Java Deploy

Java Deployment

Detection

Project is Java if:

• Maven: pom.xml exists at the root
• Gradle: gradlew or build.gradle / build.gradle.kts exists at the root

Versions

Java / JDK

1. pom.xml → maven.compiler.source or maven.compiler.target
2. build.gradle → sourceCompatibility or JavaLanguageVersion
3. .sdkmanrc or .java-version file
4. Defaults to 21

Gradle

Read from gradle/wrapper/gradle-wrapper.properties → distributionUrl.

Build

Maven

• mvn -B package -DskipTests
• Output: target/<artifact-id>-<version>.jar
• Spring Boot: target/*.jar (fat JAR)
• Offline deps: mvn -B dependency:go-offline for layer caching

Gradle

• ./gradlew build -x test (prefer wrapper when gradlew exists)
• Use --no-daemon in Docker
• Output: build/libs/<name>-<version>.jar
• Spring Boot: build/libs/*.jar (fat JAR)
• Warm deps: ./gradlew dependencies --no-daemon for layer caching

Start Command

• java -jar <jar-file>
• Spring Boot: java -jar app.jar

Port Detection

1. server.port in application.properties or application.yml
2. PORT or SERVER_PORT environment
3. Spring Boot default: 8080
4. Default: 8080

Framework Detection

Spring Boot

• Fat JAR includes embedded Tomcat
• Config: application.properties, application.yml, application-*.yml

Install Stage Optimization

• Maven: pom.xml, pom.xml children (multi-module)
• Gradle: build.gradle, build.gradle.kts, settings.gradle, gradlew, gradle/wrapper/*
• Then: src/

Caching

Use BuildKit cache mounts:

Gradle:

RUN --mount=type=cache,target=/root/.gradle \
    ./gradlew build -x test --no-daemon

Maven:

RUN --mount=type=cache,target=/root/.m2/repository \
    mvn -B package -DskipTests

Base Images

Use JRE (not JDK) for runtime.

Dockerfile Patterns

Maven + Spring Boot

FROM eclipse-temurin:21-jdk-alpine AS build
WORKDIR /app
COPY pom.xml ./
RUN mvn -B dependency:go-offline
COPY src ./src
RUN mvn -B package -DskipTests

FROM eclipse-temurin:21-jre-alpine
WORKDIR /app
COPY --from=build /app/target/*.jar app.jar
EXPOSE 8080
ENTRYPOINT ["java", "-jar", "app.jar"]

Gradle + Spring Boot

FROM eclipse-temurin:21-jdk-alpine AS build
WORKDIR /app
COPY gradlew ./
COPY gradle gradle
COPY build.gradle settings.gradle ./
RUN ./gradlew dependencies --no-daemon
COPY src ./src
RUN ./gradlew build -x test --no-daemon

FROM eclipse-temurin:21-jre-alpine
WORKDIR /app
COPY --from=build /app/build/libs/*.jar app.jar
EXPOSE 8080
ENTRYPOINT ["java", "-jar", "app.jar"]

Simple Maven (no Spring Boot)

FROM eclipse-temurin:21-jdk-alpine AS build
WORKDIR /app
COPY pom.xml ./
RUN mvn -B dependency:go-offline
COPY src ./src
RUN mvn -B package -DskipTests

FROM eclipse-temurin:21-jre-alpine
WORKDIR /app
COPY --from=build /app/target/*.jar app.jar
EXPOSE 8080
CMD ["java", "-jar", "app.jar"]

Gotchas

• Gradle wrapper gradlew must have execute permission — add RUN chmod +x gradlew in the Dockerfile if builds fail with permission denied
• --no-daemon is essential for Gradle in Docker — the daemon persists between builds, wastes memory, and can hang
• Spring Boot fat JAR glob target/*.jar may match multiple JARs (sources, javadoc) — use maven-jar-plugin with a fixed finalName or be specific with the path
• Use JRE (not JDK) for the runtime stage — JDK adds ~200MB of unnecessary build tools
• JAVA_TOOL_OPTIONS affects all stages including runtime — unset it after the build stage if used for build-only JVM flags