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Co-authored-by: Marek Novotný <>


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Co-authored-by: Marek Novotný <>
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@ge0ffrey @manstis @mbiarnes @yurloc @psiroky @mareknovotny @jervisliu @etirelli @Rikkola @hasys @Ginxo @oskopek

Developing Drools and jBPM

If you want to build or contribute to a kiegroup project, read this document.

This document will save you and us a lot of time by setting up your development environment correctly. It solves all known pitfalls that can disrupt your development. It also describes all guidelines, tips and tricks. If you want your pull requests (or patches) to be merged into main, please respect those guidelines.

If you are reading this document with a normal text editor, please take a look at the more readable formatted version.

If you discover pitfalls, tips and tricks not described in this document, please update it using the markdown syntax.

Table of content

Quick start

If you don't want to contribute to this project and you know git and maven, these build instructions should suffice:

  • To build 1 repository, for example drools:

    $ git clone
    $ cd drools
    $ mvn clean install -DskipTests
  • To build all repositories:

    $ git clone
    $ droolsjbpm-build-bootstrap/script/
    $ droolsjbpm-build-bootstrap/script/ clean install -DskipTests

If you want to contribute to this project, read the rest of this file!

Source control with Git

Installing and configuring git

  • Install git in your OS:

    • Linux: Install the package git

      $ sudo apt-get install git

      Tip: Also install gitk to visualize your git log:

      $ sudo apt-get install gitk
    • Windows, Mac OSX: Download from the git website.

      Tip for Mac OSX: Also install gitx to visualize your git log.

    • More info in GitHub's git installation instructions.

  • Check if git is installed correctly.

    $ git --version
    git version 2.21.2
  • Configure git correctly:

    $ git config --global "My Full Name"
    $ git config --global
    $ git config --global -l<user name and surname ><user email address>
    • Warning: the field is your full name, not your username.

    • Note: the field should match an email address of your github account.

    • More info on GitHub.

  • Get a github account

  • To learn more about git, read the free book Pro Git.

Getting the sources locally

Because you'll probably want to change our code, it's recommended to fork our code before cloning it, so it's easier to share your changes with us later. For more info on forking, read GitHub's help on forking.

  • First fork the repository you want to work on, for example drools:

    • Surf to the blessed repositories on github and log in.

      • Note: Every git repository can be build alone. You only need to fork/clone the repositories you're interested in (drools in this case).
    • Surf to the specific repository (drools)

    • Click the top right button Fork

    • Note: by forking the repository, you can commit and push your changes without our consent and we can easily review and then merge your changes into the blessed repository.

  • Clone your fork locally:

    # First make a directory to hold all the kiegroup projects
    $ mkdir kiegroup
    $ cd kiegroup
    # Then clone the repository you want to clone.
    $ git clone
    $ cd drools
    $ ls
    • Warning: Always clone with the SSH URL, never clone with the HTTPS URL because the latter is unreliable.

    • Note: it's highly recommended to name the cloned directory the same as the repository (which is the default), so the helper scripts work.

    • By default you will be looking at the sources of the main branch, which can be very unstable.

      • Use git checkout to switch to a more stable branch or tag:

        $ git checkout <stable version> # i.e git checkout 7.33.x
  • Add the blessed repository as upstream (if you've directly cloned the blessed repository, don't do this):

    $ git remote add upstream
    $ git fetch upstream

Working with git

  • First make a topic branch:

    $ git checkout main
    $ git checkout -b myFirstTopic
    • Don't litter your local main branch: keep it equal to remotes/upstream/main

    • 1 branch can have only 1 pull request, because the pull requests evolves as you add more commits on that branch.

