learn-co-curriculum / java-mod-5-maven-cli

Maven CLI and Packaging Projects

Learning Goals

• Use the Maven commands for cleaning, compiling, and packaging a project.

Introduction

Now that we’ve set up our POM file and the basic directory structure, we can compile and package the project into a JAR file. But before we can do that, we need to add code to our project.

Create a Simple Program

We’ll create a simple program to demonstrate compilation and packaging in Maven. We will be creating an App.java file in the src/main/java/org/example directory.

└── ~/developer/example-app
		├── pom.xml
		├── src
		│   ├── main
		│   │   ├── java
		│   │   │   └── org
		│   │   │       └── example
		│   │   │           └── App.java
		│   │   └── resources
		│   ├── target
		│   └── test
		└── target

And here’s the content of the App.java file:

package org.example;

import java.util.Scanner;

public class App
{
    public static void main( String[] args )
    {
        Scanner sc = new Scanner(System.in);
        System.out.println("What is your name?");
        String name = sc.nextLine();
        System.out.println(greet(name));
        sc.close();
    }

    public static String greet(String name) {
        return String.format("Hello, %s!", name);
    }
}

Cleaning the Target Directory

Before we compile and package the project, we want to remove the target directory. As a reminder, the target directory is where the compiled classes are stored.

Run the mvn clean command to completely remove the target directory.

Compiling the Project

We can use the mvn compile command to tell Maven to compile our project. This will create a target directory which will have a classes directory containing the compiled class.

Making a JAR File (Packaging)

We can run the mvn package command to build a JAR file. But before we can successfully run that command, we need to do a couple of things:

1. Add the maven-jar-plugin which will create the JAR file.
2. Add configuration to tell Java which .class file to run in the JAR file. In our case, it will be the App class.

Let’s modify our pom.xml file to add the plugin and the config.

<?xml version="1.0" encoding="UTF-8"?>

<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.example</groupId>
  <artifactId>example-app</artifactId>
  <version>1.0-SNAPSHOT</version>

  <name>example-app</name>
  <url>http://www.example.org</url>

  <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <maven.compiler.source>17</maven.compiler.source>
    <maven.compiler.target>17</maven.compiler.target>
  </properties>

  <dependencies>

  </dependencies>

  <build>
    <plugins>
      <plugin>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>3.10.1</version>
      </plugin>
			<!-- START NEW SECTION -->
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-jar-plugin</artifactId>
        <version>3.2.2</version>
        <configuration>
          <archive>
            <manifest>
              <mainClass>org.example.App</mainClass>
            </manifest>
          </archive>
        </configuration>
      </plugin>
			<!-- END NEW SECTION -->
    </plugins>
  </build>
</project>

We have added the following to our POM file:

<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-jar-plugin</artifactId>
  <version>3.2.2</version>
  <configuration>
    <archive>
      <manifest>
        <mainClass>org.example.App</mainClass>
      </manifest>
    </archive>
  </configuration>
</plugin>

The groupId, artifactId, and version tags have been added to add the maven-jar-plugin. The configuration tag includes instructions on which class to run when the JAR is opened by the user.

We can finally run the mvn package command to generate the JAR file in the target folder. Navigate to the target directory in your terminal or command line and run the following command to start the app:

java -jar example-app-1.0-SNAPSHOT.jar

When you run this command, the App.class gets executed.

Conclusion

We’ve learned how to clean the target folder, compile a project, and package projects into JAR files. In the later lessons, we’ll learn how to work with external libraries or dependencies.