Add a velocity template based generator

[gnodet]

extracted from Maven 4.0.0-alpha-2 https://github.com/apache/maven/tree/c07700ffc97d3967a8de77d5a1065f5e6115c00e/api/modello-plugin-velocity

https://maven.apache.org/ref/4.0.0-alpha-2/maven-api/modello-plugin-velocity/index.html

templates: https://github.com/apache/maven/tree/c07700ffc97d3967a8de77d5a1065f5e6115c00e/maven-model/src/main/mdo
plugin configuration: https://github.com/apache/maven/blob/c07700ffc97d3967a8de77d5a1065f5e6115c00e/maven-model/pom.xml#L77

[sonatype-lift]

⚠️ 14 God Classes were detected by Lift in this project. Visit the Lift web console for more details.

[hboutemy]

I'm trying to guess what this does Modello generator does, to be able to at least describe it before going more in depth into documenting it

it was created in Maven, used for Maven model:

• https://github.com/apache/maven/blame/master/maven-model/pom.xml#L78
• uses .vm templates https://github.com/apache/maven/tree/master/maven-model/src/main/mdo
• to replace previous fully generated content https://github.com/apache/maven/blame/1ca65c79fa4e25c4b7da027c8f8eef1b95ceced4/maven-model/pom.xml

[hboutemy]

it's not clear to me why this is a new Maven plugin instead of a Modello plugin that can be used by Modello Maven plugin https://codehaus-plexus.github.io/modello/

[gnodet]

it's not clear to me why this is a new Maven plugin instead of a Modello plugin that can be used by Modello Maven plugin https://codehaus-plexus.github.io/modello/

It's not a new maven plugin, but s a new modello plugin. It is used with the modello maven plugin. See https://github.com/apache/maven/pull/848/files to see how it will be used.
In maven 4.x, it is currently a standalone plugin, but if we want to integrate it into the modello, a modello plugin makes more sense, and that's what this PR is about.

[gnodet]

I've added some javadocs.
Note that using the velocity generator is more like writing a generator. And modello has not much documentation about how to use its model. The Helper helps a bit for accessing a few common things.

[kwin]

Thanks for all the updates. LGTM now.

[gnodet]

@hboutemy are you ok ?

[hboutemy]

@gnodet nice documentation, I'm starting to understand
adding an integration test in https://github.com/codehaus-plexus/modello/tree/master/modello-maven-plugin/src/it will help showing classical use cases

[hboutemy]

@gnodet looking at the only use case I know of https://github.com/apache/maven/tree/master/maven-model/src/main/mdo
I'm worried on the use case: do you really want to copy paste all the generation code from Modello and tweak it by hand?
I understand that it's easy to prototype some updates to the generated java classes, but I fear it will be unmaintainable

[gnodet]

Those are mostly new generators not supported by modello. The v4 model is immutable which is not supported, the v3 wraps the immutable v4 model, which is not supported, the reader returns the v4 immutable model , which is not supported, the transformer did not exist in maven 3.x, and the merger was hand written... So that's not tweaking the modello code, those are really new generators.
So given those are new generators, we'd have to reimplement them all in modello.

[gnodet]

However, there's still a problem with the current state in that the templates are duplicated instead of being reused. This need to be fixed.

[gnodet]

I had started working on an IT a few weeks ago, but my laptop died with the unfinished work on it (and I can't even turn that laptop on anymore....). I'll see if I can give in another try in the coming weeks.

[hboutemy]

I find the term "redirect" confusing: makes me think HTTP redirect.
Can we rename to something like "OUTPUT"?

[hboutemy]

I added documentation: it helps me understand the feature sufficiently to do the release (let's consider Maven core usage is an IT for now, I'll see later about making an internal IT)

the only little thing that I would love to change is #MODELLO-VELOCITY#REDIRECT: #MODELLO-VELOCITY#SAVE-OUTPUT-TO?

[michael-o]

What is the benefit of this over the other components?

[gnodet]

What is the benefit of this over the other components?

Ease of maintenance. Each generator can be easily replaced by a simple template that is easy to understand and modify. Also, the user can easy author or modify templates, while existing generators can not.

[michael-o]

What is the benefit of this over the other components?

Ease of maintenance. Each generator can be easily replaced by a simple template that is easy to understand and modify. Also, the user can easy author or modify templates, while existing generators can not.

So this is a replacement for the Java code?

[gnodet]

What is the benefit of this over the other components?

Ease of maintenance. Each generator can be easily replaced by a simple template that is easy to understand and modify. Also, the user can easy author or modify templates, while existing generators can not.

So this is a replacement for the Java code?

Not really, unless we ship the VTL templates and lock them. The current code is in maven master and was needed in order to be able to generate the v4 model, the v4 readers, the v3 models wrapping the v4 models, the mergers, etc... I originally started by hacking the current generators, but it was too complicated and time consuming, so I ended up writing this template generator. So all the generators could be rewritten based on a template I suppose, but this PR is here due to @hboutemy request to move it from the maven master branch to the modello project where it actually belongs.

[hboutemy]

So this is a replacement for the Java code?

yes and no

yes, it is an ultra flexible replacement for the classical generated code by existing Velocity plugins, because the new immutable Maven API creates new constraints with its non-immutable part: I'm also describing the way this new Maven API is built apache/maven#830

but in fact, this Velocity plugin ships no real code generator, the code is in the Velocity templates: each user can choose to use the classical Velocity plugins to get classical fully-Modello generated Java code (we don't remove the classical code generators), or they can choose to create their own Velocity templates to generate the code they want

In Maven core, the new Maven API forced Guillaume to create new code generators with custom Velocity templates https://github.com/apache/maven/tree/c07700ffc97d3967a8de77d5a1065f5e6115c00e/maven-model/src/main/mdo , but classical Modello plugins are used to generate xdoc and xsd

To me, this Velocity plugin is ready to merge, we just need to rename #MODELLO-VELOCITY#REDIRECT to something like #MODELLO-VELOCITY#SAVE-OUTPUT-TO or any better idea welcome

[gnodet]

To me, this Velocity plugin is ready to merge, we just need to rename #MODELLO-VELOCITY#REDIRECT to something like #MODELLO-VELOCITY#SAVE-OUTPUT-TO or any better idea welcome

I don't have commit rights in this repo, so I'm not able to merge it.

[kwin]

I don't have commit rights in this repo, so I'm not able to merge it.

Just squashed and merged it.