Importing pom in dependency management

Number945 :

Reading from the doc ,

Do not attempt to import a pom that is defined in a submodule of the current pom. Attempting to do that will result in the build failing since it won't be able to locate the pom.

Never declare the pom importing a pom as the parent (or grandparent, etc) of the target pom. There is no way to resolve the circularity and an exception will be thrown.

When referring to artifacts whose poms have transitive dependencies the project will need to specify versions of those artifacts as managed dependencies. Not doing so will result in a build failure since the artifact may not have a version specified. (This should be considered a best practice in any case as it keeps the versions of artifacts from changing from one build to the next).

I have the following doubt:

a) What does point 3 mean ?

b) In 1st point , why would maven not able to find the sub module pom ? Is not submodule build before the parent ?

In point 3 , I need more clarity on ...When referring to artifacts whose poms have transitive dependencies the project will need to specify versions of those artifacts as managed dependencies....

So, let us say we have project A which we will be imported in our project B <dependencyManagement> section . Now the people who created project A have to mention versions of all transitive dependencies (not direct) of project A in its <dependencyManagement> section ? How can anybody know those versions for all transitive dependencies of project A ?

I got another doubt with point 1. I created basically an skeletal project with a super module and sub module with basically no java code. I will hence share their poms. We will see that build gets successful while this should not be the case according to point 1.

The project structure is as follows :

The pom of super module is as follows :

<?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>

    <groupId>com.home</groupId>
    <artifactId>my-project</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>pom</packaging>

    <name>my-project</name>
    <url>http://www.example.com</url>

    <modules>
        <module>./sub-module1/pom.xml</module>
    </modules>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <maven.compiler.source>1.7</maven.compiler.source>
        <maven.compiler.target>1.7</maven.compiler.target>
    </properties>

    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>com.home</groupId>
                <artifactId>sub-module1</artifactId>
                <version>1.0-SNAPSHOT</version>
                <scope>import</scope>
            </dependency>
        </dependencies>
    </dependencyManagement>


    <licenses>
        <license>
            <name>The Apache Software License, Version 2.0</name>
            <url>http://www.apache.org/licenses/LICENSE-2.0.txt</url>
            <distribution>repo</distribution>
        </license>
    </licenses>


    <build>
        <pluginManagement><!-- lock down plugins versions to avoid using Maven defaults (may be moved to parent pom) -->
            <plugins>
                <!-- clean lifecycle, see https://maven.apache.org/ref/current/maven-core/lifecycles.html#clean_Lifecycle -->
                <plugin>
                    <artifactId>maven-clean-plugin</artifactId>
                    <version>3.1.0</version>
                </plugin>
                <!-- default lifecycle, jar packaging: see https://maven.apache.org/ref/current/maven-core/default-bindings.html#Plugin_bindings_for_jar_packaging -->
                <plugin>
                    <artifactId>maven-resources-plugin</artifactId>
                    <version>3.0.2</version>
                </plugin>
                <plugin>
                    <artifactId>maven-compiler-plugin</artifactId>
                    <version>3.8.0</version>
                </plugin>
                <plugin>
                    <artifactId>maven-surefire-plugin</artifactId>
                    <version>2.22.1</version>
                </plugin>
                <plugin>
                    <artifactId>maven-jar-plugin</artifactId>
                    <version>3.0.2</version>
                </plugin>
                <plugin>
                    <artifactId>maven-install-plugin</artifactId>
                    <version>2.5.2</version>
                </plugin>
                <plugin>
                    <artifactId>maven-deploy-plugin</artifactId>
                    <version>2.8.2</version>
                </plugin>
                <!-- site lifecycle, see https://maven.apache.org/ref/current/maven-core/lifecycles.html#site_Lifecycle -->
                <plugin>
                    <artifactId>maven-site-plugin</artifactId>
                    <version>3.7.1</version>
                </plugin>
                <plugin>
                    <artifactId>maven-project-info-reports-plugin</artifactId>
                    <version>3.0.0</version>
                </plugin>

            </plugins>
        </pluginManagement>
    </build>
</project>

The pom for sub module is as follows :

<?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>

    <groupId>com.home</groupId>
    <artifactId>sub-module1</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>pom</packaging>


    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>junit</groupId>
                <artifactId>junit</artifactId>
                <version>3.8.1</version>
            </dependency>
        </dependencies>
    </dependencyManagement>


    <dependencies>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
        </dependency>
    </dependencies>

</project>

When I do mvn clean compile , it builds fine.

Also, I see that even if sub module pom packaging type be kept as jar, scope import does not throw error. It still compiles fine.

