Java Docker container won’t run without jar file being at root

Your issue arises from how the java -jar command resolves the classpath and the working directory structure within your Docker container. When running a Spring Boot application, the app.jar file includes a BOOT-INF directory that defines where Spring Boot expects to find your compiled classes. This expectation can sometimes be disrupted if the app.jar file is located in a different directory, and relative paths become mismatched.

Here’s a detailed explanation and a solution for your problem, followed by an SEO-friendly blog post outline.

Why the Issue Occurs

1. Classpath Resolution: When running java -jar, the JVM uses the app.jar as the classpath root. However, if files like META-INF or BOOT-INF inside the JAR aren’t correctly accessible or expected paths are disrupted by the working directory structure, ClassNotFoundException can occur.
2. Dockerfile Differences: In your first Dockerfile, you set WORKDIR /app, meaning that java -jar app.jar is executed from /app. This doesn’t inherently cause issues, but Spring Boot may expect relative paths from the JAR file to match specific patterns.

Solution

To resolve the issue while keeping your organized structure:

1. Explicitly set the working directory in the CMD command.
2. Ensure the JAR’s directory structure and paths remain intact.

Here’s the corrected Dockerfile:

# === Build stage ===
FROM maven:3.9.9-eclipse-temurin-21-alpine AS builder

WORKDIR /app

COPY pom.xml ./
COPY /src /src

RUN mvn clean package -DskipTests

# === Run stage ===
FROM eclipse-temurin:21.0.5_11-jre-alpine-3.21

WORKDIR /app

COPY --from=builder /app/target/*.jar ./app.jar

EXPOSE 9090

# Explicitly set the classpath and working directory for the JAR
CMD ["java", "-cp", "/app/app.jar", "org.springframework.boot.loader.JarLauncher"]

Why This Fix Works

• WORKDIR: Ensures the container’s default directory is /app.
• -cp Option: Explicitly sets the classpath to /app/app.jar, resolving any ambiguity in Spring Boot’s classpath resolution.
• JarLauncher: This Spring Boot class properly initializes the JAR’s contents regardless of location.

The Problem

• Describe the error in detail with example logs (as in your case).
• Explain why this happens in Dockerized Spring Boot projects.

Common Mistakes

• Using default root directory for builds.
• Overlooking WORKDIR and relative paths when packaging and running JAR files.

Step-by-Step Solution

1. Correct Dockerfile with explanation of each line.
2. Explanation of java -cp vs java -jar for better understanding.
3. How the corrected setup maintains organization and functionality.

Best Practices

• Organize Docker images for readability.
• Always test classpath resolution when changing working directories.
• Validate container builds with different configurations.

Conclusion

• Recap the importance of correct Dockerfile setup for running Java applications.
• Provide final tips to debug similar issues effectively.