springboot Provide a way to customize layers with the Maven plugin

Comment From: philwebb

I'd been thinking about the layers configuration during the design of the API. Its unfortunately really hard to come up with a logic way to do it. I was originally thinking that we could define the layers in the oder that they should appear and define the items that they capture. For example:

<layers>
    <layer>
        <name>my-libs</name>
        <libs>
            <lib>org.acme:my-lib:*</lib>
        </libs>
        ...
    </layer>
</layers>

The problem with this approach is it's quite hard to define catch-all patterns that are out of order. For example, if we want to recreate ImplicitLayerResolver in config it's quite hard. We want the dependencies layer above the snapshot-dependencies but we want it to contain anything that the snapshot-dependencies doesn't capture.

The best option I could come up with is to define the layer names and their order separately from the pattern matching config.

<layer-config>
    <layers>
        <layer>dependencies</layer>
        <layer>snapshot-dependencies</layer>
        <layer>resources</layer>
        <layer>application</layer>
    </layers>
    <contents>
        <content>
            <libraries>
                <library>*:*:*SNAPSHOT</library>
            </libraries>
            <destination>snapshot-dependencies</destination>
        </content>
        <content>
            <libraries>
                <library>*:*:*</library>
            </libraries>
            <destination>dependencies</destination>
        </content>
        <content>
            <resources>
                <resource>static/*</resource>
            </resources>
            <destination>resources</destination>
        </content>
        <!-- anything not caught ends up in the last layer -->
    </contents>
</layer-config>

I'm not totally convinced that this isn't too confusing, but I'm not sure how else we can support complex matching rules.

Comment From: snicoll

The more I think of it and the more I find some analogy with the maven-assembly-plugin. The Maven dev team decided to go for an external descriptor which has several advantages.

A separate XML file tailored for layering can have a dedicated XSD with documentation. Maven is pretty limited with regards to documenting complex POJO structure (and I doubt that all IDEs have the same level of support in auto-completion).

The assembly plugin also has a concept of predefined descriptors that would match with opinions in terms of layering.

Comment From: mbhave

Regarding the ordering of the layers, I wonder if there's a way to have an optional configuration option that lets you define the order if is is different from the order that they were defined in <layers>.

Comment From: wilkinsona

<layers>
    <layer>dependencies</layer>
    <layer>snapshot-dependencies</layer>
    <layer>resources</layer>
    <layer>application</layer>
</layers>

I find this ordering confusing. I'd expect the layers to be listed from most to least frequently changing but the above is the reverse of that. In other words, I think of the layers as a representation of what the image will look like, rather than a representation of the order in which they'll be written.