Status

This RFC is currently in the DRAFT state. Nothing in this RFC has been agreed or confirmed.

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Introduction

Background

Maven uses the Project Object Model as a descriptor for the declarative build requirements of a project.

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• Jira
  server ASF JIRA serverId 5aa69414-a9e9-3523-82ec-879b028fb15b key MNG-4506

   (probably could be handled as maven-site-plugin configuration) looks to allow defining a different site deployment URL for SNAPSHOT versions of the project compared with release versions)

• Jira
  server ASF JIRA serverId 5aa69414-a9e9-3523-82ec-879b028fb15b key MNG-2216

   looks for the ability to define the default encodings to be used when reading files (and optionally when writing files)

• Jira
  server ASF JIRA columns key,summary,type,created,updated,due,assignee,reporter,priority,status,resolution serverId 5aa69414-a9e9-3523-82ec-879b028fb15b key MNG-3608

   looks for the ability to define the default encodings to be used when writing the site reporting files

  4149

   Extend POM to support encoding parameter per (test) resource ( project.build.resources[].resource.sourceEncoding )

• Jira
  server ASF JIRA serverId 5aa69414-a9e9-3523-82ec-879b028fb15b key MNG-3608

   looks for the ability to define the default encodings to be used when writing the site reporting files

The The following issues concern providing explanations of dependencies within the POM

...

• Jira
  server ASF JIRA serverId 5aa69414-a9e9-3523-82ec-879b028fb15b key MNG-5102

   looks for general purpose mix-ins

• Jira
  server ASF JIRA serverId 5aa69414-a9e9-3523-82ec-879b028fb15b key MNG-5588

   looks for an explicit pluginManagement scoped mix-in

Proposal

Existing model

The existing 4.0.0 model POM has the following high-level structure:

<project xmlns="http://maven.apache.org/POM/4.0.0"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
                      http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
 
  <!-- The Basics -->
  <groupId>...</groupId>
  <artifactId>...</artifactId>
  <version>...</version>
  <packaging>...</packaging>
  <dependencies>...</dependencies>
  <parent>...</parent>
  <dependencyManagement>...</dependencyManagement>
  <modules>...</modules>
  <properties>...</properties>
 
  <!-- Build Settings -->
  <build>...</build>
  <reporting>...</reporting>
 
  <!-- More Project Information -->
  <name>...</name>
  <description>...</description>
  <url>...</url>
  <inceptionYear>...</inceptionYear>
  <licenses>...</licenses>
  <organization>...</organization>
  <developers>...</developers>
  <contributors>...</contributors>
 
  <!-- Environment Settings -->
  <issueManagement>...</issueManagement>
  <ciManagement>...</ciManagement>
  <mailingLists>...</mailingLists>
  <scm>...</scm>
  <prerequisites>...</prerequisites>
  <repositories>...</repositories>
  <pluginRepositories>...</pluginRepositories>
  <distributionManagement>...</distributionManagement>
  <profiles>...</profiles>
</project>

The major critiques of the existing model are:

• Overly verbose and repetitive - the main pain point is that the groupId, artifactId, etc are not specified as attributes
• "I hate XML" - our current thinking is that this is really just a catch-all complaint from people who:
  • Don't like the schema / feel the schema is overly verbose
  • Want to produce an imperative build from a declarative build tool
  • Are tolling for fun and profit
• Poorly specified dependency graph resolution
• "Magic" inheritance - it can be difficult to determine how inheritance will affect the build.

The other issue with the existing model is that it is being used for two distinct purposes and as such finds it difficult to be a master of both:

• The 4.0.0 POM serves as a declarative description of the build process for a project
• The 4.0.0 POM serves as a description of the project dependency graph.

The vision of the 4.0.0 POM was that all projects would be cut from a series of standard templates (a.k.a. packaging):

• Each template would define the appropriate lifecycles and phases of those lifecycles (hopefully most templates/packagings would be sufficiently served by the three default lifecycles: default, clean and site) and each template/packaging would define the plugin bindings against the lifecycle phases. 
• Where a project needed a customized build process, the build engineer would initially explore how to develop the build process by customizing the bindings of an existing template/packaging.
• Once the build engineer had determined the correct generic process for building this type of project, the build engineer would then solidify this build process into a custom template/packaging.

