Compile Java project with Docker

Question

I am new to Docker. I have a problem with compiling the code with external library.

There is a lib folder (with the same level of "src" folder) that holding the library, such as abc.jar. I modified the Dockerfile as following. It compiled with no error but the project wasn't built with abc.jar.

# Compile our java files in this container
FROM openjdk:17-slim AS builder
COPY src /usr/src/project/src
COPY lib /usr/src/project/lib
COPY manifest.txt /usr/src/project/manifest.txt
WORKDIR /usr/src/project
RUN find . -name "*.java" | xargs javac -cp lib/abc.jar -d ./target
RUN jar cfm my_project.jar manifest.txt -C ./target/ .

# Copy the jar and test scenarios into our final image
FROM openjdk:17-slim
WORKDIR /usr/src/project
COPY --from=builder /usr/src/project/my_project.jar ./my_project.jar

manifest.txt has

Main-Class: Main
Class-Path: lib/abc.jar

Any help would be appreciated.

Thanks.

I got ClassNotFound exception when I run the program when calling the class in abc.jar. Also, I use "jar -tf my_project.jar" to view the project structure, I don't see the lib/abc.jar within my project. — user3842642
– user3842642, Commented Apr 13, 2021 at 17:13
That isn't how Java works unless you explicitly create a fat jar (e.g., via Maven/Gradle). — Dave Newton
– Dave Newton, Commented Apr 13, 2021 at 17:55
You should copy abc.jar to target image and add it to classpath when running your jar. — Sokolof
– Sokolof, Commented Apr 13, 2021 at 18:57
Hi Sokolov, thanks for helping. Could you please elaborate more on how to do that? — user3842642
– user3842642, Commented Apr 14, 2021 at 18:51

Tasos P. · Accepted Answer · 2021-04-20 14:23:55Z

3

While you use the library for compiling your project, you don't include it in the final image. In multi-stage builds, each stage starts FROM a base image (openjdk:17-slim in this case) and unless you copy over the library, the final image will not include it.

This is how to do this:

# Copy the jar and test scenarios into our final image
FROM openjdk:17-slim
WORKDIR /usr/src/project
COPY --from=builder /usr/src/project/my_project.jar ./my_project.jar
COPY --from=builder /usr/src/project/lib/abc.jar ./lib/abc.jar
RUN java -jar my_project.jar

edited Apr 20, 2021 at 14:23

answered Apr 20, 2021 at 14:18

Tasos P.

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1 Comment

user3842642 Over a year ago

It works! Appreciate for your help Tasos.

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