Build profiles are an important concept for unifying configuration differences between different environments. They also should capture information across all aspects of the POM.

Design

General Objectives

Multiple Profiles

Multiple profiles can be active simultaneously. Each profile selected is cumulative. This allows each profile to do specific things, and various combinations do not need to be created. There is no need for duplication or special inheritence of profiles.

To activate a profile, they can be added to settings.xml, or by the -p profile(s) command line option. The specification in settings.xml would be:

<activeProfiles>
  <activeProfile>jdk-1.4</activeProfile>
  <activeProfile>os-windows</activeProfile>
</activeProfiles>

Profiles for Dependencies

This allows some dependencies to be OS specific, JDK specific, etc.

Options:
1. Map a defined dependency to a particular implementation based on profile
2. Define new dependencies based on the profile.

Mapping would be as much work to specify and is less flexible, so the latter is the best choice.

Defining the profile of a dependency is not done in the dependency definition itself for the following reasons:

Proposed solution:

<profiles>
  <profile>
    <id>jdk-1.4</id>    <!-- Use the id of "default" if it is the profile to use when none is given -->
    <dependencies>
      <dependency>
        <groupId>bouncycastle</groupId>
        <artifactId>bouncycastle-provider</artifactId>
        <version>1.1.6-jdk14</version> <!-- The JDK portion may be part of the classifier instead (see other doc)
      </dependency>
      ...
    </dependencies>
    ...
  </profile>
  ...
</profiles>

This section should be added to the POM at the top level.

Profiles for Plugin Configuration

Plugin configuration and management can also be specified in the profile. Definitions here will add to the existing lists and merge as appropriate. For consistency - it behaves identically to inheritence, however regardless of the <inherit> and @inheritByDefault flag, the plugins from the current pom are always inherited into the profile as conceptually for the user, it is just overlayed, not inherited.

<profile>
  <id>jdk-1.4</id>
  <build>
    <plugins>
      <groupId/>
      <artifactId/>
      <version/>
      <configuration>
        ...
      </configuration>
    </plugins>
  </build>
</profile>

This will always bind the plugin with this version. If a version is specified in both the main plugins section and one or more profiles, the versions must match or an error occurs.

Profiles for Repositories

When switching between repositories for profiles, its possible that the snapshot and release version information is quite different.

Rather than having to maintain multiple local repositories for the segments where an alternate repository is used, just the snapshot metadata needs to be in a different location when looking to a repository from a different profile.

The resulting metadata path in the local repository is:

/groupId/artifactId/version/profileId/artifactId-version.version.txt

Another option is to write the information into one snapshot file, but that makes it different to the remote version which is undesirable and would affect timestamped checks.

The above information could possibly be duplicated in the remote repository to allow different profiles to operate on the same repository. At this point, this is not being implemented.

All elements allowed in a profile

As previously mentioned, a profile is a subset of another POM. In modello, this can be achieved by the model and profile inheriting a common base class.

The following elements will be supported by profile specifications in various locations (noted inline):

Note that build is included for the benefit of the plugin definitions, and that will bring along other elements such as finalName and outputDirectory. These can also be beneficial. One of the unexpected behaviours may be the ability to override sourceDirectory which will completely substitute a new tree. This is probably undesirable, and so may need to be forbidden by validation.

build as specified inside of a profile is not a full implementation of the traditional build POM element. This build is really another class in the model - from which the POM build is derived - and only allows the plugins and pluginManagement subelements when defined here. This sidesteps any issues with secondary validation after the pom.xml is parsed in this case.

Inheritence and Transitivity

Note that the profiles are inherited, but only in such a way as they are applied to a parent POM before the inheritence of the parent and child is performed, so the settings required are already present. This ensures the proper precedence occurs (child and its profiles win over the profile of a parent).

For this reason it is unimportant if the IDs of profiles match or not, unless they are to be specified on the command line.

Location of Profiles

The requirements listed that there would need to be all of the following:

Project based settings that apply to all users can be stored in pom.xml as previously shown.

For user specific settings that apply to a particular project, they should use a user-* profile ID. These can go into the POM (and be stored in SCM), or be in profiles.xml in the same directory (which may not be stored in the SCM).

profiles.xml are transient profiles that are for per-user customisations to an individual project. They are generally not stored in SCM, and are never deployed. Before of this, they are not inherited, except when a USD or reactor build is present that will locate them and match them to the parent POM.

For any settings that span multiple projects, the profiles need to be stored inside settings.xml instead of pom.xml.

These should take on a similar format, however is restricted to:

General configuration properties are used instead of allowing plugin configuration to be given. This avoids a settings file circumventing a POM. For example, the following general configuration may be given:

<settings>
  ...
  <profiles>
    <profile>
      <id>default</id>
      <properties>
        <tomcat5x.home>/usr/local/jakarta-tomcat-5.5.9</tomcat5x.home>
      </properties>
    </profile>
  </profiles>
</settings>

This will be made available to the POM interpolation, so could be used as configuration in a plugin:

<plugin>
  <artifactId>cactus-maven-plugin</artifactId>
  <configuration>
    <container>
      <id>tomcat5x</id>
      <home>${tomcat5x.home}</home>
    </container>
  </configuration>
</plugin>

The main drawback to this is that it requires some consistency of property naming - however this outways the problems of directly injecting plugin configuration from settings.

Preset Profiles

It should be possible to preset the activation of some profiles. Eg, JDK, OS, host and user profiles could all be made active based on the settings of the machine running the build. However, should any profiles be made active via settings.xml, this will override all of the existing ones, rather than add to them.

Likewise, specifying profiles on the command line will override all of those from settings.xml as well as any predefined ones.

This will be done by some special activation tags (the format of which is yet to be completely decided), for example:

<profile>
  <id>hsqldb</id>
  <activation>
    <property>hsqldb.enabled</property>
  </activation>
  <dependencies>
    <dependency>
      <groupId>hsqldb</groupId>
      .
      .
</profile>
<profile>
  <id>jdk-1.4+</id>
  <activation>
    <jdk>1.4+</jdk>
  </activation>
  <build>
    <pluginManagement>
      <plugin>
        <artifactId>maven-compiler-plugin</artifactId>
        <configuration>
          <source>1.4</source>
      .
      .
</profile>

Resource Filtering

Under the above design, filtering can simply be a configuration element on the resources plugin (as per Kenney's patch, already available). The profiles will allow the introduction of different filter sets.

Lifecycle Binding

The plexus use case requires that a different goal be bound to the lifecycle. However, this does not entail any changes to the lifecycle design with the above solution. Instead, the goals would simply be added to the <plugins> section as needed in each profile.

Additional Features

These are some thoughts on additional features that may or may not be made available: