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Maven

What is Maven?

  • Maven is a build tool mostly used in Java projects. By build tool it means making everything related to building the software project automatically. But maven also provides many functions like documentation, dependencies management, release...

POM (Project Object Model)

  • POM is an xml file that describes the resources of the project. This includes where is source code, test code, what external library it has. POM file describes what to build. Each project has a POM file (pom.xml) located in the root directory of the project.
  • This is an mininal POM file 
    <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">

    <groupId>com.tamco</groupId>
    <artifactId>java-gateway</artifactId>
    <version>1.0.0</version>
</project>
  • groupId is the unique ID for a project. Most often we use Java root package name of the project as the groupId. But it does not have to be the Java package name and does not need to use dot notation.
  • artifactId is the name of the project.
  • version is the version of the project.

Maven Directory Structure

root|
    |_______ src|
    |           |_______ main|
    |           |            |_______ java
    |           |            |_______ resources
    |           |            |_______ webapp
    |           |
    |           |_______ test|
    |                        |_______ java
    |                        |_______ resources
    |
    |_______ target
    |
    |_______ pom.xml
  • src is the root directory for source code and test code.
  • target is the folder that contains the results of the build process (jar or war file).

Project Dependencies

  • The Java ecosystem is very HUGE. Today we have ton ton of Java open source libs out there. Almost Java business projects re-use those libraries, so we need a way to manage those dependencies.
  • Maven provides built-in dependency management. 
    <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">

    <groupId>com.tamco</groupId>
    <artifactId>java-gateway</artifactId>
    <version>1.0.0</version>

    <dependencies>
        <dependency>
          <groupId>org.jsoup</groupId>
          <artifactId>jsoup</artifactId>
          <version>1.7.1</version>
        </dependency>
    </dependencies>
</project>

Maven Repositories

Maven has three types of repository:

  • Local repository is the directory (user-home/.m2) on developer's machine. This contains all dependency downloads. Your own project can be built and installed in local repository by running the command mvn install.
  • Central repository is the repository provided by Maven community. By default, Maven looks in this repository to download any dependencies that has not existed on local repository yet.
  • Remote repository is a repository on a web server from which Maven can download dependencies, just like the central repository. A remote repository can be located anywhere on the internet, or inside a local network.

Maven Build Liffecycles, Phases and Goals

  • When Maven builds a project it follows a Build Lifecycle. A Build Lifecycle is divided into Build Phases and Build Phases are divided in to Build Goals. Maven has 3 built-in Build Lifecycles:

    • default
    • clean
    • site

  • Each of these Build Lifecycles takes care of different aspect of building software project. The default life cycle handles everything related to compiling and packaging your project. The clean life cycle handles everyting related to removing temporary files from output directory. The site life cycle handles everything related to generating document.

  • Each life cycle is divided into sequence of build phases. And build phases are divided into build goals. So, the total build process is the sequence of build life cycle(s), build phases and goals. You can execute a whole build life cycle (like clean and site), or a build phase (like install), or a goal. Note: you cannot execute the default life cycle directly, you have to specify the phase in that life cycle.

  • The table below describes some most commonly used phases:

    Build Phase Description
    validate Validates that the project is correct and all necessary information is available. This also makes sure that the dependencies are downloaded.
    compile Compiles the source code of the project.
    test Run tests againts the compiles source code using the suitable unit testing framework.
    package Packs the compiles source code in its distributable format, such as JAR or WAR.
    install Installs the package to local repository, for use as a dependency in other projects locally.
    deploy Copy the final package to a remote repository to share with other developers and projects.

Maven Command Structure

A Maven command consists of 2 elements:

  • mvn
  • One or more build life cycles, build phases or build goals

For examlple, mvn clean package will execute all build phases of clean life cycle ang then execute the build phase package of the default life cycle.

Maven Build Profiles

Maven Plugins