Apache Maven: pom.xml explanation

In this article, we will explore and understand the major elements of “pom.xml” file

What is pom.xml in Maven?

POM stands for Project Object Model and this is specific to every project. In fact every Maven based project should have one pom.xml lying at the project’s root directory describing its various configurations relating to project

These configurations are

  • project dependencies
  • plugins with its goals
  • build profiles
  • developers involved in the project development
  • mailing lists
  • the organization and licenses
  • the URL of where the project lives
  • and other little pieces related to the project

In simple terms to summarize, pom.xml is a one-stop-shop concerning the project

Major elements of pom.xml

  • project –> root element
  • modelVersion –> mandatory element with value 4.0.0 which is currently supported version for both Maven 2 & 3
  • groupId –> this is the top-most element while considering maven repository structure and its generally kept unique amongst organization
  • artifactId –> this is similar to name of the project and this lies under <groupId> element
  • version –> this is final element in Maven co-ordinate system and generally it defines version of the project. Like, initially it can be kept 1.0 and later with couple changes version changes to 1.1 or 2.0 depending on the major/minor releases. Together with <groupId> & <artifactId> elements acts like an address and timestamp in one
  • name –> any meaningful name to the project
  • description –> description of the project
  • packaging –> this elements defines how exactly this project needs to be bundled or packaged after development. Various packaging standards in Java/JEE project developemtns are JAR or WAR or EAR. When no <packaging> element defined, then by default final products is packaged as JAR
  • url –> this elements can be used to define where exactly project going to live after development (normally it points to SVN location)
  • dependencies/dependency –> defines what are the required dependencies for this project into developments (read here about dependency scopes and transitive dependencies)
  • build/plugins –> various plugins along with their goals can be configured under <build> element (read here about Maven plugins)
  • finalName –> this is the child element under <build> element defining what will be final name of the project and a JAR or WAR or EAR will be generated under “target” folder with this name

pom.xml

<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">

	<!-- Currently supported version is 4.0.0 for Maven 2 & 3 -->
	<modelVersion>4.0.0</modelVersion>

	<!-- The Basics -->
	<groupId>...</groupId>
	<artifactId>...</artifactId>
	<version>...</version>
	<packaging>...</packaging>
	<dependencies>...</dependencies>
	<parent>...</parent>
	<dependencyManagement>...</dependencyManagement>
	<modules>...</modules>
	<properties>...</properties>

	<!-- Build Settings -->
	<build>...</build>
	<reporting>...</reporting>

	<!-- More Project Information -->
	<name>...</name>
	<description>...</description>
	<url>...</url>
	<inceptionYear>...</inceptionYear>
	<licenses>...</licenses>
	<organization>...</organization>
	<developers>...</developers>
	<contributors>...</contributors>

	<!-- Environment Settings -->
	<issueManagement>...</issueManagement>
	<ciManagement>...</ciManagement>
	<mailingLists>...</mailingLists>
	<scm>...</scm>
	<prerequisites>...</prerequisites>
	<repositories>...</repositories>
	<pluginRepositories>...</pluginRepositories>
	<distributionManagement>...</distributionManagement>
	<profiles>...</profiles>
</project>

 

3 important elements of pom.xml (Minimum requirement)

Apart from root element i.e.; <project> and <modelVersion> element, there are 3 more mandatory elements required for a minimum pom.xml to exists and run/execute successfully

These mandatory elements are groupId, artifactId, and version

Minimum pom.xml

<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>in.bench.resources</groupId>
	<artifactId>MavenTestArticle</artifactId>
	<version>0.0.1-SNAPSHOT</version>
	<name>MavenTestArticle</name>
	<description>This is test project to try out various features in Maven</description>
</project>

 

How to get effective pom.xml (Effective POM tab in Eclipse IDE)

There are two types of POM, when dealing with Maven based projects

  • Application pom.xml
  • Effective pom.xml

Application POM

Configuration we do specific to any Java/JEE projects is called “application POM”. These configurations normally contain dependencies, plugins along with their goals and build profiles

Effective POM

After configuring application POM for any Java/JEE projects, Maven constructs an “effective POM” with required plugins and inheriting mandatory properties and other elements configured in settings.xml before executing any commands or goals

To get effective POM, use Maven command (from command prompt or terminal)

mvn help:effective-pom

In Eclipse IDE, when we open pom.xml in editor there are 5 tabs available. Out of these 5 tabs, “Effective POM” tab shows the effective pom.xml

Configuring source/target JDK version in Maven compiler plugin

There are couple of plugins adds up in the effective POM, one such plugin is Maven compiler plugin. Basically this plugins dictates which version of JDK to use to compile the source code

By default, target and source setting is 1.5. But we can increase to higher level to JDK_1.6 or JDK_1.7 or JDK_1.8 depending on the business requirement by configuring (overriding) maven-compiler-plugin in our application POM

For example, let’s configure to JDK 1.7

                       <!-- plugin 2-maven compiler plugin -->
			<plugin>
				<groupId>org.apache.maven.plugins</groupId>
				<artifactId>maven-compiler-plugin</artifactId>
				<version>3.1</version>
				<configuration>
					<source>1.7</source>
					<target>1.7</target>
				</configuration>
			</plugin>

 

Inheritance from parent pom.xml concept in multi-module project

If we want to segregate different projects based on the domains, then each project will contain single pom.xml

Let’s assume AddressProject, CustomerProject and NetworkProject are three projects and their inter-dependencies are sequenced as below

NetworkProject –> CustomerProject –> AddressProject

Now we will create another project which will be parent of these three projects and here we will mention what are all the child projects with <modules>/<module> elements in pom.xml

So, if we execute any maven command in the parent pom.xml then it will start building the child projects as well (no need consider sequence, Maven takes care of it)

Also, dependencies configured in the parent POM (project) will be inherited to child POM (projects) –> just we have to mention <groupId> and <artifactId> elements and there is absolutely no need of <version> element, as this will be considered/inherited from parent POM. But if we want to override, then we have to specifically mention in the distinct child POM with <version> element

References

http://maven.apache.org/pom.html
http://maven.apache.org/pom.html#What_is_the_POM
http://maven.apache.org/pom.html#The_Basics
http://maven.apache.org/guides/introduction/introduction-to-the-pom.html
http://en.wikipedia.org/wiki/Apache_Maven#Project_Object_Model
http://www.benchresources.net/apache-maven-plugins-explanation
http://www.benchresources.net/apache-maven-dependency-scopes
http://www.benchresources.net/apache-maven-transitive-dependencies-explanation
http://maven.apache.org/plugins/maven-help-plugin/usage.html
http://maven.apache.org/plugins/maven-compiler-plugin/
http://maven.apache.org/plugins/maven-compiler-plugin/examples/set-compiler-source-and-target.html
 

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