  • Make changes, run, test and document them, then commit them:

    $ git commit -m "Fix typo in documentation"
  • Push those commits on your topic branch to your fork

    $ git push origin myFirstTopic
  • Get the latest changes from the blessed repository

    • Set your main equal to the blessed main:

      $ git fetch upstream
      $ git checkout main
      # Warning: this deletes all changes/commits on your local main branch, but you shouldn't have any!
      $ git reset --hard upstream/main
    • Start a new topic branch and set the code the same as the blessed main:

      $ git fetch upstream && git checkout -b mySecondTopic && git reset --hard upstream/main
    • If you have a long-running topic branch, merge main into it:

      $ git fetch upstream
      $ git merge upstream/main
      • If there are merge conflicts:

        $ git mergetool
        $ git commit


        $ git status
        $ gedit conflicted-file.txt
        $ git add conflicted-file.txt
        $ git commit

        Many people get confused when a merge conflict occurs, because you're in limbo. Just fix the merge conflicts and commit (even if the git seems to contain many files), only then is the merge over. Then run git log to see what happened. The many files in the merge conflict resolving commit are a side effect of non-linear history.

  • You may delete your topic branch after your pull request is closed (first one deletes remotely, second one locally):

    $ git push origin :myTopicBranch
    $ git branch -D myTopicBranch
  • Tips and tricks

    • To see the details of your local, unpushed commits:

      $ git diff origin...HEAD
    • To run a git command (except clone) over all repositories (only works if you cloned all repositories):

      $ cd ~/projects/kiegroup
      $ droolsjbpm-build-bootstrap/script/ push
      • Note: the script is working directory independent.

      • Linux tip: Create a symbolic link to the script and place it in your PATH by linking it in ~/bin:

        $ ln -s ~/projects/kiegroup/droolsjbpm-build-bootstrap/script/ ~/bin/kiegroup-git

        For command line completion, add the following line in ~/.bashrc:

        $ complete -o bashdefault -o default -o nospace -F _git kiegroup-git

Share your changes with a pull request

A pull request is like a patch file, but easier to apply, more powerful and you'll be credited as the author.

  • Creating a pull request

    • Push all your commits to a topic branch on your fork on github (if you haven't already).

      • You can only have 1 pull request per branch, so it's advisable to use topic branches to avoid mixing your changes.
    • Surf to that topic branch on your fork on github.

    • Click the button Pull Request on the top of the page.

      • Once the Pull Request is built the user who raised the PR gets an email (email address of user that is stored in github) with the result. If the user has access to the RedHat VPN he can access the links to the logs of the build and to its failed tests. In case the connection to VPN is not available the user gets the failed test in the message body (UNSTABLE build) or he gets the log file compressed as attachment (FAILED build). In case the build was SUCCESSFUL no mail is sent, only if the first build failed and the PR was fixed, so the second PR fixes the build. In these cases the user will get an email.
  • Accepting a pull request

    • Surf to the pull request page on github.

    • Review the changes

    • Click the button Merge help on the bottom of the page and follow the instructions of github to apply those changes on the blessed main.

      • Or use the button Merge if there are no merge conflicts.

If the change being proposed is affecting more than a single repository, it will require creating a pull request for each of the repositories being affected; in this case, it is required for the topic branch to share the same name across all pull requests, in order for the CI build tool to include the necessary dependencies while performing the build with the proposed change. It is also highly recommended to use the github Autolinked references in the pull request comments, in order to make these dependencies explicit and emphasized during code reviews.

Building with Maven

All projects use Maven 3 to build all their modules.

Installing Maven

  • Get Maven

  • Linux

    • Note: the apt-get version of maven is probably not up-to-date enough.

    • Linux trick to easily upgrade to future versions later:

      • Unzip maven to ~/opt/build

      • Create a version-independent link:

        $ cd ~/opt/build/
        $ ln -s apache-maven-3.6.3 apache-maven

        Next time you only have to remove the link and recreate the link to the new version.

      • Add this to your ~/.bashrc file:

        export M3_HOME="~/opt/build/apache-maven"
        export PATH="$M3_HOME/bin:$PATH"
    • Give more memory to maven:

  • Windows:

    • Give more memory to maven, so it can build the big projects too:

      • Open menu Configuration screen, menu item System, tab Advanced, button environment variables:

        set MAVEN_OPTS="-Xms256m -Xmx1024m"
  • Check if maven is installed correctly.

