Docker cache gradle dependencies

Question

I'm trying to deploy our java web application to aws elastic beanstalk using docker, the idea is to be able to run the container locally for development and testing and eventually push it up to production using git.

I've created a base image that has tomcat8 and java8 installed, the image that performs the gradle builds inherit from this base image, speeding up build process.

All works well, except for the fact that the inheriting application container that gets built using docker doesn't seem to cache the gradle dependencies, it downloads it every time, including gradlew. We build our web application using the following command:

./gradlew war

Is there some way that i can cache the files in ~/.gradle this would speed my build up dramatically.

This isn't so much of an issue on beanstalk but is a big problem for devs trying to build and run locally as this does take a lot of time, as you can imagine.

The base image dockerfile:

FROM phusion/baseimage
EXPOSE 8080
RUN apt-get update
RUN add-apt-repository ppa:webupd8team/java
RUN apt-get update
RUN echo oracle-java8-installer shared/accepted-oracle-license-v1-1 select true | sudo /usr/bin/debconf-set-selections
RUN apt-get -y install oracle-java8-installer
RUN java -version
ENV TOMCAT_VERSION 8.0.9
RUN wget --quiet --no-cookies http://archive.apache.org/dist/tomcat/tomcat-8/v${TOMCAT_VERSION}/bin/apache-tomcat-${TOMCAT_VERSION}.tar.gz -O /tmp/catalina.tar.gz
# Unpack
RUN tar xzf /tmp/catalina.tar.gz -C /opt
RUN mv /opt/apache-tomcat-${TOMCAT_VERSION} /opt/tomcat
RUN ln -s /opt/tomcat/logs /var/log/tomcat
RUN rm /tmp/catalina.tar.gz
# Remove unneeded apps
RUN rm -rf /opt/tomcat/webapps/examples
RUN rm -rf /opt/tomcat/webapps/docs
RUN rm -rf /opt/tomcat/webapps/ROOT
ENV CATALINA_HOME /opt/tomcat
ENV PATH $PATH:$CATALINA_HOME/bin
ENV CATALINA_OPTS $PARAM1
# Start Tomcat
CMD ["/opt/tomcat/bin/catalina.sh", "run"]

The application dockerfile:

FROM <tag name here for base image>
RUN mkdir ~/.gradle
# run some extra stuff here to add things to gradle.properties file
# Add project Source
ADD . /var/app/myapp
# Compile and Deploy Application, this is what is downloading gradlew and all the maven dependencies every time, if only there was a way to take the changes it makes to ~/.gradle and persist it as a cache layer
RUN cd /var/app/myapp/ && ./gradlew war
RUN mv /var/app/myapp/build/libs/myapp.war /opt/tomcat/webapps/ROOT.war
# Start Tomcat
CMD ["/opt/tomcat/bin/catalina.sh", "run"]

Answer 1

I faced this issue. As you might agree, it is a best practice to download dependencies alone as a separate step while building the docker image. It becomes little tricky with gradle, since there is no direct support for downloading just dependencies.

Option 1 : Using docker-gradle Docker image

---

We can use pre-built gradle docker image to build the application. This ensures that it's not a local system build but a build done on a clean docker image.

docker volume create --name gradle-cache
docker run --rm -v gradle-cache:/home/gradle/.gradle -v "$PWD":/home/gradle/project -w /home/gradle/project gradle:4.7.0-jdk8-alpine gradle build
ls -ltrh ./build/libs

• gradle cache is loaded here as a volume. So subsequent builds will reuse the downloaded dependencies.
• After this, we could have a Dockerfile to take this artifact and generate application specific image to run the application.
• This way, the builder image is not required. Application build flow and Application run flow is separated out.
• Since the gradle-cache volume is mounted, we could reuse the downloaded dependencies across different gradle projects.

Option 2 : Multi-stage build

---

----- Dockerfile -----

FROM openjdk:8 AS TEMP_BUILD_IMAGE
ENV APP_HOME=/usr/app/
WORKDIR $APP_HOME
COPY build.gradle settings.gradle gradlew $APP_HOME
COPY gradle $APP_HOME/gradle
RUN ./gradlew build || return 0 
COPY . .
RUN ./gradlew build

FROM openjdk:8
ENV ARTIFACT_NAME=your-application.jar
ENV APP_HOME=/usr/app/
WORKDIR $APP_HOME
COPY --from=TEMP_BUILD_IMAGE $APP_HOME/build/libs/$ARTIFACT_NAME .
EXPOSE 8080
CMD ["java","-jar",$ARTIFACT_NAME]

In the above Dockerfile

• First we try to copy the project's gradle files alone, like build.gradle, gradlew etc.,
• Then we copy the gradle directory itself
• And then we try to run the build. At this point, there is no other source code files exists in the directory. So build will fail. But before that it will download the dependencies. 
• Since we expect the build to fail, I have tried a simple technique to return 0 and allow the docker to continue execution
• this will speed up the subsequent build flows, since all the dependencies are downloaded and docker cached this layer. Comparatively, Volume mounting the gradle cache directory is still the best approach.
• The above example also showcases multi-stage docker image building, which avoid multiple docker build files.