Dependency management and Maven

Maven is great and mature. There is always a solution on almost everything. The main case you might stumble on organisation projects is dependency management. Instead of each project having it’s own dependencies you want a centralised way to inherit those dependencies.

In those case you declare on the parent prom the managed dependencies. In my example I just want to include the Akka stream dependencies.

<?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>org.example</groupId>
	<artifactId>maven-dependency-management</artifactId>
	<packaging>pom</packaging>
	<version>1.0-SNAPSHOT</version>

	<properties>
		<akka.version>2.5.31</akka.version>
		<akka.http.version>10.1.11</akka.http.version>
		<scala.binary.version>2.12</scala.binary.version>
	</properties>

	<modules>
		<module>child-one</module>
	</modules>


	<dependencyManagement>
		<dependencies>
			<dependency>
				<groupId>com.typesafe.akka</groupId>
				<artifactId>akka-stream_2.12</artifactId>
				<version>${akka.version}</version>
			</dependency>
			<dependency>
				<groupId>com.typesafe.akka</groupId>
				<artifactId>akka-http_2.12</artifactId>
				<version>${akka.http.version}</version>
			</dependency>
			<dependency>
				<groupId>com.typesafe.akka</groupId>
				<artifactId>akka-http-spray-json_2.12</artifactId>
				<version>${akka.http.version}</version>
			</dependency>
		</dependencies>
	</dependencyManagement>

</project>

What I use is the dependency management block.

Now the child project would be able to include those libraries without specifying the version. Having the version derived and managed is essential. Many unpleasant surprises can come if a version is incompatible.

Now on to the child module the versions are declared without the version since it is the child module.

<?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>maven-dependency-management</artifactId>
		<groupId>org.example</groupId>
		<version>1.0-SNAPSHOT</version>
	</parent>
	<modelVersion>4.0.0</modelVersion>

	<artifactId>child-one</artifactId>

	<dependencies>
		<dependency>
			<groupId>com.typesafe.akka</groupId>
			<artifactId>akka-stream_2.12</artifactId>
		</dependency>
		<dependency>
			<groupId>com.typesafe.akka</groupId>
			<artifactId>akka-http_2.12</artifactId>
		</dependency>
		<dependency>
			<groupId>com.typesafe.akka</groupId>
			<artifactId>akka-http-spray-json_2.12</artifactId>
		</dependency>
	</dependencies>

</project>

On another note sometimes we want to use another project’s dependency management without that project being our parent. Those are cases where you need to include the dependency management from a parent project when you already have a parent project.

<?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>org.example</groupId>
	<artifactId>independent-project</artifactId>
	<version>1.0-SNAPSHOT</version>

	<dependencyManagement>
		<dependencies>
			<dependency>
				<artifactId>maven-dependency-management</artifactId>
				<groupId>org.example</groupId>
				<version>1.0-SNAPSHOT</version>
				<type>pom</type>
				<scope>import</scope>
			</dependency>
		</dependencies>
	</dependencyManagement>

	<dependencies>
		<dependency>
			<groupId>com.typesafe.akka</groupId>
			<artifactId>akka-stream_2.12</artifactId>
		</dependency>
		<dependency>
			<groupId>com.typesafe.akka</groupId>
			<artifactId>akka-http_2.12</artifactId>
		</dependency>
		<dependency>
			<groupId>com.typesafe.akka</groupId>
			<artifactId>akka-http-spray-json_2.12</artifactId>
		</dependency>
	</dependencies>
</project>

As you can see in the block

	<dependencyManagement>
		<dependencies>
			<dependency>
				<artifactId>maven-dependency-management</artifactId>
				<groupId>org.example</groupId>
				<version>1.0-SNAPSHOT</version>
				<type>pom</type>
				<scope>import</scope>
			</dependency>
		</dependencies>
	</dependencyManagement>

We included the dependency management from another project, which can be applied to inherit dependencies from multiple projects.