How to build the simplest multi module Maven project using Windows command line?

Question

I have multi module project which is very simple.

Directory structure:

   C:\acme-project\parent
   C:\acme-project\alpha   
   C:\acme-project\beta

Logical structure:

     parent
    /     \
 alpha <- beta

I cannot build the beta because it depends on the alpha. And Meven doesn't want to build the alpha during the beta's build process!

C:\acme-project\beta> mvn package
[INFO] Scanning for projects...
[INFO]                                                                         
[INFO] ------------------------------------------------------------------------
[INFO] Building beta 1.0-SNAPSHOT
[INFO] ------------------------------------------------------------------------
[WARNING] The POM for com.company:alpha:jar:1.0-SNAPSHOT is missing, no dependency information available
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 1.263 s
[INFO] Finished at: 2017-03-28T15:00:34+03:00
[INFO] Final Memory: 16M/220M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal on project beta: Could not resolve dependencies for project com.company:beta:jar:1.0-SNAPSHOT: The following artifacts could not be resolved: com.company:alpha:jar:1.0-SNAPSHOT: Could not find artifact com.company:alpha:jar:1.0-SNAPSHOT -> [Help 1]
[ERROR] 
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR] 
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/DependencyResolutionException

My POMs :

C:\acme-project\parent\parent.pom :

<?xml version='1.0' encoding='UTF-8'?>
<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 http://maven.apache.org/maven-v4_0_0.xsd">
  <modelVersion>4.0.0</modelVersion>

  <groupId>com.company</groupId>
  <artifactId>parent</artifactId>
  <version>1</version>
  <packaging>pom</packaging>

  <modules>
    <module>../alpha</module>
    <module>../beta</module>    
  </modules>
</project>

C:\acme-project\alpha\alpha.com :

<?xml version="1.0" encoding="UTF-8"?>
<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 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <parent>
    <artifactId>parent</artifactId>
    <groupId>com.company</groupId>
    <version>1</version>
    <relativePath>../parent/pom.xml</relativePath>
  </parent>
  <modelVersion>4.0.0</modelVersion>

  <artifactId>alpha</artifactId>
  <version>1.0-SNAPSHOT</version>

</project>

C:\acme-project\beta\beta.pom :

<?xml version="1.0" encoding="UTF-8"?>
<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 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>

  <parent>
    <groupId>com.company</groupId>
    <artifactId>parent</artifactId>
    <version>1</version>
    <relativePath>../parent/pom.xml</relativePath>
  </parent>

  <artifactId>beta</artifactId>
  <version>1.0-SNAPSHOT</version>
  <packaging>jar</packaging>

  <dependencies>
    <dependency>
      <groupId>com.company</groupId>
      <artifactId>alpha</artifactId>
      <version>1.0-SNAPSHOT</version>
    </dependency>
  </dependencies>
</project>

I can build the acme-project running mvn package in C:...\parent. But in case of more complex project parent could have 100 sub-modules. And obviously I don't want to build 98 modules (others that beta doesn't depend on) just to make Maven happy.

Answer 1

If you want to build just the projects that beta depends on, go to the root aggregator (i.e., the project with all the <modules>; parent in your case) and then ask Maven to build only beta and its dependencies in the current reactor:

cd parent
mvn package --projects com.company:beta --also-make

FYI, this can be abbreviated:

mvn package -pl :beta -am

In your minimal example, the above command will still build everything (as beta depends on alpha and implicitly on the parent), but in larger reactors it will build just the minimal subset necessary to build beta.