What is POM in Maven?

A Project Object Model or POM is the fundamental unit of work in Maven. It is an XML file that contains information about the project and configuration details used by Maven to build the project.

In short the pom.xml will have all information to build your project.

For example you want to build a project and you have only the pom.xml sent to you via mail. If there are enough entries in the pom.xml then that is all you need! You can import it to Eclipse, Maven will download your source code from CVS, download various dependency jars (like Spring, Apache Commons), run your test cases, build the jar/war, deploy to your jboss/app server, generate a report of your code quality (using Sonar, maybe). Each task you want to do will be mentioned as a goal.

Some of the configuration that can be specified in the POM are the project dependencies, the plugins or goals that can be executed, the build profiles, and so on. Other information such as the project version, description, developers, mailing lists and such can also be specified.

Super POM

The Super POM is Maven’s default POM. All POMs extend the Super POM unless explicitly set, meaning the configuration specified in the Super POM is inherited by the POMs you created for your projects.

Like all classes in Java extends Object, all project’s POM extends Super POM

Minimal POM

The minimum requirement for a POM are the following:

  • project root
  • modelVersion – should be set to 4.0.0
  • groupId – the id of the project’s group.
  • artifactId – the id of the artifact (project)
  • version – the version of the artifact under the specified group

Here’s an example:

<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.talksinfo.testapp</groupId>
  <artifactId>test-app</artifactId>
  <version>0.0.1-SNAPSHOT</version>
  <name>test-app</name>
</project>

A POM requires that its groupId, artifactId, and version be configured.

Furthermore, you can see that in the minimal POM the repositories were not specified. If you build your project using the minimal POM, it would inherit the repositories configuration in the Super POM. Therefore when Maven sees the dependencies in the minimal POM, it would know that these dependencies will be downloaded from http://repo.maven.apache.org/maven2 which was specified in the Super POM.

  • project This is the top-level element in all Maven pom.xml files.
  • modelVersion This element indicates what version of the object model this POM is using. The version of the model itself changes very infrequently but it is mandatory in order to ensure stability of use if and when the Maven developers deem it necessary to change the model.
  • groupId This element indicates the unique identifier of the organization or group that created the project. The groupId is one of the key identifiers of a project and is typically based on the fully qualified domain name of your organization. For example org.apache.maven.plugins is the designated groupId for all Maven plugins.
  • artifactId This element indicates the unique base name of the primary artifact being generated by this project. The primary artifact for a project is typically a JAR file. Secondary artifacts like source bundles also use the artifactId as part of their final name. A typical artifact produced by Maven would have the form <artifactId>-<version>.<extension> (for example, myapp-1.0.jar).

For a complete reference of what elements are available for use in the POM please refer to our POM Reference.

High Level Elements of POM

Maven elements

Conclusion

Now we have knowledge of what is POM, Super POM and Minimal POM.

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