Java Docker

You are a Java/Spring Boot DevOps engineer. The user wants to create an optimized Docker image for their Java or Spring Boot application using best practices.

What to check first

• Verify your project has a pom.xml (Maven) or build.gradle (Gradle) file in the root
• Check that you have Docker installed: docker --version
• Confirm your Java application has a main class and can run: mvn clean package or gradle build

Steps

1. Add the Google Jib Maven plugin to your pom.xml in the <plugins> section under <build> to enable containerization without writing a Dockerfile
2. Configure Jib with your target image registry (Docker Hub, GCR, or local Docker daemon) using <configuration> tags
3. Set the base image to eclipse-temurin:21-jre or openjdk:21-slim for a lightweight JVM runtime
4. Configure Jib to split layers: app dependencies, resources, and classes separately for better caching
5. Add JVM flags like -XX:+UseG1GC and -XX:MaxRAMPercentage=75.0 in Jib's <jvmFlags> for production optimization
6. Build the image using mvn compile jib:build for remote registry or mvn compile jib:dockerBuild for local Docker
7. Test the image: docker run --rm -p 8080:8080 your-image-name:latest
8. Verify logs and health endpoints respond correctly before pushing to production

Code

<!-- Add to pom.xml -->
<project>
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.example</groupId>
  <artifactId>spring-boot-app</artifactId>
  <version>1.0.0</version>

  <properties>
    <maven.compiler.source>21</maven.compiler.source>
    <maven.compiler.target>21</maven.compiler.target>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
  </properties>

  <build>
    <plugins>
      <plugin>
        <groupId>com.google.cloud.tools</groupId>
        <artifactId>jib-maven-plugin</artifactId>
        <version>3.4.0</version>
        <configuration>
          <from>
            <image>eclipse-temurin:21-jre</image>
          </from>
          <to>
            <image>docker.io/your-username/spring-boot-app</image>
            <tags>latest,1.0.0</tags>
          </to>
          <container>
            <jvmFlags>
              <jvmFlag>-XX:+UseG1GC</jvmFlag>
              <jv

Note: this example was truncated in the source. See the GitHub repo for the latest full version.

Common Pitfalls

• Treating this skill as a one-shot solution — most workflows need iteration and verification
• Skipping the verification steps — you don't know it worked until you measure
• Applying this skill without understanding the underlying problem — read the related docs first

When NOT to Use This Skill

• When a simpler manual approach would take less than 10 minutes
• On critical production systems without testing in staging first
• When you don't have permission or authorization to make these changes

How to Verify It Worked

• Run the verification steps documented above
• Compare the output against your expected baseline
• Check logs for any warnings or errors — silent failures are the worst kind

Production Considerations

• Test in staging before deploying to production
• Have a rollback plan — every change should be reversible
• Monitor the affected systems for at least 24 hours after the change