Transitive dependency excludes (MNG-2315)

It is sometimes useful to clip a dependency's transitive dependencies. A dependency may have incorrectly specified scopes, or dependencies that conflict with other dependencies in your project. Using wildcard excludes makes it easy to exclude all a dependency's transitive dependencies. In the case below you may be working with the maven-embedder and you want to manage the dependencies you use yourself, so you clip all the transitive dependencies:

<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 https://maven.apache.org/xsd/maven-4.0.0.xsd">
  ...
  <dependencies>
    <dependency>
      <groupId>org.apache.maven</groupId>
      <artifactId>maven-embedder</artifactId>
      <version>3.1.0</version>
      <exclusions>
        <exclusion>
          <groupId>*</groupId>
          <artifactId>*</artifactId>
        </exclusion>
      </exclusions>
    </dependency>
    ...
  </dependencies>
  ...
</project>

Reactor changes

The Maven reactor is now pluggable (MNG-5578). This feature required the introduction of a new custom Guice scope called @SessionScoped as we need to inject the MavenSession into the reactor right after the creation the MavenSession and before the execution of the build. The reactor behaves as a special type of artifact repository that provides the current behaviour users expect when building multi-module projects. New implementations can provide different semantics which can potentially provide new ways for multi-module projects to build, or to fix/add new features like resolving test JARs properly from the reactor. Now that the implementation is pluggable we can always default to the existing behaviour and allow people to try new behavior with a feature toggle.

The reactor now limits the projects that are exposed internally when --projects is used (MNG-5557). What happened previously is that all the projects in the reactor would still be available for resolution even if you restricted the number of projects being built using --projects. This behaviour is now explicit: what you specify using --projects is what you will build and resolve against, if a project you need is not present in the constrained set the local repository will be consulted. There are cases where you may still want access to the projects that are normally part of the build but not currently being built, and those projects are tracked (MNG-5582) and available in the MavenSession. It is also now possible to exclude projects from the reactor when using the -am and -amd options (MNG-5230). If you select a project to build and its dependents (-amd) you can exclude one or more of the dependents with a command like the following: mvn -amd --projects foo,bar,!dependent-of-bar.

These combined changes above will allow us to make new implementation of the Maven reactor with improved/different behaviour. For example, this could potentially allow you to step into a directory and run Maven and have Maven pick up the projects surrounding the project you're building instead of going to local repository. We can experiment with different implementations and let users toggle between them until we ultimately decide on the right behaviour. Lots of user complain about the behaviour of the reactor and this allows stepwise improvement.

AbstractMavenLifecycleParticipant changes (MNG-5389)

The AbstractMavenLifecycleParticipant has a new afterSessionEnd() method which will be executed when the build is complete. This feature was added to account for end of build deployment in a parallel build. The current logic employed in many plugins is to look for the last project in the array of projects that are in the reactor, but during a parallel build there is no guaranteed execution order and breaks in plugins which assume there is. There needs to be a way to execute logic when the build is definitively complete and AbstractMavenLifecycleParticipant.afterSessionEnd() provides this.

New Builder interface (MNG-5575)

There is a new Builder interface which classes can implement to encapsulate a strategy for building projects. The existing strategies for building Maven serially and in parallel are now Builder implementations. It's now possible for others to implement additional strategies for building projects. This is a provisional interface and may change in the near future but will stabilize by Maven 4.0.0.

Continuous delivery friendly versions (MNG-5576)

A simple change to prevent Maven from emitting warnings about versions with property expressions. Allowed property expressions in versions include ${revision}, ${changelist}, and ${sha1}. These properties can be set externally, but eventually a mechanism will be created in Maven where these properties can be injected in a standard way. For example you may want to glean the current Git revision and inject that value into ${sha1}. This is by no means a complete solution for continuous delivery but is a step in the right direction.

Custom lifecycle mapping (MNG-5581)

Implementations of the new LifecycleMappingDelegate interface can create custom lifecycles. The delegate has access to the existing lifecycles so it can create projections of existing lifecycles or create completely new ones. A simple example might be to create a lifecycle called test-only which might extract only the Surefire execution from the standard lifecycle and execute it.