In this vision, almost all 4.0.0 POMs should basically consist of the following structure:

<project>
  <modelVersion>4.0.0</modelVersion>
  <parent> <!-- most projects should inherit from a parent pom of some sort --> 
    <groupId>...</groupId>
    <artifactId>...</artifactId>
    <version>...</version>
    <relativePath/>
  </parent>
  <artifactId>...</artifactId> <!-- most projects should inherit the parent's groupId -->
  <version>...</version>
  <packaging><!-- THIS IS THE IMPORTANT BIT--></packaging>
  <dependencies>
    ... <!-- Here is the project dependencies -->
  </dependencies>
  <build> <!-- no custom plugin configuration or bindings -->
    <extensions> <!-- this is only needed if not inherited from the parent -->
      <extension>
        <groupId>...</groupId>
        <artifactId>...</artifactId>
        <version>...</version>
      </extension>
    </extensions>
  </build>
</project>

In other words, when using the 4.0.0 POM in accordance with its initial vision, there should be at most 25 lines of boilerplate above the specification of the project dependencies and in the ideal case that boilerplate can be reduced to ~14 lines which specify:

• the versions of Maven which the POM is compatible with (the modelVersion)
• the parent to inherit from (3 lines of information due to the use of XML elements instead of attributes)
• the identity of this project (2 lines of information if inheriting the groupId from the parent)
• the template/packaging that this project is built with

When we inspect real world POMs however, we see that this pattern is almost never followed. Instead of producing custom templates/packaging most projects instead just fight with a standard template/packaging. The end result of this kind of fighting is POMs that run into the 10,000+ LOC levels with many plugin bindings and overloading of an existing lifecycle binding and profiles used to enable additional side-build processes. The reasons cited for these long POMs include:

• "It is too hard to make a custom template/packaging"
• "This is a one-off project, we will never make another of this type, therefore it doesn't make sense to produce a custom template/packaging" 

Changes

This section details the rationale for all the changes to the POM format.

Dual usage

The most important change for the 5.0.0 POM is to split the dual usage:

• The 5.0.0 POM will be used as a declarative description of the build processes of the project. 
• The description of the project artifact dependency graphs will be provided by the Project Dependency Trees schema proposal.

DECISION: The POM is for Building, the Project Dependency Trees is for consumption of artifacts

AFFECTS: 

XML vs custom DSL

The project dependency trees schema will be XML because that is designed to be a machine generated document that is for consumption primarily by machines but needs to remain easily parsable by humans. The choice of XML is dictated by the requirement to enable multiple tools to have a level of forward compatibility and, at this time, the only cross-technology tool that can deliver a mapping is XSLT. For this reason the Project Dependency Trees schema will be an XML format.

As the 5.0.0 POM will only be used by Maven, and as the 5.0.0 POM will require Maven 5.0+ to build, there is no longer a strict requirement to retain the XML format for the 5.0.0 POM.

There are, however, a number of advantages to continuing with the XML based format at least for the 5.x release train of Maven:

• Most editors already have syntax highlighting and completion support for XML, e.g. when closing an element
• A significant number of editors can use the XML schema to provide enhanced completion support, e.g. providing contextual suggestions for elements and attributes
• Familiarity of the existing user base, e.g. the current users of Maven are already used to the XML based-syntax
• Reduces the number of code paths to allow Maven 5.0+ to parse the 4.0.0 POM, e.g. it will significantly aid adoption of Maven 5.0+ if you can build Maven 2/3 projects with Maven 5.0+

The single biggest reason for retaining XML, however, is that we expect the build model will need to evolve. With the Project Dependency Trees schema, we need to provide for backwards compatibility (i.e. newer clients need to be able to parse older schemas) and limited forward compatibility (i.e. older clients need to be able to parse newer schemas). With the POM, we only need to provide for limited backwards compatibility (i.e. newer versions of Maven need to be able to parse a defined range of older schemas) without forwards compatibility (i.e. older versions of Maven will not be able to build newer POM modelVersions). Ideally we want the range of backwards compatibility to reach back as far as the 4.0.0 POM. Retaining XML as a POM format allows for technology such as XSLT to be used to map say a 5.0.0 POM into a 5.3.0 POM which would thus reduce the number of parsers that would be required to be included within Maven (we will still need a custom parser for the 4.0.0 POM, and it is likely that Maven 6.0+ would need custom two parsers one for the 4.0.0 POM and one for the 5.x.y POMs). A custom DSL would force tool vendors to produce syntax parsers for each and every model version.

DECISION: The 5.0.0 POM will be XML

AFFECTS: 

Elements vs Attributes

There seems to be universal agreement to use attributes where possible. The reason for choosing elements in the 4.0.0 was purely a technical limitation of the Modello toolchain at the time.

DECISION: The 5.0.0 POM will use XML attributes for data that cannot have child data. At a minimum the groupId/artifactId/platformId/version/classifier/type information of project/parent/dependencies/plugins/extensions will be defined using attributes.

AFFECTS: 

Customizing build behavior / One-off projects

The term packaging in the 4.0.0 POM is used for two distinct purposes: defining the type of the primary artifact and defining the base template of the build process. The Project Dependency Trees schema removes the concept of a primary artifact by providing the dependency trees of all attached artifacts, thus a single Maven project that produces a .jar, .war and even say a secondary "skinny" .war will have the appropriate dependency trees for each artifact declared in the PDT. This contrasts with the 4.0.0 POM which only defined the dependencies of the primary artifact and relied on build tooling convention to infer what contextual transitive dependencies should be extracted by consumers from the POM.

Thus, in the 5.0.0 POM we only really want to specify the template for the lifecycles and default bindings. Given that the use case for this data is purely as a template, it makes sense to change the name to template.

DECISION: The 5.0.0 POM will use the term template rather than packaging.

AFFECTS: 

One of the requirements that a lot of projects have is cross-cutting inheritance. There is general agreement that mix-ins are the way to achieve this.

DECISION: The 5.0.0 POM will allow for mix-ins

AFFECTS: 

The evidence of the use of Maven shows that there seems to be a significant number of projects that believe themselves to be "one-off" projects that will not benefit from expressing the build logic in a reusable template. We need to provide a way to allow projects to easily change their effective lifecycle and plugin bindings

DECISION: The 5.0.0 POM will provide the ability to define custom lifecycles directly within the POM

AFFECTS: 

DECISION: The 5.0.0 POM will provide the ability to override and completely clear the plugin bindings against individual lifecycles

AFFECTS: 

Custom scopes

One of the blockers for custom scopes has been the requirement that the 4.0.0 POM be used for both the declarative build description and the consumer's dependency graph construction. Any custom scopes introduced into the POM would either break or confuse clients that relied on the assumed 5 scopes defined in the 4.0.0 POM. The Project Dependency Trees schema removes use case of consumption of the POM by consumers of the artifacts produced by the project. This has the effect of completely removing the limits on scopes. The 4.0.0 scopes will likely remain the conventions as interoperability with older plugins as well as conventions in the default configurations of plugins will simplify their use, but in those cases where a project needs to define and consume its own scopes it should be possible to permit it.

DECISION: The 5.0.0 POM will allow the definition and consumption of custom scopes directly within the POM, parent POM, mixins, or templates

AFFECTS: 

The system scope was a Java-centric special scope experiment that hit issues with consumption of dependencies across multiple platforms.

DECISION: The 5.0.0 POM will not provide any special case behaviour for a scope named system

AFFECTS: 

Build vs Reporting

If we look at the two use cases of building with Maven 2/3 there are actually two distinct use-cases:

• Building the project artifacts
• Building the project site/documentation

This was shaped by having two configuration sections in the 4.0.0 POM, build and reporting. One of the issues with these two sections is that they did not have parity of configurability. Specifically, the reporting section did not have a pluginManagement. The solution of having the pluginManagement from the build section apply to the reporting section feels incorrect as now the child element of one is affecting another.

