In gradle docker image, why gradle doesn't use system JDK version?

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I used the hello word Java app produced with gradle init. And I want to run it in docker. Here is my docker file:

FROM gradle:7.5.1-jdk11 AS builder
COPY . /home/gradle/src
WORKDIR /home/gradle/src
RUN gradle installDist

FROM openjdk:18.0.2.1-oracle
RUN microdnf install findutils
COPY --from=builder /home/gradle/src/build/install/app/ /app/
WORKDIR /app

CMD bin/app

It runs ok only when the second running image has a Java18. I'm really confused here, the first builder image has Java11. Why the gradle in image isn't configured to use system's JDK version? If it always uses it's own JDK. Why bother shipping a JDK11 with the gradle image?

Also, docker noob question: There are so many JDK / JRE images out there, how do you guys pick which one to use? Any preference ranking or something similar?

I did another try. Only use the first part of the docker file

FROM gradle:7.5.1-jdk11 AS builder
COPY . /home/gradle/src
WORKDIR /home/gradle/src
RUN gradle installDist

Then docker build --tag second-try . Then docker run -it second-try /bin/bash

I went into the build/install/app/lib folder and unpacked the app.jar jar xf app.jar to get the compiled class file. Then javap -v App.class | grep major to get the compiling JDK version. The version is 62 which is Java18.

docker gradle java
2 Answers

Why the gradle in image isn't configured to use system's JDK version?

It is actually the case. Gradle, in the builder image, is using the system JDK. You can test it by running java --version and gradle --version inside the container:

$ docker run -it gradle:7.5.1-jdk11 /bin/sh
# java --version
openjdk version "11.0.16.1" 2022-08-12
OpenJDK Runtime Environment Temurin-11.0.16.1+1 (build 11.0.16.1+1)
OpenJDK 64-Bit Server VM Temurin-11.0.16.1+1 (build 11.0.16.1+1, mixed mode, sharing)
# gradle --version

------------------------------------------------------------
Gradle 7.5.1
------------------------------------------------------------

Build time:   2022-08-05 21:17:56 UTC
Revision:     d1daa0cbf1a0103000b71484e1dbfe096e095918

Kotlin:       1.6.21
Groovy:       3.0.10
Ant:          Apache Ant(TM) version 1.10.11 compiled on July 10 2021
JVM:          11.0.16.1 (Eclipse Adoptium 11.0.16.1+1)
OS:           Linux 5.10.104-linuxkit aarch64

The final image is running OpenJDK 18, which can run Java 11 bytecode.

user_18076670

Ok, I figured it out. I overlooked the fact that gradle installDist produce the build/install/<app>/bin/<app> according to the folder name.

So when COPY . /home/gradle/src, I included local built jar files which is under build/install/app/bin/src because my local folder name is app. But when installDist runs during docker image build, the folder name is src. The output is build/install/src/bin/src.

Below is the fixed docker file which can be run with Java17

FROM gradle:7.5.1-jdk11 AS builder
COPY . /home/gradle/src
WORKDIR /home/gradle/src
RUN gradle clean
RUN gradle installDist

FROM openjdk:17-oracle
RUN microdnf install findutils
COPY --from=builder /home/gradle/src/build/install/src/ /app/
WORKDIR /app

CMD bin/src

Adding a gradle clean is a best practice that will throw error if the path is wrong.

Thanks to Ortomala Lokni, I now know a way to get the shell running in docker so that I can do more debugging

user_168091330
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