Starting with the Maven 2.1 release, there are new Maven command line options which allow you to manipulate the way that Maven will build multi-module projects. These new options are:
- -rf, --resume-from
- Resume reactor from specified project
- -pl, --projects
- Build specified reactor projects instead of all projects
- -am, --also-make
- If project list is specified, also build projects required by the list
- -amd, --also-make-dependents
- If project list is specified, also build projects that depend on projects on the list
Example Multi-module Project
This project approximates the structure of a real-world enterprise project: the sample-model project contains a set of foundational model objects used throughout the system, the sample-util project would contain utility code, the sample-persist project would contain logic that deals with persisting objects to a database, and the other projects would all be combined to produce the various GUI and Web-based interfaces that comprise a very complex system. This next figure captures the dependencies between each of these sample modules.
If you go into the sample-parent/ project directory and run mvn clean, you will see that the Maven Reactor reads all of the project dependencies and comes up with the following build order for these projects as shown in below:
[INFO] Reactor build order: [INFO] sample-parent [INFO] sample-model [INFO] sample-persist [INFO] sample-services [INFO] sample-util [INFO] sample-security [INFO] sample-admin-webapp [INFO] sample-webapp [INFO] sample-rest [INFO] sample-client-connector [INFO] sample-gui [INFO] sample-admin-gui
The -rf or --resume-from option can come in handy if you want to tell the Maven Reactor to resume a build from a particular project. This can be useful if you are working with a large multimodule project and you want to restart a build at a particular project in the Reactor without running through all of the projects that precede it in the build order.
$ mvn --resume-from sample-client-connector install [INFO] Scanning for projects... [INFO] Reactor build order: [INFO] sample-client-connector [INFO] sample-gui [INFO] sample-admin-gui ...
Specifying a Subset of Projects
The -pl or --projects option allows you to select a list of projects from a multimodule project. This option can be useful if you are working on a specific set of projects, and you'd rather not wait through a full build of a multi-module project during a development cycle. If you only wanted Maven to build the sample-rest and sample-client-connector project, you would use the following syntax from the sample-parent/ directory:
$ mvn --projects sample-client-connector,sample-rest install [INFO] Scanning for projects... [INFO] Reactor build order: [INFO] sample-rest [INFO] sample-client-connector
Making a Subset of Projects
If you wanted to run a portion of the larger build, you would use the -pl or --projects option with the -am or --also-make option. When you specify a project with the -am option, Maven will build all of the projects that the specified project depends upon (either directly or indirectly). Maven will examine the list of projects and walk down the dependency tree, finding all of the projects that it needs to build.
$ mvn --projects sample-services --also-make install [INFO] Scanning for projects... [INFO] Reactor build order: [INFO] sample-parent [INFO] sample-model [INFO] sample-persist [INFO] sample-services
Making Dependent Projects
While the -am command makes all of the projects required by a particular project in a multi-module build, the -amd or --also-make-dependents option configures Maven to build a project and any project that depends on that project. When using --also-make-dependents, Maven will examine all of the projects in our reactor to find projects that depend on a particular project. It will automatically build those projects and nothing else.
$ mvn --projects sample-services --also-make-dependents install [INFO] Scanning for projects... [INFO] Reactor build order: [INFO] sample-services [INFO] sample-admin-webapp [INFO] sample-webapp [INFO] sample-rest
Resuming a "make" Build
Combining --project, --also-make, and --resume-from provides you with the ability to refine your build even further.
mvn --projects sample-webapp --also-make \ --resume-from sample-services install [INFO] Scanning for projects... [INFO] Reactor build order: [INFO] sample-services [INFO] sample-util [INFO] sample-security [INFO] sample-webapp
In this example, the build is resumed from sample-services which omits the sample-persist and sample-model projects from the build. If you are focused on individual components and you need to accelerate your build times, using these advanced reactor options together is a great way to skip portions of your large multi-module project build. The --resume-from argument also works with --also-make-dependents.