Basic Concepts of Apache Maven

My last article on maven described maven basic setup. This article covers basic concepts of maven self. 

Introduction

Maven is a project management framework (or tool).

This definition is quite abstract and don’t justifies the richness and complexity of Maven. It turns out often it’s not easy to describe software tools. So the idea of Software is central element. The effort was to adopt best practices let’s say principles:

• Convention over configuration
• Declarative execution
• Reuse of build logic
• Coherent organization of dependencies

Let’ see how maven adopts these. Here are the key concepts maven provides:

• Set of build standards
• Standard life cycles for building, testing and so on
• Default Tasks
• Common declarative Project Object Model (POM)
• Dependencies description and management
• Artifact repository
• Modular Design (Plug-ins)

Maven provides a comprehensive model that can be applied to all software projects. That model uses a common project “language”, and the software tool is just a supporting element within this model.

Maven is not like Ant

Because i’m was long time a Apache Ant user (and still i am) it’s important for me to emphasize some key differences in maven philosophy.

• Pre-defined (non just project wide) build standards, with pre-defined default life cycles, directory layout and more. Can say maven has predefined assumptions how to build your project (I don’t say that i like it in any sence)
• It’s possible to build any given project without having to understand how the individual plug-ins works.
• Maven provides declarative style. Instead of defining procedures your configure it.

That is in general. Let’s look deeper where and how the configuration is done.

Maven Project Object Model (POM)

A File named pom.xml is XML representation of “Project Object Model” an always a central place of any maven project. We can think of it as maven project definition file. Or as maven POM Reference states

It is a one-stop-shop for all things concerning the project. In fact, in the Maven world, a project need not contain any code at all, merely a pom.xml

In dedail every Project has basics…

<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.codehaus.mojo</groupId>
  <artifactId>my-project</artifactId>
  <version>1.0</version>

<packaging>jar</packaging>

</project>

These three elements defines the ID of an artifact and in effect of the particular build results.
groupId:artifactId:packaging:version

If packaging is omitted “jar” is assumed. The Apache Maven POM Reference covers all the aspects of the POM. Here are just sections listed.

• Basics
  • groupId
  • projectId
  • version
  • packaging
  • parent – Define inheritance to parent project.
  • modules – declares child modules.
  • dependencies – Dependencies of a project.
  • dependencyManagement – allows maintenance application-wide dependency maintenance.
  • properties
• Project Information (For human eyes, no meaning for maven)
  • name
  • description
  • inceptionYear
  • licenses
  • organizations
  • developers
  • contributors
• Build Settings: Changes in Lifecycles done here.
  • build
    • defaultGoal
    • directory
    • finalName – suitable for artifact name
    • filters
    • resources – used for configuration of resource paths and destinations.
    • testResources
    • plugins -  allows registration and configuration of custom build-steps.
    • pluginManagement
    • directories
    • extensions
  • Reporting
    • plugins
• Environment Settings
  • issueManagement
  • ciManagement – Continous Integration management.
  • mailingLists
  • scm – Source Revision Control Systems configuration.
  • repositories – artefat repositories configuration.
  • pluginRepositores – is here to define separate repositories for Maven Plugins (not used widely)
  • distributionManagement – manges distribution on releases.
    • downloadUrl
    • status
    • repository
    • snapshotRepository
    • site
    • relocation
• Profiles

The POM-Tree gives a quick overview, what it ist all bout. Of course you don’t have to be familiar with all that parts of the tree. It ist usual to start working with simple pom and extending it step by step if default behavior is not sufficient. Therefore pom.xml represents static project configuration, let’s look how this configuration apply on build-time.

Lifecycles

Lifecycle is essential concept of maven. There are tree pre-defined Lifecycles Clean, Build and Site. Please read Lyfecycle Refence for more details. Every lifecycle defines an ordered set of Phases. If Clean-Lifecycle and Site-Lifecycle defines 3 and 4 Phases respectively. The main Build-Lifecycle has 23!:

1. validate
2. initialize
3. generate-sources
4. process-sources
5. generate-resources
6. process-resources
7. compile
8. process-classes
9. generate-test-sources
10. process-test-sources
11. generate-test-resources
12. process-test-resources
13. test-compile
14. process-test-classes
15. test
16. prepare-package
17. package
18. pre-integration-test
19. integration-test
20. post-integration-test
21. verify
22. install 
23. deploy

These build phases are executed sequentially in the given order. To do all those, you only need to call the last build phase to be executed, in this case, deploy:
mvn deploy
and
mvn install
will execute all phases except deploy and do on. It is also possible to call phases of different lifecycles e.g.
mvn clean install
this command will traverse into all of the lifecycles and run clean including all of the prior steps then install in the same way.

Maven Goals

A Goal represents a specific task (finer than a build phase) which actually caries out the amount of work.

• A goal may be bound to zero or more build phases. If a build phase has no goals bound to it, that build phase will not execute
• A goal not bound to any build phase could be executed outside of the build lifecycle by direct invocation

Example:
mvn clean dependency:copy-dependencies package
dependency:copy-dependencies is an unbound goal, which is executed as it is. clean and package are phases and executed like phases with predecessors.
Moreover, if a goal is bound to one or more build phases, that goal will be called in all those phases.

Therefore it’s getting essential to know how that Built-in Lifecycle bindings between phases and goals looks like. And then to know how are binding to the default plugins. In the Lifecycle List above you can see all the default bounded goals. Effectively these goals are executed every time in default configuration.
So it helps to know where to find the configuration information about them. Here ist the short list:

Basic Bindings for Build-Lifecycle

Below are the default life-cycle bindings for the jar packaging.

Profiles

Profiles are specify some subset of  POM definition. Piratically profiles modify the POM at build time, and are meant to be used to give context dependent parameters. Fro example they allow to define test and production context without creating several different POM definitions. Profile-definitions  is defined inside of pom.xml or settings.xml following same inheritance rules as the rest of POM file (read above).

Profiles can be used explicitly form command line

mvn groupId:artifactId:goal -P profile-1, profile-2

or defined as default active inside settings.xml

<settings>
  ...
  <activeProfiles>
    <activeProfile>profile-1</activeProfile>
  </activeProfiles>
  ...
</settings>

But in case of pom definition it is considerable to use use “smart activation” of a profile. Example below show’s example profile definition which will be activated automatically on apple mac computers. The defined Profiled with id default_mac only set property: build.style="Macintosh"

<profile>
   <id>default_mac</id>
   <activation>
       <os>
        <family>mac</family>
      </os>
  </activation>
   <properties>
      <build.style>Macintosh</build.style>
  </properties>
</profile>

Check more examples.

Summary

I hope this helps someone to go quick and dirty familiar with maven. For details pleas ask questions or see following resources.

• Maven Getting Startet guide.
• Maven Documentation by Sonatype.