For future readers I will summarize the answer. Both points 1 and 2 holds true but it will give error only when POM1 (parent or super module) imports POM2 (sub module or child module) and then POM2 needs POM1 for resolution of dependencies.

In point 1 , this won't be found. In point 2 , it will be found due to inheritance but will create a cycle.

Below I give example so that people can verify.

Super module 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>

    <groupId>com.home</groupId>
    <artifactId>my-project</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>pom</packaging>

    <name>my-project</name>
    <url>http://www.example.com</url>

    <modules>
        <module>./sub-module1/pom.xml</module>
    </modules>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <maven.compiler.source>1.7</maven.compiler.source>
        <maven.compiler.target>1.7</maven.compiler.target>
    </properties>

    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>com.home</groupId>
                <artifactId>sub-module1</artifactId>
                <version>1.0-SNAPSHOT</version>
                <type>pom</type>
                <scope>import</scope>
            </dependency>
            <dependency>
                <groupId>org.jsoup</groupId>
                <artifactId>jsoup</artifactId>
                <version>1.8.3</version>
            </dependency>
        </dependencies>
    </dependencyManagement>

    <dependencies>
        <dependency>
                <groupId>junit</groupId>
                <artifactId>junit</artifactId>
        </dependency>
    </dependencies>


    <licenses>
        <license>
            <name>The Apache Software License, Version 2.0</name>
            <url>http://www.apache.org/licenses/LICENSE-2.0.txt</url>
            <distribution>repo</distribution>
        </license>
    </licenses>


    <build>
        <pluginManagement><!-- lock down plugins versions to avoid using Maven defaults (may be moved to parent pom) -->
            <plugins>
                <!-- clean lifecycle, see https://maven.apache.org/ref/current/maven-core/lifecycles.html#clean_Lifecycle -->
                <plugin>
                    <artifactId>maven-clean-plugin</artifactId>
                    <version>3.1.0</version>
                </plugin>
                <!-- default lifecycle, jar packaging: see https://maven.apache.org/ref/current/maven-core/default-bindings.html#Plugin_bindings_for_jar_packaging -->
                <plugin>
                    <artifactId>maven-resources-plugin</artifactId>
                    <version>3.0.2</version>
                </plugin>
                <plugin>
                    <artifactId>maven-compiler-plugin</artifactId>
                    <version>3.8.0</version>
                </plugin>
                <plugin>
                    <artifactId>maven-surefire-plugin</artifactId>
                    <version>2.22.1</version>
                </plugin>
                <plugin>
                    <artifactId>maven-jar-plugin</artifactId>
                    <version>3.0.2</version>
                </plugin>
                <plugin>
                    <artifactId>maven-install-plugin</artifactId>
                    <version>2.5.2</version>
                </plugin>
                <plugin>
                    <artifactId>maven-deploy-plugin</artifactId>
                    <version>2.8.2</version>
                </plugin>
                <!-- site lifecycle, see https://maven.apache.org/ref/current/maven-core/lifecycles.html#site_Lifecycle -->
                <plugin>
                    <artifactId>maven-site-plugin</artifactId>
                    <version>3.7.1</version>
                </plugin>
                <plugin>
                    <artifactId>maven-project-info-reports-plugin</artifactId>
                    <version>3.0.0</version>
                </plugin>

            </plugins>
        </pluginManagement>
    </build>
</project>

Sub Module 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>


    <groupId>com.home</groupId>
    <artifactId>sub-module1</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>pom</packaging>


    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>junit</groupId>
                <artifactId>junit</artifactId>
                <version>3.8.1</version>
            </dependency>
        </dependencies>
    </dependencyManagement>
    <dependencies>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
        </dependency>
        <dependency>
            <groupId>org.jsoup</groupId>
            <artifactId>jsoup</artifactId>
        </dependency>
    </dependencies>

</project>

VonC :

b) In 1st point , why would maven not able to find the sub module pom ? Is not submodule build before the parent ?

The submodule is not installed/deployed in a GAV referential (your maven cache or a remote Nexus repo)

That means any project which would reference a submodule pom would not find said pom in a submodule GAV, since what was built, installed and deployed was the main project GAV.

As khmarbaise adds in the comments:

You should never do a <scope>import</scope> inside a project which is already defined as a submodule which means is listed one <module>..</module> entry. Because the resolution of the import scope will be done before the rest which will result in a fail of the build.

And:

a) What does point 3 mean ?

It means the <dependencies> section should declare some <dependency> with only <group> and <artifact>, not <version>.
The version will be fetched from the imported <dependencyManagement>.
See also "Tracking managed dependency versions in Maven".
As illustrated here, managed dependency is used to lock the version at the parent pom.

Importing pom in dependency management

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