DECISION: The 5.0.0 POM will treat global plugin configuration defaults as a top level concern and have a tag equivalent to pluginManagement at the top level of the POM.

AFFECTS: 

If we seek to find a generic solution to the split between the build and reporting sections in the POM, it becomes apparent that these are all really just ways of defining bindings of plugins to the phases of various lifecycles. The build section defines the bindings against the default lifecycle, while the reporting section defines the bindings against the site lifecycle (Note: this is a simplification as the reporting plugins are actually partly invoked by the site plugin and thus are not actually specifically bound to the lifecycle, rather the site:site goal is bound to the site phase of the site lifecycle and that goal is responsible for invoking the reporting plugin goals)

As a side-effect of making it easier to produce custom lifecycles, we probably need to be able to make it easier to manage the bindings of plugins for the custom lifecycles.

DECISION: The 5.0.0 POM will remove the distinction between build and reporting relying rather on lifecycle specific binding declarations

AFFECTS: 

Project Object Model

<project> element

The Project Object Model consists of a top level <project> tag with child elements

<project modelVersion="5.0.0" [groupId="..."] artifactId="..." [version="..."] template="...">
  [<parent [groupId="..."] [artifactId="..."] [version="..."] [relativePath="..."]/>]
  [<mixin [groupId="..."] [artifactId="..."] [version="..."] [relativePath="..."]/>]
  [<extensions [mode="override|inherit"]>
    ...
  </extensions>]
  [<lifecycle id="..." mode="override|inherit">
    ...
  </lifectcle>]
  ...
</project>

The following are mandatory elements:

• modelVersion attribute - containing the model version of the POM.
• artifactId attribute - containing the artifactId of the project
• template attribute - containing the identifier of the template / packaging that will be used as the initial basis for this project's conventions

The following are optional elements:

• groupId attribute - containing the groupId of the project. If this attribute is missing then the parent element must be present and the groupId will be inherited from the parent project.
• version attribute - containing the version of the project. If this attribute is missing then the parent element must be present and the version will be inherited from the parent project.
• parent element (cardinality 0-1) - containing at a minimum either the GAV of the parent project or the relative path to the parent project. Where more than the minimum required information is supplied, the additional information will be used to validate the parent project reference.
• mixin elements (cardinality 0-N) - containing at a minimum either the GAV of the mix-in project or the relative path to the mix-in project. Where more than the minimum required information is supplied, the additional information will be used to validate the mix-in project reference.
• extensions element (cardinality 0-1) - containing the extensions to enable for this project.
• lifecycle elements (cardinality 0-N) - containing lifecycle customisations for this project.

<parent> element

The parent element identifies the parent project from which conventions will be inherited.

<parent [groupId="..."] [artifactId="..."] [version="..."] [relativePath="..."]/>

Technically from a schema perspective all attributes are optional, however there are two minimum valid sets of attributes:

• If the relativePath attribute is present, no other attributes are required:

  Code Block
  xml xml
  <parent relativePath="..."/>

  This indicates that the parent project can be found on disk at the supplied relative path and the conventions should be inherited from that project. Specifying the additional attributes of groupId or artifactId while the version attribute is unspecified will indicate that the build should fail if the project at the supplied relative path does not match the specified groupId or artifactId. If all three of the groupId, artifactId and version attributes are missing then a mismatch at the supplied relative path will not be fatal as the parent can be resolved from the reactor/repository.

• Specifying the GAV of the parent project:

  Code Block
  xml xml
  <parent groupId="..." artifactId="..." version="..."/>

  This indicates that the parent project should be resolved from the reactor / repository. If the relativePath element is present then in prior to the the reactor / repository the project at the specified relative path will be validated against the supplied groupId, artifactId and version and used in the event of a match.

The following are the attributes:

• groupId attribute - containing the groupId of the parent project. 
• artifactId attribute - containing the artifactId of the parent project
• version attribute - containing the version of the parent project. 
• relativePath attribute - containing the relative path to the parent project.

<mixin> element

The mixin element identifies additional projects from which conventions will be inherited.

