Coordinates

Maven coordinates are use to unqiuely identify maven artifacts, the below is used to create the corrdinate. The coordinate is used for share your projects and in the dependencies/dependency in the POM file.

• groupId - unique id, which normally is the reverse of your domain name for example uk.co.datadisk
• artifactId - normally the project name
• version - the specific version of the project

The typical version breakdown could be below but this depends on what your company implements

• Major version - generally the first number
• Minor version - generally the second number
• Incremental version - genrally know as the patch number, normally the third number
• Build version - companies might use a build number
• Qualifier - examples are beta, alpha, etc

The snapshot versions are important and we will look in depth later and you will see why, I will provide a link

• Tells maven this is a development version, its not stable
• Informs developers what versions they should be working on
• Maven will automatically fetch the latest SNAPSHOT

<groupId>uk.co.datadisk</groupId>
<artifactId>hello-world</artifactId>
<version>0.0.1-SNAPSHOT</version>

Repositories

Repositories are a location where your maven artifacts are stored, there are a number of different primary types

• Local - is on your local filesystem of drive (windows), normally in /<user home>/.m2/repository
• Central - is a public repository hosted by Maven
• Remote - other repositories which could be private or remote (Oracle, Atlassian, JBoss, etc)

When you execute maven it will first look in the local repository (cached copy), then using the dependency it will then look either at Maven entral or a 3rd party repository. All older versions of any artifacts will also be stored unless you clear them down.

Local Repository	Maven Central

Wagon

Maven Wagon is an API for transporting artifacts (to or from a repo), this allows for different providers to communicate with a Maven repository, HTTP/HTTPS (File, FTP,SSH/SCP, WebDAV and SCM) are generally used and you also have a proxy setup if your company uses proxies. The below shows you that one API is used for all communication (HTTP, HTTPS, SSH, etc)

Project Object Model (POM)

The POM file is pom.xml, its a XML file that complies with a schema which defines the rules, what elements can be used, etc. The POM also has inheritance capabilities (parent POM) and thus you only need to define properties once. You can show the complete POM file (including all the inheritance) using the command effective-pom.

mvn help:effective-pom

Dependencies

Dependencies are artifacts that your project requires to compile/run, there are three terms that you need to understand

• Dependency - an artifact that your project requires
• Transitive Dependency - a dependency of a artfact that your project requires, this can go many levels, dependency -> dependency -> dependency, etc. Cyclic dependencies are NOT supported.
• Dependency Management - a management system that allows versions to be used for different artifact versions.
• Dependency Mediation - determines what version to use when multiple versions of the same dependency are encountered, Maven uses the nearest definition is used, try not to use multiple versions use inheritance (parent POM).
• Exclude/Optional - you can exclude or make optional specific dependencies

Dependencies have scope with in a Maven project, also there are three classpaths (compile, run and test) the scopes are

• Compile - (default) available on all classpaths and propagated to down stream projects
• Provided - like compile but expected to be provided by the JDK or container at runtime
• Runtime - needed for runtime only
• Test - only used on test classpath and not transitive
• System - similar to provided but JAR is added to system explicity via file path
• Import - imports dependency of POM

Dependencies are handled by the Maven dependency plugin and there are a few commands you can use

mvn dependency:tree

mvn dependency:go-offline

mvn dependency:purge-local-repository

mvn dependency:sources

Directory Layout

Maven uses a standard directory outlay, below is a Maven project created by Intellij, the src directory contains both the source code (main/java directory) for the application and test files (test/java). Java packages will be created in the src/main/java directory.

Intellij	Maven standard directory layout

Build Lifecycles

Maven uses plugins to build an application this consists of phases and goals, lifecycles and phases are the framework which call plugin goals in a sequence.

• Lifecycle - is a group of phases, which deploy and distribute the project
• Phase - each phase can have one or more goals, it represents a stage in the Maven build.
• Goal - a goal is responsible for a specific task

Maven has a number of pre-defined lifecycles

• clean - removes all built artifacts from the working directory
• default - performs the build and deployment of your project
• site - creates a website for your project,

To see the phases for a particular lifestyle you can use the help:describe command, you can see that in the below screenshot the clean lifecycle uses 3 phases, the pre-clean and post-clean are not binded to anything but the clean phase uses a plugin goal called clean.

mvn help:describe -Dcmd=clean

The default lifecycle consists of the following top-level goals, the commonly used

• Validate - checks the pom.xml for any validation errors
• Compile - compiles the source code
• Test - compiles the test source code and runs it
• Package - packages the compiled files into the specified packaging type (pom.xml)
• Verify - runs the integration tests
• Install - installs the build into the local respoitory
• Deploy - deploys the build into a shared Maven repository

Intellij has a very handy tool that allow you to look and run lifecycles within your project, you can also see the plugins

There are many phases in the default lifecycle, the complete list is below

I will talk more about lifecycles in another section.

Wrapper

The Maven wrapper is a wrapper around Maven that allows you to distribute your Maven version with the project, to install use the below command, you use the mvnw command instead of the normal mvn command

mvn -N io.takari:maven:wrapper
mvn -N io.takari:maven:wrapper -Dmaven=3.5.2                  ## specific version

Archetypes

A archetype is a predefined Maven project template, when you select a specific archetype it will setup the structure and create the neccessary files, there are many archetypes

Looking at the simple archetype you can see the structure and the files that will be created

If you want to create a specific archetype its best to either use an your IDE (see intellij below) or go to the Maven website documentation to retrieve the command, there are many archetypes available including very old ones.