    $ mvn --version
    Apache Maven 3.6.3 (...)
    Java version: 1.8.0_112

    Note: the enforcer plugin enforces a minimum maven and java version.

Running the build

  • Go into a project's base directory, for example drools:

    $ cd ~/projects/kiegroup
    $ ls 

    the repositories displayed should be like listed here. repository_list

    $ cd drools
    $ ls
    ...  drools-core  drools-cdi  pom.xml ...

    Notice you see a pom.xml file there. Those pom.xml files are the heart of Maven.

  • Run the build:

    $ mvn clean install -DskipTests

    The first build will take a long time, because a lot of dependencies will be downloaded (and cached locally).

    It might even fail, if certain servers are offline or experience hiccups. In that case, you'll see an IO error, so just run the build again.

    If you consistently get Could not transfer artifact ... Connection timed out and you are behind a non-transparent proxy server, configure your proxy server in Maven.

    After the first successful build, any next build should be fast and stable.

  • Try running a different profile by using the option -D<profileActivationProperty>:

    $ mvn clean install -DskipTests -Dfull

    There are 3 profile activation properties:

    • none: Fast, for during development

    • full: Slow, but builds everything (including documentation). Used by Jenkins and during releases.

    • productized: activates branding changes for productized version

  • To run a maven build over all repositories (only works if you cloned all repositories):

    $ cd ~/projects/kiegroup
    $ droolsjbpm-build-bootstrap/script/ -DskipTests clean install
    • Note: the script is working directory independent.
  • You can use to compile a specific repository and all repositories that your target repository depends on. This is done using the --target-repo option which will invoke script to discover cross-repository project dependencies. Use --repo-list to specify custom list of repositories. These options work for too.

  • Warning: The first mvn build of a day will download the latest SNAPSHOT dependencies of other kiegroup projects, unless you build all those kiegroup projects from source. Those SNAPSHOTS were built and deployed last night by Jenkins jobs.

    • If you've pulled all changes (or cloned a repository) today, this is a good thing: it saves you from having to download and build all those other latest kiegroup projects from source.

    • If you haven't pulled all changes today, this is probably a bad thing: you're probably not ready to deal with those new snapshots.

      In that case, add -nsu (= --no-snapshot-updates) to the mvn command to avoid downloading those snapshots:

      $ mvn clean install -DskipTests -nsu

      Note that using -nsu will also make the build faster.

Running tests

  • All modules

    $ cd ~/projects/kiegroup/drools
    $ mvn test [-Dtest=ATestClassName]

Running code-coverage checks

JaCoCo plugin allows to measure code-coverage for any child of droolsjbpm-build-bootstrap. The check binds to the verify phase and for the plugin to run, the code-coverage profile has to be enabled.

  • From the module/project folder run command:

    $ mvn clean verify -Pcode-coverage
  • The coverage report is then generated in ./target/site/jacoco/index.html

Running Pitest mutation coverage analysis

Mutation coverage is used to measure how good the tests are at making assertions about the tested code. It is a good idea to check the mutation coverage of tests added together with any changes, be it a newly developed feature or a bug fix. Code coverage is analyzed for free as part of the mutation analysis.

To analyze the complete module:

$ mvn verify -Dmutation-coverage

To limit analyzed classes to a sub-package:

$ mvn verify -Dmutation-coverage -DtargetClasses=org.drools*

The HTML report will be stored in local/pit-reports/ directory. Currently, it is not possible to get a report aggregated over multiple modules. Learn more about using Pitest.

Configuring Maven

To deploy snapshots and releases to nexus, you need to add this to the file ~/.m2/settings.xml:


Furthermore, you'll need nexus rights to be able to do this.

More info in the guide to get started with Maven.

Requirements for dependencies

Any dependency used in any KIE project must fulfill these hard requirements:

  • The dependency must have an Apache 2.0 compatible license.

    • Good: BSD, MIT, Apache 2.0

    • Avoid: EPL, LGPL

      • Especially LGPL is a last resort and should be abstracted away or contained behind an SPI.

