[jira] Commented: (MAVEN-1656) Doco that explains the protocols required to allow maven.xml or any plugin.jelly to inject values into or retrieve values from another plugin

["Brett Porter (JIRA)" <ji...@codehaus.org>]

    [ http://jira.codehaus.org/browse/MAVEN-1656?page=comments#action_43970 ] 

Brett Porter commented on MAVEN-1656:
-------------------------------------

explanation I gave Andy. To date I haven't been able to distill this into a user readable form:

Variables in a plugin are in its own, separate, jelly context.
However, this context is only created when the plugin is first loaded,
which is when it is first used.

Unfortunately, maven:set does not initialise the plugin context. It
was intended to, but caused some other problems that are documented in
the JIRA issue. So, you cannot use maven:set (or get) before the
plugin context is initialised.

When the plugin context is initialised, it uses the variables from the
current context, so anything set with j:set (no scope) is added. So
using this before the plugin gets used works, but not afterwards.
Using scope="parent" always works, as it puts it into the root
context, shared by all - as long as the plugin has not overridden the
variable in its own context.

The plugin is initialised when:
a) the goal you are in has a preGoal on a goal from that plugin
b) attainGoal has been called for that plugin
c) you have a namespace dependency on the plugin (This seems to have
been unreliable and only work if you actually use a tag from the
plugin though - hence the dependency handle)

It is much better to use a or b if possible, then use maven:set/get
exclusively. If you know the plugin has not been initialised and need
to set a var, you must use j:set without a scope. j:set with a scope
of parent is a last resort that will work in most but not all
situations.

It's a mess, and that's why m2 was a rewrite :)

> Doco that explains the protocols required to allow maven.xml or any plugin.jelly to inject values into or retrieve values from another plugin
> ---------------------------------------------------------------------------------------------------------------------------------------------
>
>          Key: MAVEN-1656
>          URL: http://jira.codehaus.org/browse/MAVEN-1656
>      Project: maven
>         Type: Bug
>   Components: documentation
>     Versions: 1.1-beta-1
>  Environment: All
>     Reporter: Andy Glick

>
>
> Brett recently posted a suggestion to someone who asked how to access the contents of "plugin a" from the context of "plugin b" or from maven.xml. Brett stated that "plugin a" would have to be initialized before it could be accessed, and that from what I could understand he recommended 1 or 2 ways to do that:
> 1) reference the artifact plugin's namespace in the project tag 
> 2) specify <j:set target="plugin-name" scope="parent" var="some.plugin.var" value="some.value"/>  (I think that this is the proper syntax, but I'm not sure :-).
> Thierry Lach, who asked the question this time around, reported that using the artifact namespace didn't work in his instance, but that setting the scope to parent did. I'm hoping that we can put together a fairly comprehensive explanation about what is going on here, and if that means explaining why the project gave up on Jelly and switched to M2, then so be it.
> See http://news.gmane.org/gmane.comp.jakarta.turbine.maven.user for the postings
> Given that during the last couple of months Vincent Massol recommended that people use the now deprecated <dependency-handle> mechanism on the mavenbook.org site (see Tip #2), I should think that there ought to be a priority on documenting this issue in an obvious place, probably the FAQ, but also on the Maven Jelly tags get and set entries. 
> If this is actually explicitly documented somewhere, I'm sorry to have wasted anyone's time. I have seen several plugins that have successfully used the artifact namespace tag, so there must be some way that people have learned this. I simply wish that I knew how they had done so. ;-) 
> I'm willing to write the doco if someone would explain the details.

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