Java on Previewops

Previewops builds whatever your Dockerfile produces and runs it as a container, so Java needs no special support. What it does need is what every stack needs: read the injected PORT and listen on 0.0.0.0.
Read the port System.getenv("PORT")
Bind all interfaces --server.address=0.0.0.0

Dockerfile

FROM eclipse-temurin:21-jdk AS build
WORKDIR /src
COPY . .
RUN ./mvnw -q package -DskipTests

FROM eclipse-temurin:21-jre
COPY --from=build /src/target/app.jar /app.jar
ENV PORT=8080
CMD ["sh", "-c", "java -jar /app.jar --server.port=$PORT --server.address=0.0.0.0"]

ENV PORT=8080 is only a default. Previewops overrides it at deploy time, so the app must read the value rather than assume it.

Application code

A typical setup with Spring Boot:

# application.properties
# Spring Boot does not read PORT automatically — it looks for SERVER_PORT
# or server.port. Map it explicitly, or pass --server.port=$PORT on the
# command line as the Dockerfile above does.
server.port=${PORT:8080}
server.address=0.0.0.0

Java-specific pitfalls

Configuration

If your Dockerfile is not at the repository root, point at it from .previewops.yaml:

provider: cloud-run
dockerfile: ./docker/Dockerfile
port: 8080
ttl_hours: 24

See configuration.md for every available option.

Do not gate startup behind a migration

Language-agnostic, and the hardest failure here to diagnose:

# Avoid this
CMD ["sh", "-c", "run-migrations && start-server"]

If the migration cannot reach the database — a paused instance, an expired connection string, a cold-start timeout — it never exits, the server never starts, and the port never opens. The deploy itself already succeeded, so the preview shows as Live while nothing answers. Run migrations as a separate, visible step instead.