      • Test scope dependencies pose no problem if they are EPL or LPGL.

    • Forbidden: no license, GPL, AGPL, proprietary license, field of use restrictions ("this software shall be used for good, not evil"), ...

      • Even test scope dependencies cannot use these licenses.
    • To check the ALS compatibility license please visit these links:Similarity in terms to the Apache License 2.0  How should so-called "Weak Copyleft" Licenses be handled

  • The dependency shall be available in Maven Central or JBoss Nexus.

    • Any version used must be in the repository Maven Central and/or JBoss (Nexus) Public repository group

      • Never add a <repository> element in a pom.xml.

      • Note: JBoss Public repository group mirrors,, ... Most jars are available there.

    • Why?

      • Build reproducibility. Any repository server we use, must still run 7 years from now.

      • Build speed. More repositories slow down the build.

      • Build reliability. A repository server that is temporarily down can break builds.

    • Workaround to still use a great looking jar as a dependency:

      • Get that dependency into JBoss Nexus as a 3rd party library.
  • The dependency must be able to run on any JVM 1.8 and higher.

    • It must be compiled for Java target 1.8 or lower (even if it's compiled with JDK 7 or JDK 8).

    • It must not use any JDK APIs that were not yet available in Java 1.8.

  • Do not release the dependency yourself (by building it from source).

    • Why? Because it's not an official release, by the official release guys.

      • A release must be 100% reproducible.

      • A release must be reliable (sometimes the release person does specific things you might not reproduce).

  • No security issues (CVE's) reported on that version of the dependency

    • We don't expect you to check this manually: The victims enforcer plugin will automatically fail the build if a known bad dependency is used.
  • The sources are publicly available

    • We may need to rebuild the dependency from sources ourselves in future. This may be in the rare case when the dependency is no longer maintained, but we need to fix a specific CVE there. The other reason is that productisation needs to be able to easily rebuild the dependency internally.

    • Make sure the dependency's pom.xml contains link to the source repository (scm tag).

  • The dependency needs to use reasonable build system

    • Since we may need to rebuild the dependency from sources, we also need to make sure it is easily buildable. Maven or Gradle are acceptable as build systems.

Any dependency used in any KIE project should fulfill these soft requirements:

  • Edit dependencies in kie-parent.

    • Dependencies in subprojects should avoid overwriting the dependency versions of kie-parent
  • Prefer dependencies with the groupId org.jboss.spec over those with the groupId javax.*.

    • Dependencies with the groupId javax.* are unreliable and are missing metadata. No one owns/maintains them consistently.

    • Dependencies with the groupId org.jboss.spec are checked and fixed by JBoss.

  • Only use dependencies with an active community.

    • Check for activity in the last year through Open Hub.
  • Less is more: less dependencies is better. Bloat is bad.

    • Try to use existing dependencies if the functionality is available in those dependencies

      • For example: use poi instead of jexcelapi if poi is already a KIE dependency
  • Do not use fat jars, nor shading jars.

    • A fat jar is a jar that includes another jar's content. For example: weld-se.jar which includes org/slf4j/Logger.class

    • A shaded jar is a fat jar that shades that other jar's content. For example: weld-se.jar which includes org/weld/org/slf4j/Logger.class

    • Both are bad because they cause dependency tree trouble. Use the non-fat jar instead, for example: weld-se-core.jar

There are currently a few dependencies which violate some of these rules. If you want to add a dependency that violates any of the rules above, get approval from the project leads.

Regenerating Protobuf Files

Some modules include Protobuf files (like drools-core and jbpm-flow). Every time a .proto file is changed, the java files have to be regenerated. In order to do that, on the module that contains the files to be regenerated, execute the following command:

$ mvn exec:exec -Dproto

After testing the regenerated files, don't forget to commit them.

