Chapter 22: Jenkins Language Projects — Java to Python

22.3 Java (Maven) — start to end

By Ravindra Bagale · Last updated: 25 October 2026

Why: the problem

Think of the HDFC Bank internet banking team. Most of their backend is Java, and Java projects are built with Maven. A developer says: it works on my laptop. But the release manager needs proof from a clean machine, every time, with tests. Maven reads one file, pom.xml, which lists the Java version, the libraries and the test tool. One command, mvn package, compiles the code, runs the tests and makes a jar file. A jar is a zip of compiled Java classes that the server runs with java -jar. In this lesson you build the badge rule in Java, test it with JUnit, run it, then push it so Jenkins does the same on an agent and keeps the jar as an artifact.

What we will build

You will create badge-java with Maven, write Badge.java and a JUnit test, run mvn package locally, push with a Jenkinsfile, see green, then break a test to see red, and finally publish test reports in Jenkins.

What: every term in plain English

• Maven — The Java build tool. Reads pom.xml.
• pom.xml — Project Object Model. Lists name, version, Java version and libraries.
• groupId / artifactId — The company name and the project name, like com.example and badge-java.
• JUnit — The common Java test library.
• Surefire — The Maven plugin that runs tests and writes XML reports.
• jar — The packaged app: a zip of .class files.
• target/ — The folder where Maven writes classes, reports and the jar.
• ~/.m2 — Maven cache of downloaded libraries on the machine.

How: step by step

1. Make the project

   mkdir -p ~/devops-lab/ch22/badge-java/src/main/java/lab ~/devops-lab/ch22/badge-java/src/test/java/lab && cd ~/devops-lab/ch22/badge-java
   

   What it does: Standard Maven folders.

   Output

   (no output)
   

2. Write the files

   Create pom.xml, Badge.java, BadgeTest.java from Example 1
   

   What it does: Copy exactly.

   Output

   (no output)
   

3. Test and package

   mvn -B -q package
   

   What it does: Compile, test, jar.

   Output

   (quiet: no output on success)
   

4. See the jar

   ls -l target/*.jar
   

   What it does: The artifact.

   Output

   -rw-r--r-- 1 ravi ravi 3712 Oct 25 10:12 target/badge-java-1.0.0.jar
   

5. Run it

   java -jar target/badge-java-1.0.0.jar 120
   

   What it does: Our rule in Java.

   Output

   gold
   

6. Push with Jenkinsfile

   git init -q -b main && git add . && git commit -qm "Java badge" && git remote add origin GIT_URL && git push -q -u origin main
   

   What it does: GIT_URL is your lab repo.

   Output

   (no output)
   

7. Build in Jenkins

   badge-java-pipeline, Build Now
   

   What it does: Same commands on the agent.

   Output

   Tests run: 3, Failures: 0
   BUILD SUCCESS
   Finished: SUCCESS
   

Example 1: The full Maven project

Stack. Java 17, Maven, JUnit 5

Goal. The full Maven project.

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0">
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.example</groupId>
  <artifactId>badge-java</artifactId>
  <version>1.0.0</version>
  <properties>
    <maven.compiler.release>17</maven.compiler.release>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
  </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-surefire-plugin</artifactId>
        <version>3.2.5</version>
      </plugin>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-jar-plugin</artifactId>
        <version>3.4.1</version>
        <configuration>
          <archive><manifest><mainClass>lab.Badge</mainClass></manifest></archive>
        </configuration>
      </plugin>
    </plugins>
  </build>
</project>

src/main/java/lab/Badge.java

package lab;

public class Badge {
    public static String level(int points) {
        if (points >= 100) return "gold";
        if (points >= 50) return "silver";
        return "bronze";
    }
    public static void main(String[] args) {
        int p = args.length == 0 ? 0 : Integer.parseInt(args[0]);
        System.out.println(level(p));
    }
}

src/test/java/lab/BadgeTest.java

package lab;

import static org.junit.jupiter.api.Assertions.assertEquals;
import org.junit.jupiter.api.Test;

class BadgeTest {
    @Test void gold()   { assertEquals("gold", Badge.level(100)); }
    @Test void silver() { assertEquals("silver", Badge.level(50)); }
    @Test void bronze() { assertEquals("bronze", Badge.level(0)); }
}

What each line does

• maven.compiler.release 17 — Compile for Java 17.
• scope test — JUnit is only for tests, not shipped in the jar.
• surefire 3.2.5 — A version that finds JUnit 5 tests.
• mainClass — So java -jar knows where to start.
• assertEquals — Test fails if the two values differ.

Green output

[INFO] Tests run: 3, Failures: 0, Errors: 0, Skipped: 0
[INFO] Building jar: /home/ravi/devops-lab/ch22/badge-java/target/badge-java-1.0.0.jar
[INFO] BUILD SUCCESS

Expected result. A tested jar from one command.

Example 2: The Jenkinsfile with test reports and artifact

Stack. Jenkins, Maven

Goal. The Jenkinsfile with test reports and artifact.

