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Java on Miren

The JVM isn't auto-detected, so you deploy Java apps with a Dockerfile.miren that builds a runnable jar and runs it on a JRE image. This guide uses Spring Boot with Maven as the example; the same pattern works for Gradle, Kotlin, Scala, and Clojure — build a jar, then java -jar it.

Let your agent do this

Ask your AI coding agent to "set up this Spring Boot app on Miren" after installing the Miren agent skills. It adds the Dockerfile.miren, binds the server to 0.0.0.0:$PORT, wires up environment variables, and deploys — using this page as its reference.

Do you need a Dockerfile?​

Yes. Miren doesn't auto-detect the JVM, so add a Dockerfile.miren to your project root. Miren builds from it instead of guessing the stack — see Using Dockerfile.miren.

Want native support?

Miren auto-detects and builds common stacks (Python, Node, Bun, Go, Ruby, Rust) without a Dockerfile. This language isn't one of them yet — if you'd like first-class support, request it.

Bind to the injected port​

Miren injects PORT and routes traffic to it. Spring Boot reads server.port, so map PORT to it in src/main/resources/application.properties:

server.port=${PORT:8080}
server.address=0.0.0.0

${PORT:8080} uses the PORT environment variable when present and falls back to 8080 for local development. Frameworks other than Spring bind however they normally do — the key is to read PORT and listen on 0.0.0.0.

The Dockerfile​

Create Dockerfile.miren in your project root. The build caches dependencies before copying source so rebuilds are faster:

# ----- Build stage -----
FROM maven:3.9-eclipse-temurin-21 AS builder
WORKDIR /app
COPY pom.xml .
RUN mvn -q -B dependency:go-offline
COPY src src
RUN mvn -q -B -DskipTests package

# ----- Runtime stage -----
FROM eclipse-temurin:21-jre
WORKDIR /app
COPY --from=builder /app/target/app.jar app.jar
EXPOSE 8080
CMD ["java", "-jar", "app.jar"]

Set <finalName>app</finalName> in your pom.xml <build> so the jar has a stable name, or adjust the COPY to match your artifact.

.dockerignore​

.git
target

Set up the app​

Even with a Dockerfile.miren, Miren needs at least one service defined — it doesn't use the image's CMD as the start command. Add a Procfile:

web: java -jar /app/app.jar

Then create .miren/app.toml naming your app and deploy from your project root:

name = "java-bench"
miren deploy
Deploying without a service fails

If no service is defined, the build succeeds but the deploy stops with no services defined: please define at least one service in a Procfile or .miren/app.toml.

Environment variables​

Set variables with miren env set — -e for plain values, -s for secrets (masked in output and logs). Spring maps env vars to properties (SPRING_DATASOURCE_URL → spring.datasource.url):

miren env set -e SPRING_PROFILES_ACTIVE=prod
miren env set -s SPRING_DATASOURCE_URL

You can also declare variables in .miren/app.toml:

[[env]]
key = "SPRING_PROFILES_ACTIVE"
value = "prod"

See App Configuration — Environment Variables.

Agent quick reference​

  • Detection: none — requires Dockerfile.miren
  • Build: mvn -DskipTests package (or Gradle); run the jar on a JRE image
  • Service is required: define a Procfile (web: java -jar /app/app.jar) — the image CMD is not used
  • Port: Spring server.port=${PORT:8080} + server.address=0.0.0.0; other frameworks read PORT and bind 0.0.0.0
  • Env vars: miren env set -e/-s; Spring maps SPRING_* env vars to properties
  • Database: optional [addons.miren-postgresql] injects DATABASE_URL

Next steps​