How do you force a maven MOJO to be executed only once at the end of a build?

if (!getProject().isExecutionRoot()) {
        return ;
}

/**
 * The projects in the reactor.
 *
 * @parameter expression="${reactorProjects}"
 * @readonly
 */
private List reactorProjects;

public void execute() throws MojoExecutionException {

    // only execute this mojo once, on the very last project in the reactor
    final int size = reactorProjects.size();
    MavenProject lastProject = (MavenProject) reactorProjects.get(size - 1);
    if (lastProject != getProject()) {
        return;
    }
   // do work
   ...
}

boolean result = mavenSession.getExecutionRootDirectory().equalsIgnoreCase(basedir.toString());

/**
 * The Maven Project Object
 *
 * @parameter expression="${project}"
 * @required
 * @readonly
 */
protected MavenProject project;


/**
 * The Maven Session.
 *
 * @parameter expression="${session}"
 * @required
 * @readonly
 */
protected MavenSession session;

...


@Override
public void execute() throws MojoExecutionException, MojoFailureException
{
    //Register the event handler right at the start only
    if (project.isExecutionRoot())
        registerEventMonitor();
    ...
}


/**
 * Register an {@link EventMonitor} with Maven so that we can respond to certain lifecycle events
 */
protected void registerEventMonitor()
{
    session.getEventDispatcher().addEventMonitor(
            new EventMonitor() {

                @Override
                public void endEvent(String eventName, String target, long arg2) {
                    if (eventName.equals("reactor-execute"))
                        printSummary();
                }

                @Override
                public void startEvent(String eventName, String target, long arg2) {}

                @Override
                public void errorEvent(String eventName, String target, long arg2, Throwable arg3) {}


            }
    );
}


/**
 * Print summary at end
 */
protected void printSummary()
{
    ...
}