Apache Maven

What is Maven?

Apache Maven is project management tool which is following the concept of a project object model (POM), The POM file to manage project’s build, dependency and documentation. The most powerful feature is able to download the project dependency libraries automatically

Key features of Maven

Apache Maven can be used in environments where common build tools like GNU Make or Apache Ant were used. The key features of Maven are:

• Convention over configuration: Maven tries to avoid as much configuration as possible, by choosing real world default values and supplying project templates (archtypes).
• Dependency management: it is possible to define dependencies to other projects. During the build, the Maven build system resolves the dependencies and, if needed, it also builds the dependent projects.
• Repository: project dependencies can be loaded from the local file system, from the Internet or public repositories. The company behind the Maven project also provides a central repository called Maven Central.
• Extensible via plug-ins: The Maven build system is extensible via plug-ins, which allows keeping the Maven core small. The Maven core does for example not know how to compile Java source code, this is handled by the compiler plug-in.

Maven Repository

A maven repository is a directory of packaged JAR file with pom.xml file. Maven searches for dependencies in the repositories. There are 3 types of maven repository:

1. Local Repository
2. Central Repository
3. Remote Repository

 Maven searches for the dependencies in the following order:

Local repository then Central repository then Remote repository.

If dependency is not found in these repositories, maven stops processing and throws an error.

1) Maven Local Repository

Maven local repository is located in your local system. It is created by the maven when you run any maven command.

By default, maven local repository is %USER_HOME%/.m2 directory.

Update location of Local Repository

We can change the location of maven local repository by changing the settings.xml file. It is located in MAVEN_HOME/conf/settings.xml, for example: E:\apache-maven-3.1.1\conf\settings.xml.

2) Maven Central Repository

Maven central repository is located on the web. It has been created by the apache maven community itself.

The path of central repository is: http://repo1.maven.org/maven2/.

The central repository contains a lot of common libraries that can be viewed by this url http://search.maven.org/#browse.

3) Maven Remote Repository

Maven remote repository is located on the web. Most of libraries can be missing from the central repository such as JBoss library etc, so we need to define remote repository in pom.xml file.

  

Maven pom.xml file

POM is an acronym for Project Object Model. The pom.xml file contains information of project and configuration information for the maven to build the project such as dependencies, build directory, source directory, test source directory, plugin, goals etc.

Maven reads the pom.xml file, then executes the goal.

Before maven 2, it was named as project.xml file. But, since maven 2 (also in maven 3), it is renamed as pom.xml.

A multi project pom file includes a modules section. This section tells Maven which project are part of the build.

In the build section of the pom you can define plugins for which are needed for the build.

Elements of maven pom.xml file

For creating the simple pom.xml file, you need to have following elements:

Element	Description
project	It is the root element of pom.xml file.
modelVersion	It is the sub element of project. It specifies the modelVersion. It should be set to 4.0.0.
groupId	It is the sub element of project. It specifies the id for the project group. Defines a unique base name of the organization or group that created the project.
artifactId	It is the sub element of project. It specifies the id for the artifact (project). An artifact is something that is either produced or used by a project. Examples of artifacts produced by Maven for a project include: JARs, source and binary distributions, and WARs.
version	It is the sub element of project. It specifies the version of the artifact under given group.

Maven pom.xml file with additional elements

Here, we are going to add other elements in pom.xml file such as:

Element	Description
packaging	defines packaging type such as jar, war etc.
name	defines name of the maven project.
url	defines url of the project.
dependencies	defines dependencies for this project.
dependency	defines a dependency. It is used inside dependencies.
scope	defines scope for this maven project. It can be compile, provided, runtime, test and system.

Maven life cycle

Every build follows a specified life cycle. Maven comes with a default life cycle that includes the most common build phases like compiling, testing and packaging.

The following lists gives an overview of the important Maven life cycle phases.