<mixin [groupId="..."] [artifactId="..."] [version="..."] [relativePath="..."]/>

Technically from a schema perspective all attributes are optional, however there are two minimum valid sets of attributes:

• If the relativePath attribute is present, no other attributes are required:

  Code Block
  xml xml
  <mixin relativePath="..."/>

  This indicates that the mix-in project can be found on disk at the supplied relative path. Specifying the additional attributes of groupId or artifactId while the version attribute is unspecified will indicate that the build should fail if the project at the supplied relative path does not match the specified groupId or artifactId. If all three of the groupId, artifactId and version attributes are missing then a mismatch at the supplied relative path will not be fatal as the mix-in can be resolved from the reactor/repository.

• Specifying the GAV of the mix-in project:

  Code Block
  xml xml
  <mixin groupId="..." artifactId="..." version="..."/>

  This indicates that the mix-in project should be resolved from the reactor / repository. If the relativePath element is present then in prior to the the reactor / repository the project at the specified relative path will be validated against the supplied groupId, artifactId and version and used in the event of a match.

The following are the attributes:

• groupId attribute - containing the groupId of the mix-in project. 
• artifactId attribute - containing the artifactId of the mix-in project
• version attribute - containing the version of the mix-in project. 
• relativePath attribute - containing the relative path to the mix-in project.

<extensions> element

The extensions element identifies extensions to be enabled for this project.

<extensions [mode="override|inherit"]>
  [<extension [groupId="..."] [artifactId="..."] [version="..."] [relativePath="..."]/>]
  ...
</extensions> 

There is one attribute:

• mode attribute - when specified as override then any inherited extensions are ignored and the full set of extensions to be enabled is contained within this element. When specified as inherit - the default - then the inherited extensions are merged with the extensions contained within this element. In the case of duplicate groupId:artifactId entries, the version declared in this project will take precedence.

There can be 0-N extension elements.

<extension> element

The extension element identifies additional projects containing extensions to enable for the project.

<extension [groupId="..."] [artifactId="..."] [version="..."] [relativePath="..."]/>

Technically from a schema perspective all attributes are optional, however there are two minimum valid sets of attributes:

• If the relativePath attribute is present, no other attributes are required:

  Code Block
  xml xml
  <extension relativePath="..."/>

  This indicates that the extension project can be found on disk at the supplied relative path. Specifying the additional attributes of groupId or artifactId while the version attribute is unspecified will indicate that the build should fail if the project at the supplied relative path does not match the specified groupId or artifactId. If all three of the groupId, artifactId and version attributes are missing then a mismatch at the supplied relative path will not be fatal as the extension can be resolved from the reactor/repository.

• Specifying the GAV of the extension project:

  Code Block
  xml xml
  <extension groupId="..." artifactId="..." version="..."/>

  This indicates that the extension project should be resolved from the reactor / repository. If the relativePath element is present then in prior to the the reactor / repository the project at the specified relative path will be validated against the supplied groupId, artifactId and version and used in the event of a match.

The following are the attributes:

• groupId attribute - containing the groupId of the extension project. 
• artifactId attribute - containing the artifactId of the extension project
• version attribute - containing the version of the extension project. 
• relativePath attribute - containing the relative path to the extension project.

TODO resolve the inheritance problem

We have multiple places where things are coming from...

• parent
• mix-ins
• extensions

we need a model of inheritance that is easy for people to understand.

First stab:

• complete each parent/mixin's project model before inheriting.
  • PROs:
    • We do not have as much complexity, you just help:effective-pom on the parent, and mixins to see what they are defining and hence pulling in
  • CONs
    • What about mixin version conflict? If the parent brings in one mixin and then we explicitly state another version of that mixin and finally explicitly state a 3rd mixin that transitive states a third version of the same mixin.
      If we flatten first, it is very likely that we could have some cruft left over from one of the other versions

Second stab:

• process parent inheritance by injecting pseudo mixin nodes from the parent and then de-dup
  • CONs
    • I'm not even sure what I am saying here

I think we need to build the tree of mixins, resolve the closest version (with tree pruning) and then apply them...

