Starting With Apache Maven

Apache Maven is a build or a total project management tool that scores over its predecessors such as ANT for Java builds with the following advantages:

• Useful when multiple JAR files are required for builds
• Resolving recursive dependencies during build time
• Creating project structures that are standardized
• Building, publishing, and deployment of applications
• Supports lifecycle of an application along with plugins

I have created an alumni project that is a dynamic web archive with a Servlet that says “Hello, Alumni”. 

Requirements

• Apache Maven 3.2.5

• Apache Tomcat 8.0.9

• JDK 1.7.0 / JRE 1.7.0

• Notepad++ 6.6.7

Maven Basics

1. Setting Up Maven

First, download Apache Maven 3.2.5. 

Next, set the path variables for M2 and M2_HOME.

Set your M2_HOME environment variable to point to the root of the Apache Maven installation. Also, the M2 environment variable will point to the bin folder under the Apache Maven installation. For example, if you have installed Apache Maven under d:\apache-maven-3.2.5, you would have to set the variables as following:

set M2_HOME= d:\apache-maven-3.2.5

set M2= d:\apache-maven-3.2.5\bin

You must additionally set Maven in your path as follows.

set PATH=%PATH%;%M2%

2. Configuring Maven (Creating a Project)

Now run the mvn archetype:generate command from the place where you want to create a web project. Start off with this command to download all templates from the Internet. This has to be used initially so that all known project templates can be downloaded to the local system. It will prompt you at the end of a successful execution of this command for a number. Enter the number 529, which is used for ‘maven-archetype-webapp’. Make sure that you have an Internet connection — otherwise, you will not see all the archetype listings. Maven Archetype is a templating toolkit that is provided along with Maven.

3. Creating the Alumni Servlet

We need to add a folder, ‘java’, under ‘src\main’ before we can code our Java Servlet. Under main, add the folders ‘me’, followed by ‘sumithpuri’ followed by ‘maven’ – each under the other; so that the directory structure looks like this:

Now, write the java servlet source file AlumniServlet.java using Notepad++ as follows:

4. Building Alumni Project

If we now try to build the project using ‘mvn compiler:compile’, we will get the following errors because we have not added the dependent JAR files for Java Servlets.

5. Dependency Management

We can add the dependent JARs so that the project can compile. We can do so in the pom.xml – The explanation for how Maven will download this JAR to the local repository and the details of the POM (Project Object Model) are provided later in this blog.

6. (Re) Building/Compiling the Alumni Project

Use ‘mvn compiler:compile’ to compile the web application.

7. Packaging and Deploying Alumni Project

Modify the web.xml to add the Servlet using the following configuration.

Use ‘mvn war:war’ to package the web application.

Finally, drop this under the Apache Tomcat ‘webapps’ folder and type the URL  to see the output on the browser as:

Note: Make sure you rename the default index.jsp to index.bkp to render the Servlet output.

Conclusion

Maven provides a simple and extensible way to compile, build and deploy applications and allows us to manage the total build life-cycle of applications.

Maven Theory

Maven Coordinates

The following are known as the maven coordinates (the ones that are the most important) for any project.

• GroupId: The name with which we can refer groups of such projects. e.g.: alumni_project

• ArtifactId: The name with which the JAR or WAR will be created. e.g.: alumni

• Version: The version number of the current project

• Package: The package of the resulting classes, usually under src\java. e.g.: me.sumithpuri.maven

Maven Lifecycle

The most important lifecycle stages or phases for a Maven project include the following.

• Validate: Validates whether the project coordinates and pom.xml are valid, or else it generates errors.

• Compile: Compiles the project and generates errors if it cannot be compiled.

• Test: Unit tests the code and does not require that the code be packaged.

• Package: Packages and generates the artifact, such as a JAR.

• Install: Installs the generated artifact, such as a JAR in the local repository.

Maven Plugins

Maven works in the form of plugins and follows the general format as follows: mvn plugin:goal

Some of the plugins include JAR, compiler, and surefire. An example of a Maven command is as follows: mvn compiler:compile.

Maven Repository (Architecture)

Maven follows a two-repository model, wherein it maintains a local repository (which is created when you execute the first maven command). It downloads dependencies from the remote repository onto this local repository and then builds projects. The architecture looks like the following:

• The Maven project is compiled; it checks in the local repository for dependencies.
• If dependencies do not exist, Maven downloads them from the remote repository.
• Once dependencies are downloaded, they are installed in the local repository.
• Once all dependencies are met, Maven compiles the code and the project.

Project Object Model

Maven is able to achieve dependency management through the existence of Project Object Model or pom.xml files within each of the modules. At each module level, we can provide a pom.xml that contains the build environment information, build settings information, dependency relationships between POMs, and general project information. Also, there is a concept of inheritance and effective POMs (where the effective POM is a resultant of the inheritance hierarchy). 

A Sample POM

Super Project Object Model 

Located inside the Maven installation is a Super POM (pom.xml), usually in the maven-model-builder-<x.y.z>.jar files, which is the under the %M2_HOME%\lib\ folders. This contains the details that all other modules’ POMs inherit from. This also contains the location of the remote repository, which is, by default https://repo.maven.apache.org/maven2. It usually has four main sections – central repository details, plugin repository details, build details, and plugin management details.