Implemented Java packaging with Maven

[mardukbp]

• Refactored the make target java to package daff using Maven
• Added packaging/java/pom.xml needed for packaging and publishing with Maven

Maven has to be installed using a package manager (e.g. apt install maven).

In order to publish daff to Maven Central, the file ~/.jreleaser/config.properties with the following contents is required:

JRELEASER_NEXUS2_USERNAME="<your-sonatype-account-username>"
JRELEASER_NEXUS2_PASSWORD="<your-sonatype-account-password>"
JRELEASER_GPG_PASSPHRASE="<your-pgp-passphrase>"
JRELEASER_GPG_PUBLIC_KEY="/path/to/public.gpg"
JRELEASER_GPG_SECRET_KEY="/path/to/private.gpg"
JRELEASER_GITHUB_TOKEN="<your-github-token>"

In order to get a Sonatype account, follow the steps in this guide.

The GPG public key has to be published to a key server:

gpg --keyserver keyserver.ubuntu.com --send-keys <key id>

The public and private keys can be exported to files with the following commands:

gpg --output public.pgp --armor --export username@email-host
gpg --output private.pgp --armor --export-secret-key username@email-host

The GitHub token can be generated in GitHub/User Profile/Settings/Developer settings.

Once all configuration is in place, execute the following commands from the directory java_bin/daff (from JReleaser's docs):

1) Verify release & deploy configuration

mvn jreleaser:config

2) Stage all artifacts to a local directory

mvn -Ppublication

3) Deploy and release

mvn jreleaser:full-release

[mardukbp]

Could you take the helpful notes you have in #192 (comment) and place them in a Java section of a RELEASE.md so I can find them again easily in future?

Done :)

[mardukbp]

Awesome! Thank you for your dedication and your time!