Build part of a multi-module project

Build one module

Select the module with -pl, followed by its directory:

mvn verify -pl core

Maven builds core and nothing else. The modules core depends on are not part of this build, so Maven takes them from the local repository, like any other dependency. If you have never installed them, the build stops:

[ERROR] Failed to execute goal on project core: Could not resolve dependencies for project com.example.shop:core:jar:1.0-SNAPSHOT

Either install the whole project once with mvn install in the root directory, so that -pl finds the other modules in the local repository, or build the modules it needs along with it, as in the next section. An installed module stays at the version you installed until you install it again.

Maven 4: When Maven 4 packages a subproject, it keeps a copy of the jar in .mvn/target/project-local-repo in the root directory, and uses that copy when the subproject is not part of a later build. mvn clean removes it again. Without an install or an earlier package, -pl core fails on Maven 4 as well.

Build a module with the modules it needs

Add -am (also make) to build the selected module together with every module it depends on, directly or indirectly:

mvn verify -pl app -am

[INFO] Reactor Summary for shop 1.0-SNAPSHOT:
[INFO] 
[INFO] shop ............................................... SUCCESS [  0.001 s]
[INFO] api ................................................ SUCCESS [  0.546 s]
[INFO] core ............................................... SUCCESS [  0.014 s]
[INFO] app ................................................ SUCCESS [  0.010 s]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS

The parent shop is built too, because app inherits from it. None of the project's own modules has to come from the local repository, which makes this the safe way to build one module.

Build a module with the modules that use it

Add -amd (also make dependents) after changing a module, to check that the modules depending on it still compile and pass their tests:

mvn verify -pl core -amd

[INFO] Reactor Summary for core 1.0-SNAPSHOT:
[INFO] 
[INFO] core ............................................... SUCCESS [  0.586 s]
[INFO] app ................................................ SUCCESS [  0.017 s]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS

-amd does not add the modules core needs, so api still comes from the local repository. Use both options, -pl core -am -amd, to build core, what it needs and what uses it.

Build several modules or leave one out

-pl takes a comma-separated list, -pl api,app. To build everything except some modules, put ! before them:

mvn verify -pl '!app'

The quotes keep shells such as Bash from treating ! as a history command. - works as well as !: -pl -app.

Maven 4: Selecting a subproject that is itself an aggregator selects its subprojects too, and excluding it excludes them. Maven 3 selects or excludes only the aggregator. Add -N (--non-recursive) on Maven 4 to select the aggregator alone.

Resume a failed build

When a module fails, Maven skips the modules after it and prints where to pick up again:

[ERROR] After correcting the problems, you can resume the build with the command
[ERROR]   mvn <args> -rf :core

Fix the problem and run the same command with -rf (resume from). Maven skips the modules that came before core in the build order:

mvn verify -rf :core

The modules before it are not rebuilt, so core takes them from the local repository, as with -pl.

Maven 4: Maven 4 suggests mvn [args] -r instead. With -r (--resume), Maven reads the modules that failed or were skipped from target/resume.properties in the root directory, and builds only those. After -fae, that is the failed modules and their dependents, not everything that came after the first failure. Add -am to build the modules they need as well: mvn verify -r -am. Maven 3 does not know -r and stops with Unrecognized option: -r.

Build from a module's directory

Running Maven in a module's directory builds that module alone, like -pl, and needs the other modules installed:

cd app
mvn verify

Maven 4: Maven 4 knows the whole project when you start in a subproject's directory. mvn verify -am in app builds shop, api, core and app, and mvn verify -amd in api builds api, core and app. Maven 3 ignores the rest of the project and builds only the module in the current directory.