DeployMojo.java

/**
 * When building with multiple threads, reaching the last project doesn't have to mean that all projects are ready
 * to be deployed
 */
private static final AtomicInteger readyProjectsCounter = new AtomicInteger();
    
...

public void execute()
        throws MojoExecutionException, MojoFailureException {
    boolean addedDeployRequest = false;
    if (skip) {
        getLog().info("Skipping artifact deployment");
    } else {
        failIfOffline();

        DeployRequest currentExecutionDeployRequest =
                new DeployRequest().setProject(project).setUpdateReleaseInfo(isUpdateReleaseInfo()).setRetryFailedDeploymentCount(getRetryFailedDeploymentCount()).setAltReleaseDeploymentRepository(altReleaseDeploymentRepository).setAltSnapshotDeploymentRepository(altSnapshotDeploymentRepository).setAltDeploymentRepository(altDeploymentRepository);

        if (!deployAtEnd) {
            deployProject(currentExecutionDeployRequest);
        } else {
            deployRequests.add(currentExecutionDeployRequest);
            addedDeployRequest = true;
        }
    }

    boolean projectsReady = readyProjectsCounter.incrementAndGet() == reactorProjects.size();
    if (projectsReady) {
        synchronized (deployRequests) {
            while (!deployRequests.isEmpty()) {
                deployProject(deployRequests.remove(0));
            }
        }
    } else if (addedDeployRequest) {
        getLog().info("Deploying " + project.getGroupId() + ":" + project.getArtifactId() + ":"
                + project.getVersion() + " at end");
    }
}