Convention over configuration

Convention over configuration is one of the main design philosophies behind Apache Maven. Let's go through a few examples.

A complete Maven project can be created using the following configuration file (pom.xml):

<project>
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.packt</groupId>
  <artifactId>sample-one</artifactId>
  <version>1.0.0</version>
</project>

Note

The Maven POM file starts with the <project> element. Always define the <project> element with the schema. Some tools can't validate the file without it:

<project xmlns=http://maven.apache.org/POM/4.0.0
         xmlns:xsi=………
         xsi:schemaLocation="…">

The pom.xml file is the heart of any Maven project and is discussed in detail in Chapter 2, Understanding the Project Object Model (POM). Copy the previous configuration element and create a pom.xml file out of it. Then, place it in a directory called chapter-01, and then create the following child directories under it:

• chapter-01/src/main/java
• chapter-01/src/test/java

Now, you can place your Java code under chapter-01/src/main/java and test cases under chapter-01/src/test/java. Use the following command to run the Maven build from where the pom.xml is:

$ mvn clean install

This little configuration that you found in the sample pom.xml file is tied up with many conventions:

• Java source code is available at {base-dir}/src/main/java
• Test cases are available at {base-dir}/src/test/java
• The type of the artifact produced is a JAR file
• Compiled class files are copied to {base-dir}/target/classes
• The final artifact is copied to {base-dir}/target
• http://repo.maven.apache.org/maven2, is used as the repository URL.

If someone needs to override the default, conventional behavior of Maven, then it is possible too. The following sample pom.xml file shows how to override some of the preceding default values:

<project>
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.packt</groupId>
  <artifactId>sample-one</artifactId>
  <version>1.0.0</version>
  <packaging>jar</packaging>

  <build>    
    <sourceDirectory>${basedir}/src/main/java</sourceDirectory>              
    <testSourceDirectory>${basedir}/src/test/java               
                                         </testSourceDirectory>     
    <outputDirectory>${basedir}/target/classes
                                             </outputDirectory>     
  </build>
</project>