• validate - checks if the project is correct and all information is available
• compile - compiles source code in binary artifacts
• test - executes the tests
• package - takes the compiled code and package it, for example
• integration-test - takes the packaged result and executes additional tests, which require the packaging
• verify - performs checks if the package is valid
• install - install the result of the package phase into the local Maven repository
• deploy - deploys the package to a target, i.e. remote repository

If you instruct Maven to execute a phase, it executes all existing phases in the pre-defined sequence until has executed the defined phase. All relevant goals are executed during this process. A goal is relevant for a phase if the Maven plug-in or the pom binds this goal to the corresponding life cycle phase.

Maven Plugins

The maven plugins are central part of maven framework, it is used to perform specific goal.

According to Apache Maven, there are 2 types of maven plugins.

1. Build Plugins
2. Reporting Plugins

Build Plugins

These plugins are executed at the time of build. These plugins should be declared inside the element.

Reporting Plugins

These plugins are executed at the time of site generation. These plugins should be declared inside the element.

Maven Core Plugins

A list of maven core plugins is given below:

Plugin	Description
clean	clean up after build.
compiler	compiles java source code.
deploy	deploys the artifact to the remote repository.
failsafe	runs the JUnit integration tests in an isolated classloader.
install	installs the built artifact into the local repository.
resources	copies the resources to the output directory for including in the JAR.
site	generates a site for the current project.
surefire	runs the JUnit unit tests in an isolated classloader.
verifier	verifies the existence of certain conditions. It is useful for integration tests.

What is archetype?

Archetype is the maven plugin. It creates the project structure.

We can create a simple maven example by executing the archetype:generate command of mvn tool.

To create a simple java project using maven, you need to open command prompt and run thearchetype:generate command of mvn tool.

Syntax

The syntax to generate the project architecture is given below:

1. mvn archetype:generate -DgroupId=groupid -DartifactId=artifactid   
2. -DarchetypeArtifactId=maven-archetype-quickstart -DinteractiveMode=booleanValue  

Example

The example to generate the project architecture is given below:

1. mvn archetype:generate -DgroupId=com.teachtojava -DartifactId=SampleExample   
2. -DarchetypeArtifactId=maven-archetype-quickstart -DinteractiveMode=false  

Note: Here, we are using maven-archetype-quickstart to create simple maven core project. if you use maven-archetype-webapp, it will generate a simple maven web application.

Advantages of Using Maven over Ant

• Managing dependencies.
• Convention over configuration – configuration is very minimal
• Multiple/Repeated builds can be achieved.
• Focus on automation.
• Plugin management.
• Testing - ability to run JUnit and other integration test suites.
• Making the development process transparent.
• Provision to check the status of each build.
• Avoiding inconsistent setups.
• Standard and uniformed infrastructure among projects.

Differences Between ANT and Maven

Maven is a build automation tool used mainly for java projects from apache.

Apache Ant is a Java library and command-line tool whose mission is to drive processes described in build files.

Some of the differences as below.

Project structure: Ant has not a defined project convention, you can put things in any place, and later instruct ant for know where the things are. Maven has a project conventions and has several archetypes for predefined projects, so maven is easier because it knows where things are if you follow the project convention.

Execution way

Ant is a procedural tool, you have to tell it when, what and how it has to do all the things: compile, then copy, then package, then deploy, etc… Maven is a declarative tool, you only have to declare your project object model (pom) and put your source code and resources in the correct folder, maven will take care of the rest for you.

Lifecycle management

Ant do not have a lifecycle management, you have to declare some goals and define which of those goals run first, what run later and so on manually. Maven has a lifecycle management.

Dependency management

Ant does not have any mechanism to manage dependencies, you have to manage it manually: download all dependencies, place the dependencies in a folder and later copy it to the packaged artifacts, maven has a mechanism to manage dependencies, you have a local repository that acts like a cache and you can define some remote repositories in order to download more dependencies, you only have to define all your dependencies needed and maven will download it for you.