PreviewDrop for Spring Boot

Deploy previews for Spring Boot apps

Push a branch, get a live preview URL. PreviewDrop builds your fat JAR, boots the embedded server, and posts the link to the pull request, usually within a couple of minutes — no CI scripts, no serverless Java wrappers.

TL;DR

Connect your repo, push. PreviewDrop detects Spring Boot, builds the JAR with Maven or Gradle, runs it on the JRE, and posts a TLS-terminated preview URL to the PR. Embedded Tomcat or Netty runs exactly as in production — your @Scheduled jobs, connection pools and a warm JVM are all intact, not folded into a Lambda handler.

What the build needs to do

You don't write this. PreviewDrop detects Spring Bootfrom your repo and builds the container for you — a Dockerfile in your repo is not used. The reference below shows the install, build and start steps a working Spring Boot image needs. If detection gets one wrong, set the matching Install, Build or Start command in Project Settings. Your app must listen on $PORT.

Reference (not read by PreviewDrop)
FROM eclipse-temurin:21-jdk AS build
WORKDIR /app
COPY .mvn/ .mvn
COPY mvnw pom.xml ./
RUN ./mvnw dependency:go-offline -B
COPY src ./src
RUN ./mvnw clean package -DskipTests

FROM eclipse-temurin:21-jre
WORKDIR /app
COPY --from=build /app/target/*.jar app.jar
EXPOSE 8080
# PreviewDrop injects PORT; Spring binds to it via server.port
CMD ["sh", "-c", "java ${JAVA_OPTS} -Dserver.port=${PORT:-8080} -jar app.jar"]

Environment variables

Set these in the Variables tab of your PreviewDrop project. They're encrypted at rest and injected into every preview container at start.

Variable	Example value	Note
SPRING_PROFILES_ACTIVE	preview	Activates an application-preview.yml so previews can use a lighter config than prod.
JAVA_OPTS	-XX:MaxRAMPercentage=75	Cap the heap so the JVM fits the preview container instead of grabbing the whole host.
SPRING_DATASOURCE_URL	jdbc:postgresql://host:5432/db	Point at a shared dev DB, or use an in-memory H2 profile for UI-only previews.

Common gotchas

Bind to the injected PORT, not hard-coded 8080

PreviewDrop routes traffic to whatever port your app listens on via the PORT env var. Pass -Dserver.port=${PORT} (as in the reference Start command) or set SERVER_PORT. A hard-coded port still works as long as it matches what the proxy expects, but reading PORT is the safe default.

Commit the Maven wrapper with the executable bit

The build calls ./mvnw, which must be committed and executable. Windows checkouts often drop the exec bit — restore it once with git update-index --chmod=+x mvnw so the Linux build can run it. (Gradle: swap in ./gradlew bootJar.)

Skip the test suite in the image build

The reference build above uses -DskipTests. Preview builds should be fast — your full suite belongs in CI, not on the critical path to a preview URL. Keeping the build lean keeps builds fast. If detection runs your tests, set the Build command in Project Settings.

Cap the JVM heap

By default the JVM sizes its heap from total host memory, which can exceed a preview container and trigger an OOM kill. Set JAVA_OPTS=-XX:MaxRAMPercentage=75 (or an explicit -Xmx256m) so the heap stays inside the container limit.

Why not Vercel or Netlify?

Vercel and Netlify have no JVM runtime. “Serverless Java” options wrap a single handler in a Lambda, but you lose the embedded Tomcat/Netty server, @Scheduled jobs, hosted background workers, and a warm JVM with live connection pools. PreviewDrop runs your fat JAR directly — the same container shape as production, used as a per-PR deploy preview.

Preview your Spring Boot app in 5 minutes

Connect a repo, push a branch, get a URL. No credit card, no trial clock.

Start free