Jenkinsfile

pipeline {
  agent any
  tools { maven 'M3'; jdk 'jdk17' }
  options { timestamps(); buildDiscarder(logRotator(numToKeepStr: '20')) }
  stages {
    stage('Checkout') { steps { checkout scm } }
    stage('Build')    { steps { sh 'mvn -B -DskipTests compile' } }
    stage('Test')     { steps { sh 'mvn -B test' } }
    stage('Package')  { steps { sh 'mvn -B -DskipTests package' } }
    stage('Live') {
      steps {
        sh 'mkdir -p "$HOME/lab-live/badge-java" && cp target/*.jar "$HOME/lab-live/badge-java/"'
        sh 'java -jar "$HOME/lab-live/badge-java/badge-java-1.0.0.jar" 120'
      }
    }
  }
  post {
    always  { junit 'target/surefire-reports/*.xml' }
    success { archiveArtifacts artifacts: 'target/*.jar', fingerprint: true }
  }
}

What each line does

• tools — Use Maven and JDK set in Manage Jenkins, Tools.
• logRotator — Keep only 20 builds so the disk does not fill.
• junit — Reads Surefire XML and shows a test trend chart.
• fingerprint — Tracks which build made this jar.
• Live java -jar — Smoke test: the jar really runs.

Green output

[Pipeline] stage (Test)
[INFO] Tests run: 3, Failures: 0, Errors: 0, Skipped: 0
[Pipeline] stage (Live)
gold
Recording test results
Archiving artifacts
Finished: SUCCESS

Expected result. The job page shows Test Result: 0 failures and a download link for the jar.

Example 3: Break a test and read the red build

Stack. Maven

Goal. Break a test and read the red build.

change

sed -i 's/points >= 100/points > 100/' src/main/java/lab/Badge.java
git commit -qam "Bug: 100 is no longer gold" && git push -q

What each line does

• >= to > — Now exactly 100 points gives silver.
• push — Jenkins runs again.

Green output

[ERROR] Tests run: 3, Failures: 1, Errors: 0, Skipped: 0
[ERROR] BadgeTest.gold:7 expected: [gold] but was: [silver]
[ERROR] BUILD FAILURE
Finished: FAILURE

Expected result. Test stage red, Package and Live skipped. Read the expected and was values, fix the code, push again.

Example 4: Speed up Maven builds with a cache in a Docker agent

Stack. Jenkins, Docker, Maven

Goal. Speed up Maven builds with a cache in a Docker agent.

Jenkinsfile

pipeline {
  agent {
    docker {
      image 'maven:3.9-eclipse-temurin-17'
      args '-v $HOME/.m2:/root/.m2'
    }
  }
  stages {
    stage('Package') { steps { sh 'mvn -B package' } }
  }
}

What each line does

• maven:3.9-eclipse-temurin-17 — Official image with Maven and Java 17. No install on the agent.
• -v $HOME/.m2:/root/.m2 — Keep downloaded libraries between builds.

Green output

First build:  Total time:  41.203 s
Second build: Total time:  6.118 s

Expected result. Libraries download once. Builds get about 6 times faster.

Verify: did it work?

1. Jar exists

   ls target/badge-java-1.0.0.jar
   

   What it does: Built.

   Output

   target/badge-java-1.0.0.jar
   

2. Reports exist

   ls target/surefire-reports/*.xml
   

   What it does: For the junit step.

   Output

   target/surefire-reports/TEST-lab.BadgeTest.xml
   

3. Live copy runs

   java -jar ~/lab-live/badge-java/badge-java-1.0.0.jar 60
   

   What it does: Copied by Jenkins.

   Output

   silver
   

When it is red 1

[ERROR] Source option 5 is no longer supported. Use 7 or later.

Cause. No Java version in pom.xml.

Fix, step by step

1. Add maven.compiler.release 17 in properties.

When it is red 2

Tests run: 0, Failures: 0

Cause. Old Surefire does not see JUnit 5 tests.

Fix, step by step

1. Set maven-surefire-plugin to 3.2.5 or newer.
2. Test class names must end with Test.

When it is red 3

no main manifest attribute, in target/badge-java-1.0.0.jar

Cause. The jar does not say which class to start.

Fix, step by step

1. Add mainClass in maven-jar-plugin configuration.

When it is red 4

Could not transfer artifact ... from/to central: Connect timed out

Cause. The agent cannot reach Maven Central.

Fix, step by step

1. Check proxy or firewall.
2. Use a company mirror like Nexus in settings.xml.

Cleanup

1. Clean build files

   mvn -q clean
   

   What it does: Removes target/.

   Output

   (no output)
   

Practice tasks

Homework: try at home

Recap

• Maven reads pom.xml.
• mvn package compiles, tests and makes a jar.
• Publish JUnit XML with junit.
• Archive the jar with a fingerprint.
• Cache ~/.m2 for speed.

FAQ

What is the difference between mvn install and mvn package?

package makes the jar in target. install also copies it to ~/.m2 for other local projects.

Should CI use -B?

Yes. Batch mode removes colours and prompts.

Should I skip tests in CI?

Never in the Test stage. Skip only when you package after tests already ran.

Where do company libraries come from?

A private repository like Nexus or Artifactory set in settings.xml.

What is mvn verify?

Runs tests plus extra checks after package, like integration tests.

Why is the first build slow?

Maven downloads all libraries once.

Interview questions

1. Explain the Maven lifecycle.

   validate, compile, test, package, verify, install, deploy; each phase runs the ones before it.

2. How do you show test results in Jenkins?

   junit step on Surefire XML.

3. How do you speed up Maven in CI?

   Cache ~/.m2, use a mirror, run in parallel with -T.

4. What is a SNAPSHOT version?

   A changing development version; releases have fixed versions.

5. How do you trace which build made a jar?

   archiveArtifacts with fingerprint.

6. What does BUILD FAILURE with Tests run: 0 suggest?

   Surefire did not detect tests: version or naming problem.

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