Then after that, we can start to build the tree of extensions...

This is a pain!

TODO write this up... I'm just dumping stuff I have done on the mail thread here to TODO write this up... I'm just dumping stuff I have done on the mail thread here to make it easier to collaborate:

<project modelVersion="5.0.0" [groupId="..."] artifactId="..." [version="..."] packagingtemplate="...">
  [<parent groupId="..." artifactId="..." [version="..."] [relativePath="...']/>]

  [<mixin groupId="..." artifactId="..." [version="..."]/>]
  [<mixin groupId="..." artifactId="..." [version="..."]/>]
  ...
  [<mixin groupId="..." artifactId="..." [version="..."]/>]

  [<lifecycle id="..." mode="override|inherit">
    <phase id="..." [after="..." | before="..."]/>
    <phase id="..." [after="..." | before="..."]/>
    ...
    <phase id="..." [after="..." | before="..."]/>

  </lifecycle>]
  [<lifecycle id="...">
    ...
  </lifecycle>]
  ...
  [<lifecycle id="...">
    ...
  </lifecycle>]

  [<scope id="compile" [mode="override|inherit"]>
    <dependency groupId="..." artifactId="..." [platformId="..."] version="..." [classifier="..."] type="..."/> <!-- type is mandatory-->
    <dependency groupId="..." artifactId="..." [platformId="..."] version="..." [classifier="..."] type="..."/>
    ...
    <dependency groupId="..." artifactId="..." [platformId="..."] version="..." [classifier="..."] type="..."/>
  </scope>]
  [<scope id="...">
    ...
  </scope>]
   ...
  [<scope id="...">
    ...
  </scope>]

  [<extensions [mode="override|inherit"]>
    <extension groupId="..." artifactId="...
  [<scope id" version="..."/>
    ...
  </scope>extensions>]

  [<plugins [mode="override|inherit"]>
    <!-- this is what pluginManagement was -->
    <plugin groupId="..." artifactId="..." version="...">
      ...
    </plugin>
    ...
  </plugins>]

  [<bindings [mode="override|inherit"]>
    <!-- this is what plugins was, we make explicit here that this is the binding of executions into the lifecycles -->
  </bindings>]

  [<platform id="..." [mode="override|inherit"]>
    <activation>
      <!-- define how we determine that this platform can be built in the current environment -->
    </activation>
    <!-- allow platform specific mixins -->
    [<mixin groupId="..." artifactId="..." [version="..."]/>]
    <!-- allow platform specific lifecycles -->
    [<lifecycle id="...">
      ...
    </lifecycle>]

    <!-- allow platform specific dependencies -->
    [<scope>
      ...
    </scope>]

    <!-- allow platform specific bindings... but plugin management is from the root only -->
    [<bindings>
      ...
    </bindings>]

    <!-- allow most of the other root tags except platform and packaging and deployment config -->
  </platform>]
  [<platform id="...">
    ...
  </platform>]
  ...
  [<platform id="...">
    ...
  </platform>]

  <!-- packagingtemplate is only allowed in poms with an id of "parent" or "mixin". It allows a parent/mixin to be used by different packagingtemplate ids and define specialized defaults -->
  [<packaging<template id="...">
    [<mixin groupId="..." artifactId="..." [version="..."]/>]
    <!-- allow platform specific lifecycles -->
    [<lifecycle id="...">
      ...
    </lifecycle>]

    <!-- allow platform specific dependencies -->
    [<scope>
      ...
    </scope>]

    <!-- allow platform specific bindings... but plugin management is from the root only -->
    [<bindings>
      ...
    </bindings>]

    <!-- allow most of the other root tags except platform and packaging and deployment config -->
  </packaging>template>]
  [<packaging<template id="...">
    ...
  </packaging>template>]
  ...
  [<packaging<template id="...">
    ...
  </packaging>template>]

  <!-- unsure if we still need profiles -->
  <!-- perhaps we still need properties -->
  <!-- TBD deployment config, repositories, etc -->

</project>
  

Some things that came to mind, in no particular order:

...