Build tools and Maven

In plain English

Maven is a build tool that compiles your Java code, runs tests and packages the result using a single configuration file. The pom.xml is that configuration file, and it declares the project coordinates, which are a groupId, artifactId and version that name the project. A dependency is an external library your project needs, and you list each one in the pom so Maven downloads it and adds it to the classpath. The Maven lifecycle is a fixed sequence of phases such as compile, test and package that run in order, where each phase triggers the ones before it. Packaging a runnable jar means building a jar file that includes a main class in its manifest, so a user can start the program with a simple java -jar command.

Step by step

1. 1Create a pom.xml at the root of the project.
2. 2Set the groupId, artifactId, version and packaging.
3. 3Add each library you use as a dependency with its scope.
4. 4Run mvn test, then mvn package to build a jar.
5. 5Configure the jar plugin manifest and run java -jar on the output.

<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>com.edubyte</groupId>
    <artifactId>java-lessons</artifactId>
    <version>1.0.0</version>
    <packaging>jar</packaging>

    <properties>
        <maven.compiler.release>17</maven.compiler.release>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.junit.jupiter</groupId>
            <artifactId>junit-jupiter</artifactId>
            <version>5.10.2</version>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-jar-plugin</artifactId>
                <version>3.4.1</version>
                <configuration>
                    <archive>
                        <manifest>
                            <mainClass>com.edubyte.Main</mainClass>
                        </manifest>
                    </archive>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

Common mistakes

• Hardcoding a dependency jar on the classpath instead of declaring it in the pom makes the build impossible for other developers to reproduce.
• Running mvn package and expecting tests to be skipped is wrong, because package runs the test phase first and fails the build on a failing test.
• Forgetting the mainClass in the manifest produces a jar that cannot be started with java -jar, because the JVM does not know where to begin.
• Leaving the version out of the coordinates breaks dependency resolution, because Maven needs the full groupId, artifactId and version to identify an artifact.