What I wish I knew about Maven years ago

@aalmiray | andresalmiray.com
Andres
Almiray
Seasoned Sourceror

@ Oracle

Java Champion

Hackergarten

github.com/aalmiray

Override Project
Properties

mvn -Dkey=value
• Properties may be deﬁned

• In a <properties> block

• By plugins

• Property values deﬁned on the command line have
precedence

mvn G:A:V:goal

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!"

Dependency
Resolution

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.

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

Use the <dependencyManagement> block!

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

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

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

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

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.

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

mvn -am -pl

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

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.

Aggregating POMs

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.

File Structure & POM

BOM Files

BOM Deﬁnition

BOM Consumption

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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What I wish I knew about Maven years ago

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