This document provides an overview of Maven concepts including: - Defining and overriding project properties - Invoking plugins on the command line - Dependency resolution and managing dependencies - Partial project execution using the -pl and -am flags - Aggregating multi-module projects using parent POMs - Defining and consuming Bill of Materials (BOM) files

1. What I wish I Knew About

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Andres
Almiray
Seasoned Sourceror
@ Oracle
Java Champion
Hackergarten
github.com/aalmiray

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Override Project
Properties

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mvn -Dkey=value
• Properties may be defined
• In a <properties> block
• By plugins
• Property values defined on the command line have
precedence

10. DEMO

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mvn G:A:V:goal

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Invoke plugins on the go
• Any plugin may be applied using the long notation
G:A:V:goal, in other words
mvn com.github.ekryd.echo-maven-plugin:echo-maven-
plugin:1.2.0:echo -Decho.message="Hello World!"

13. DEMO

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Dependency
Resolution

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Dependencies
• Dependencies defined in a <dependencies> block are
known as direct dependencies.
• Dependencies brought by direct dependencies are known
as transitive dependencies.
• Maven resolves dependencies by locality
- direct in the current POM. Last found wins (closest).
- direct in the parent hierarchy.
- transitive in the hierarchy based on number of hops to
reach it. First found wins.

21. DEMO

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Semantic Versioning?
https://twitter.com/rfscholte/status/1243226856914063360

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Use the <dependencyManagement> block!

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http://maven.apache.org/enforcer/maven-enforcer-plugin/

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maven-enforcer-plugin
https://twitter.com/aalmiray/status/1243515942560313350

26. http://andresalmiray.com/maven-dependencies-pop-quiz-results

28. http://andresalmiray.com/maven-dependencies-pop-quiz-results

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Surviving Dependency Hell with Maven
https://saturnism.me/talk/surviving-dependency-hell-maven/

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mvn verify
• The Reactor knows how to build all requirements.
• The install goal should only be used when artifacts must
be published to the local repository.
• Invoking clean diminishes the reuse of intermediate
results -> defeats incremental builds.

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Broken Buildtools and Bad Behaviors;
The Maven Story by Robert Scholte
https://www.youtube.com/watch?v=2HyGxtsDf60

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mvn -am -pl

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Partial Execution
• Maven runs multi-project builds inside a Reactor.
• The Reactor executes goals for all projects.
• mvn -pl <paths> executes the given goals for all projects
in <paths>
• mvn -am -pl <paths> also executes for all prerequisites in
the Reactor

39. DEMO

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Restrictions
• All invoked plugin goals must exist in every project!
• Define common plugins in parent POMs using a
combination of <pluginManagement> and <plugins>.
• You’d be forced to define dummy values for plugins in
some cases.

42. DEMO

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Aggregating POMs

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Aggregating POMs
• These POM files define a <modules> section.
• They are typically also parent POMs but they DO NOT
have to be!
• Also, values in <module> are paths, not project names.

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File Structure & POM

46. DEMO

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BOM Files

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BOM Definition

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BOM Consumption

50. DEMO

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Resources
• https://blogs.oracle.com/developers/mastering-maven-
the-series
• https://cguntur.me/2020/05/20/understanding-apache-
maven-the-series
• https://kordamp.org/kordamp-gradle-plugins

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