Maven: The Definitive Guide

To start a new Maven project, use the Maven Archetype plugin from the command line.

$ mvn archetype:create -DgroupId=org.sonatype.mavenbook.ch03 \
                                         -DartifactId=simple \
                                         -DpackageName=org.sonatype.mavenbook
[INFO] Scanning for projects...
[INFO] Searching repository for plugin with prefix: 'archetype'.               
[INFO] artifact org.apache.maven.plugins:maven-archetype-plugin: checking for 
       updates from central
[INFO] -----------------------------------------------------------------------
[INFO] Building Maven Default Project
[INFO]    task-segment: [archetype:create] (aggregator-style)
[INFO] --------------------------------------------------------------------
[INFO] [archetype:create]
[INFO] artifact org.apache.maven.archetypes:maven-archetype-quickstart: \
       checking for updates from central
[INFO] Parameter: groupId, Value: org.sonatype.mavenbook.ch03
[INFO] Parameter: packageName, Value: org.sonatype.mavenbook
[INFO] Parameter: basedir, Value: /Users/tobrien/svnw/sonatype/examples
[INFO] Parameter: package, Value: org.sonatype.mavenbook
[INFO] Parameter: version, Value: 1.0-SNAPSHOT
[INFO] Parameter: artifactId, Value: simple
[INFO] * End of debug info from resources from generated POM *
[INFO] Archetype created in dir: /Users/tobrien/svnw/sonatype/examples/simple

mvn is the Maven 2 command. archetype:create is called a Maven goal. If you are familiar with Apache Ant, a Maven goal is analogous to an Ant target; both describe a unit of work to be completed in a build. The -Dname=value pairs are arguments that are passed to the goal and take the form of -D properties, similar to the system property options you might pass to the Java Virtual Machine via the command line. The purpose of the archetype:create goal is to quickly create a project from an archetype. In this context, an archetype is defined as “an original model or type after which other similar things are patterned; a prototype.”^[2] A number of archetypes are available in Maven for anything from a simple Swing application to a complex web application. In this chapter, we are going to use the most basic archetype to create a simple skeleton starter project. The plugin is the prefix archetype, and the goal is create.

Once we've generated a project, take a look at the directory structure Maven created under the simple directory:

simple/
simple/pom.xml
      /src/
      /src/main/
          /main/java
      /src/test/
          /test/java

This generated directory adheres to the Maven Standard Directory Layout. We’ll get into more details later in this chapter, but for now, let’s just try to understand these few basic directories:

	The Maven Archetype plugin creates a directory that matches the artifactId. Simple. This is known as the project’s base directory.
	Every Maven project has what is known as a Project Object Model (POM) in a file named pom.xml. This file describes the project, configures plugins, and declares dependencies.
	Our project's source code and resources are placed under src/main. In the case of our simple Java project this will consist of a few Java classes and some properties file. In another project, this could be the document root of a web application or configuration files for an application server. In a Java project, Java classes are placed in src/main/java and classpath resources are placed in src/main/resources.
	Our project's test cases are located in src/test. Under this directory, Java classes such as JUnit or TestNG tests are placed in src/test/java, and classpath resources for tests are located in src/test/resources.

The Maven Archetype plugin generated a single class org.sonatype.mavenbook.App, which is a 13-line Java class with a static main function that prints out a message:

package org.sonatype.mavenbook;

/**
 * Hello world!
 *
 */
public class App 
{
    public static void main( String[] args )
    {
        System.out.println( "Hello World!" );
    }
}

The simplest Maven archetype generates the simplest possible program: a program which prints "Hello World!" to standard output.