Maven - pom.xml

POM = Project Object Model

The pom.xml is the master configuration file for a Maven project. Every project has exactly one pom.xml.

Here is what the pom.xml for the HelloWorldJARArch project looks like:

 <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.confucius</groupId>  
  <artifactId>HelloWorldJARArch</artifactId>  
  <version>1.0-SNAPSHOT</version>  
  <packaging>jar</packaging>  
   
  <name>HelloWorldJARArch</name>  
  <url>http://maven.apache.org</url>  
   
  <properties>  
   <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>  
  </properties>  
   
  <dependencies>  
   <dependency>  
    <groupId>junit</groupId>  
    <artifactId>junit</artifactId>  
    <version>3.8.1</version>  
    <scope>test</scope>  
   </dependency>  
  </dependencies>  
 </project>  
   

First, note that unlike an Ant build.xml, the pom.xml DOES NOT contain step-by-step instructions for the build.

It declares the packaging as a 'jar' - that is all Maven needs to know.

Other than that, it contains some metadata - group id, artifact id, version - the three unique coordinates of any Maven project - we will learn more about Maven co-ordinates later.

Then it contains the descriptive name of the project (you can give it any descriptive name - like, "My first Maven project") and a URL in case this project had a web page.

Then come the properties - Maven properties are value placeholder, like properties in Ant. Their values are accessible anywhere within a POM by using the notation ${X}, where X is the property.

Finally, one of the most important parts of any pom.xml -- the dependencies.

Just like Ivy, the dependencies are a list of all the external jars your project depends on. Each dependency is specified by a unique combination of groupId, artifactId and version. This unique combination is called the co-ordinates of the jar.

By default, Maven will look for dependency jars by their co-ordinates in the central Maven repository. It is possible to tell Maven to look in other repositories as well.