IMPORTANT: before trying to regenerate the protobuf java files, you must install the protobuf compiler (protoc) in your machine. Please follow the instructions. You can download it from here:

For Linux/Mac, you have to compile it yourself as there are no binaries available. Follow the instructions in the README file for that.

kiegroup project structure

Project hierarchy

CI Information

See CI Information document You can check Kiegroup organization repositories CI status from Chain Status webpage

Team communication

To develop a great project as a team, we need to communicate efficiently as a team.

Team workflows

  • Fixing a community issue in JIRA:

    • Find/create the issue in JIRA (Drools, OptaPlanner, jBPM)

    • Fix the issue and push those changes to the appropriate branch(es) on github.

      • If you don't have push permissions, create a pull request (PR). See Using pull requests for more info.
    • Change the Status to Resolved.

      • When you file a PR, do not mark the issue as Resolved until the PR gets merged. Link the PR to the JIRA issue and wait till someone reviews the changes.

      • Once the reporter verifies the fix, he changes Status to Closed. Or we bulk change it to Closed after a year.

Knowing what's going on

  • Subscribe to the Drools Development Google Group and check it daily.

    • Start a new topic for every important organizational or structural decision.

    • If you (accidentally) push a change that can severely hinder or disrupt other developers (such as a compilation failure), notify the Development group.

  • Subscribe to the RSS feeds.

    • It's recommended to subscribe at least to the RSS feeds of the project/repositories you're working on.

    • Prefer an RSS reader which shows which RSS articles you've already read, such as:

      • Thunderbird

        • Open menu File, menu item Subscribe.

        • Tip: create a new, separate directory for each feed: some feeds (such as about the project you are working on) are more important to you than others.

    • Subscribe to jira issue changes:

    • Subscribe to github repository commits:

      • droolsjbpm-build-bootstrap

        • Example how to build a right URL:<repository>/commits/main.atom

        • where you will find <repository> used in kiegroup here: repositories

  • Join us on Zulip: Chat

Writing documentation

  • Optionally install a DocBook editor to write documentation more comfortably, such as:

    • oXygen

      • Open menu Options, menu item Preferences....

      • Click tree item Global

        • Combobox Line separator: Unix-like
      • Click tree item Format

        • Checkbox Detect indent on open: off

        • Checkbox Indent with tabs: off

        • Combobox Indent size: 2

        • Textfield Line width - Format and Indent: 120

    • XMLmind

      • Open menu Options, menu item Preferences....

      • Click tree item Save

        • Combobox Encoding: UTF-8

        • Textfield Identation: 2

        • Textfield Max. line length: 120

        • Checkbox Before saving, make a backup copy of the file: off

          • To avoid committing backups to source control.

          • Source control history is better than backups.

  • To generate the html and pdf output run maven with -Dfull:

    $ cd kiegroup
    $ cd optaplanner/optaplanner-docs
    $ mvn clean install -Dfull
    $ firefox target/docbook/publish/en-US/html_single/index.html
  • Read and follow the documentation guidelines.

  • The Drools Expert manual uses railroad diagrams.

    These are generated from a BNF file into images files with the application Ebnf2ps, Automatic Railroad Diagram Drawing


  • Why do you not accept @author lines in your source code?

    • Because the author tags in the java files are a maintenance nightmare

      • A large percentage is wrong, incomplete or inaccurate.

      • Most of the time, it only contains the original author. Many files are completely refactored/expanded by other authors.

      • Git is accurate, that is the canonical source to find the correct author.

    • Because the author tags promote code ownership, which is bad in the long run.

      • If people work on a piece they perceive as being owned by someone else, they tend to:

        • only fix what they are assigned to fix, instead of everything that's broken

        • discard responsibility if that code doesn't work properly

        • be scared of stepping on the feet of the owner.

      • For more motivation, see this video on How to get a healthy open source project?

    • Credit to the authors is given:

      • on the team page

        • Please contact Geoffrey (or any of us) if you want to add/change/expand your entry in the team page. Don't be shy!
      • on the blog

        • Write an article about the improvements you did! Contact us if you don't have write authorization on the blog yet.
      • with Open Hub which also has statistics

      • in the GitHub web interface.