为什么 Jenkins 允许指定多个 git 分支？

Question

Multiple times now I have stumbled over why does Jenkins support multiple "branches to build", considering that the help text does not recommend it, what would be a valid use-case?

Edit: I am referring to why there is the "Add branch" button in a single Job, not Multi-Branch.

Answer 1

I cannot explain what a use-case would be, but from reading the source code there seems to be an "advanced use-case" in the git plugin which selects a single branch* (ref) and schedules a new build for the other ones.

It must be noted that scheduling new builds is limited to jobs whose implementation inherits from AbstractProject , like Free-Style Jobs. Note well that Pipeline Jobs are not AbstractProjects !

* There are "build choosers" available which order or limit the matched branches/refs.

Selection code excerpt:

    /**
     * If the configuration is such that we are tracking just one branch of one repository
     * return that branch specifier (in the form of something like "origin/master" or a SHA1-hash
     *
     * Otherwise return [@code null}.
     */
    @CheckForNull
    private String getSingleBranch(EnvVars env) {
        // if we have multiple branches skip to advanced usecase
        if (getBranches().size() != 1) {
            return null;
        }

        // [...]
    }

    // [...]

    /**
     * Determines the commit to be built in this round [...]
     */
    private @NonNull Build determineRevisionToBuild(/* ... */) {

        // [...]
    
        if (candidates.isEmpty() ) {
            final String singleBranch = environment.expand( getSingleBranch(environment) );


            candidates = getBuildChooser().getCandidateRevisions(
                    false, singleBranch, git, listener, buildData, context);
        }


        if (candidates.isEmpty()) {
            // getBuildCandidates should make the last item the last build, so a re-build
            // will build the last built thing.
            throw new AbortException("Couldn't find any revision to build. Verify the repository and branch configuration for this job.");
        }


        Revision marked = candidates.iterator().next();

( source )

scheduling new build:

        if (candidates.size() > 1) {
            log.println("Multiple candidate revisions");
            if (checkForMultipleRevisions) {
                Job<?, ?> job = build.getParent();
                if (job instanceof AbstractProject) {
                    AbstractProject project = (AbstractProject) job;
                    if (!project.isDisabled()) {
                        log.println("Scheduling another build to catch up with " + project.getFullDisplayName());
                        if (!project.scheduleBuild(0, new SCMTrigger.SCMTriggerCause("This build was triggered by build "
                                + build.getNumber() + " because more than one build candidate was found."))) {
                            log.println("WARNING: multiple candidate revisions, but unable to schedule build of " + project.getFullDisplayName());
                        }
                    }
                }
            }
        }

( source )

Answer 2

You have a more descriptive explanation in the latest version of the plugin.

Let me answer your question. This really boils down to the question why would you want to create multiple branches in Git? A typical example that is applicable to a CICD pipeline is. Let's say you have some code that will be deployed to different environments. So inorder to represent different environments you can create branches. For example develop , test , production etc. Developers will work on develop branch when the development is completed it will be propagated upto production. Develop branch will be merged to test and then to production. This is where Jenkins comes into the picture. On each commit to the relevant branch you need to deploy this code to a relevant environment and to do this you can create a single pipeline to handle the deployment process.

For example look at the following pipeline.

 stage('Deliver for development') {
            when {
                branch 'development'
            }
            steps {
                sh './jenkins/scripts/deliver-for-development.sh'
                input message: 'Finished using the web site? (Click "Proceed" to continue)'
                sh './jenkins/scripts/kill.sh'
            }
        }
        stage('Deploy for production') {
            when {
                branch 'production'
            }
            steps {
                sh './jenkins/scripts/deploy-for-production.sh'
                input message: 'Finished using the web site? (Click "Proceed" to continue)'
                sh './jenkins/scripts/kill.sh'
            }
        }