Guide to Working with Multiple Modules

As seen in the introduction to the POM, Maven supports project aggregation in addition to project inheritance. This section outlines how Maven processes projects with multiple modules, and how you can work with them more effectively.

The Reactor

The mechanism in Maven that handles multi-module projects is referred to as the reactor. This part of the Maven core does the following:

  • Collects all the available modules to build
  • Sorts the projects into the correct build order
  • Builds the selected projects in order

Reactor Sorting

Because modules within a multi-module build can depend on each other, it is important that The reactor sorts all the projects in a way that guarantees any project is built before it is required.

The following relationships are honoured when sorting projects:

  • a project dependency on another module in the build
  • a plugin declaration where the plugin is another modules in the build
  • a plugin dependency on another module in the build
  • a build extension declaration on another module in the build
  • the order declared in the <modules> element (if no other rule applies)

Note that only "instantiated" references are used - dependencyManagement and pluginManagement elements will not cause a change to the reactor sort order

Command Line Options

No special configuration is required to take advantage of the reactor, however it is possible to customize its behavior.

The following command line switches are available:

  • -r - ignore the modules declared in the current project, and instead build the list of projects listed after the -r switch (which may include wildcards)
  • --resume-from - resumes a reactor the specified project (e.g. when it fails in the middle)
  • --also-make - build the specified projects, and any of their dependencies in the reactor
  • --also-make-dependents - build the specified projects, and any that depend on them
  • --fail-fast - the default behavior - whenever a module build fails, stop the overall build immediately
  • --fail-at-end - if a particular module build fails, continue the rest of the reactor and report all failed modules at the end instead
  • --non-recursive - do not use a reactor build, even if the current project declares modules and just build the project in the current directory

Refer to the Maven command line interface reference for more information on these switches.

The Reactor Plugin

For versions of Maven prior to Maven 2.1, or for additional capabilities with the reactor such as building only the modules with SCM changes, the Reactor plugin can be used to further customize the execution of the projects. For information on how to use this, refer to the Reactor Plugin documentation.