Maven Java gitlab CI Explanation

.gitlab-ci.yml

# Utilize the docker container image tagged as "maven:latest"
# see - https://hub.docker.com/_/maven
# What is a container - https://www.docker.com/resources/what-container
image: maven:latest

# We now specify the differnt stages of our CI build. We have 3 stages
stages:
  - build
  - test
  - package

# Here we specify variables which may be used later
# These variables will be available via the CLI as $VARIABLE_NAME
variables:
  MAVEN_CLI_OPTS: " --batch-mode"
  MAVEN_OPTS: "-Dmaven.repo.local=./../.m2/repository"

# Here we are indicating which paths on the filesystem we would like to cache
# i.e. keep for later. This gives us the option to have faster builds.
# In this specific instance we are caching the maven repo directory which means
# maven will not have download packages from the internet next time it runs
cache:
  paths:
    - .m2/repository/

# Here we are specify a task that will run at our build stage
build:
# we specify the stage at which this will run
  stage: build
# Here we choose to specify the list of CLI commands to run to accomplish the task
  script:
# We need to build and "install" the library project first since our other project depends on it
# let us navigate to the directory with our library project
    - cd welcome-lib 
    - mvn $MAVEN_CLI_OPTS compile # let us compile
    - mvn $MAVEN_CLI_OPTS install # let us install it in our local maven repository
    - cd .. # let us go back up on directory
    - cd welcome-service # let us go into our app directory
    - mvn $MAVEN_CLI_OPTS compile # let us compile our app and it should find the previously installed lib project

test:
  stage: test # this task will run at the test stage
  script:
# again this task starts from the base directory
    - cd welcome-service # navigate to the app directory
    - mvn $MAVEN_CLI_OPTS test # run any tests specified in this application

package:
  stage: package # this task will run at our package stage
  script:
# again this task starts from the base directory
    - cd welcome-service # navigate to the app directory
# let us run this command to convert the app into a runnable jar, this is possible because of settings in our pom.xml
    - mvn $MAVEN_CLI_OPTS package 
# let us indicate that we would like to save the result of this stage as an artefact
  artifacts:
    paths: # we must specify the location of all artefacts line by line
    - welcome-service/target/welcome-service-0.0.1-SNAPSHOT-jar-with-dependencies.jar
    expire_in: 1 week # this artefact will be saved on gitlab for one week