Java Docker Skill

Containerize Java applications with optimized Dockerfiles and JVM settings.

Overview

This skill covers Docker best practices for Java including multi-stage builds, JVM container settings, security hardening, and layer optimization.

When to Use This Skill

Use when you need to:

• Create optimized Java Dockerfiles
• Configure JVM for containers
• Implement security best practices
• Reduce image size
• Set up health checks

Topics Covered

Dockerfile Optimization

• Multi-stage builds
• Layer caching strategy
• Spring Boot layered JARs
• Dependency caching

JVM Container Settings

• UseContainerSupport
• MaxRAMPercentage
• GC selection
• Exit on OOM

Security

• Non-root users
• Read-only filesystem
• Vulnerability scanning
• Secrets handling

Quick Reference

# Multi-stage optimized Dockerfile
FROM eclipse-temurin:21-jdk-alpine AS builder

WORKDIR /app

# Cache dependencies
COPY pom.xml .
COPY .mvn .mvn
RUN mvn dependency:go-offline -B

# Build and extract layers
COPY src ./src
RUN mvn package -DskipTests && \
    java -Djarmode=layertools -jar target/*.jar extract

# Runtime stage
FROM eclipse-temurin:21-jre-alpine

# Security: non-root user
RUN addgroup -S app && adduser -S app -G app
USER app

WORKDIR /app

# Copy layers in order of change frequency
COPY --from=builder /app/dependencies/ ./
COPY --from=builder /app/spring-boot-loader/ ./
COPY --from=builder /app/snapshot-dependencies/ ./
COPY --from=builder /app/application/ ./

# JVM container settings
ENV JAVA_OPTS="-XX:+UseContainerSupport \
    -XX:MaxRAMPercentage=75.0 \
    -XX:+ExitOnOutOfMemoryError \
    -XX:+UseG1GC"

EXPOSE 8080

HEALTHCHECK --interval=30s --timeout=3s --start-period=30s \
    CMD wget -qO- http://localhost:8080/actuator/health/liveness || exit 1

ENTRYPOINT ["sh", "-c", "java $JAVA_OPTS org.springframework.boot.loader.launch.JarLauncher"]

JVM Container Flags

# Recommended production settings
JAVA_OPTS="
  -XX:+UseContainerSupport
  -XX:MaxRAMPercentage=75.0
  -XX:InitialRAMPercentage=50.0
  -XX:+ExitOnOutOfMemoryError
  -XX:+HeapDumpOnOutOfMemoryError
  -XX:HeapDumpPath=/tmp/heapdump.hprof
  -XX:+UseG1GC
  -Djava.security.egd=file:/dev/./urandom
"

Base Image Comparison

Image	Size	Security	Use Case
temurin:21-jre	~200MB	Good	General use
temurin:21-jre-alpine	~100MB	Good	Size-optimized
distroless/java21	~80MB	Best	Production

Security Best Practices

# Non-root user
RUN addgroup -S app && adduser -S app -G app
USER app

# Read-only filesystem
# (Configure at runtime with --read-only)

# No shell access with distroless
FROM gcr.io/distroless/java21-debian12

# Health check
HEALTHCHECK --interval=30s --timeout=3s \
    CMD wget -qO- localhost:8080/actuator/health || exit 1

Troubleshooting

Common Issues

Problem	Cause	Solution
OOMKilled	Heap > limit	Set MaxRAMPercentage
Slow startup	Large image	Multi-stage build
Permission denied	Root required	Fix file permissions
No memory info	Old JVM	Update to Java 11+

Debug Checklist

□ Check container memory limits
□ Verify JVM sees container limits
□ Review health check configuration
□ Scan image for vulnerabilities
□ Test with resource constraints

Usage

Skill("java-docker")

Related Skills

• java-maven-gradle - Build integration
• java-microservices - K8s deployment