figure out which packages you need to expose like that and which ones can remain internal. How a developer of Jopr plugins can figure this out is not entirely clear.

-- Bruno

On Fri, Jul 31, 2009 at 10:58 AM, Bruno Wassermann < bruno.wassermann@googlemail.com > wrote:

Hm,

I checked the plugin name in my depend, added an explicit useCases=true, but it still doesn't have TomcatConfig loaded at runtime. Is there, like for Eclipse plug-ins, a concept of internal and external (visible to other plugins that have some relationship with this plugin) classes and packages? I haven't seen any evidence for that, but just checking.
Are dependencies transitive? If we have plugins A, B, C and B depends on C and A depends on B and C, is it sufficient for A to declare its dependency on B with the dependency on C being implied?

Here's an excerpt from Axis' rhq-plugin.xml. Maybe you can spot something obvious that's wrong here?

<plugin
name="Axis"
displayName="Axis Server"
package="uk.ac.ucl.cs.sse.plugins.axis"
description="Discovery, measurment and management of Axis and deployed Web services"
xmlns:xsi=" http://www.w3.org/2001/XMLSchema-instance "
xmlns="urn:xmlns:rhq-plugin"
xmlns:c="urn:xmlns:rhq-configuration">

<depends plugin="JMX" />
<depends plugin="Tomcat" useClasses="true" />

<server
name="Axis Server"
...

-- Bruno

On Fri, Jul 31, 2009 at 3:41 AM, < mazz@redhat.com > wrote:

It sounds like what you are doing is correct. If you have a plugin "A" and it has some classes that you want available to plugin "B", then plugin "B"'s descriptor should have a <depends> tag on plugin "A":

<plugin name="B" ...>
<depends plugin="A" useClasses="true" />
...
</plugin>

What this does is put A's plugin classloader as the parent to B's plugin classloader. You do not have to do anything else other than put that <depends> in your descriptor (no need to put the plugin jar in your plugin jar's lib directory).

FYI: The useClasses, if not defined, is inferred on the last <depends> tag listed in the descriptor. If you only have one <depends> tag, its useClasses is assumed true if its not specified.

BTW: recently, alot of work has gone into trunk that performs more classloading "stuff". See: http://jopr.org/confluence/display/RHQ/Plugin+Dependencies+and+Class+Loaders

----- Original Message -----
From: "Bruno Wassermann" < bruno.wassermann@googlemail.com >
To: "jopr-dev" < jopr-dev@lists.jboss.org >
Sent: Thursday, July 30, 2009 5:46:05 PM GMT -05:00 US/Canada Eastern
Subject: Re: [jopr-dev] Plugin Dependencies

Whoops, it should rather be <depends plugin="xyz" useClasses="true/>, right? Sorry about the premature question :(

-- Bruno

On Thu, Jul 30, 2009 at 10:41 PM, Bruno Wassermann < bruno.wassermann@googlemail.com > wrote:

Hi,

This may be a typical newbie question...

I want the Axis plugin to be able to reuse some classes defined in the Tomcat plugin, namely TomcatConfig. To achieve this lofty goal I add the following to Axis.rhq-plugin.xml: <depends plugin="Tomcat"/>.
However, at runtime the class loader for the Axis plugin doesn't seem to have loaded Tomcat's classes. Asking the agent to run a discovery scan, the Axis plugin reports a NoClassDefFoundError for org.jboss.on.tomcat.helper.TomcatConfig.

What am I getting wrong here? Do I have to manually add the Tomcat plugin's jar file to my plugin?

Many thanks,

-- Bruno

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