get notified when a maven project is imported

How do you create and open the project?

I just create a dir, write some pom files to the dir, then call com.intellij.ide.impl.ProjectUtil#openOrImport

I just create a directory, write some pom files to the directory, then call com.intellij.ide.impl.ProjectUtil.openOrImport

Sorry if this comment is duplicated, my previous one is "Pending Approval" which I haven't seen before.

You can try the following:

MavenUtil.runWhenInitialized(openedProject, () -> {
  ApplicationManager.getApplication().executeOnPooledThread(() -> {
    MavenProjectsManager projectsManager = MavenProjectsManager.getInstance(openedProject);
    List<Module> modules = projectsManager.importProjects();
    //
  });
});

Also, since 2017.2 the following listener will be available https://github.com/JetBrains/intellij-community/blob/50a1dba045fd3ff57e2ce825da077afa4225110a/plugins/maven/src/main/java/org/jetbrains/idea/maven/project/MavenImportListener.java#L31 

Awesome, thanks. Unfortunately the code you gave me in the previous comment returned immediately, and the modules were not available.

For now I settled for a "module added" listener.

@Vladislav Soroka How do I register that listener?

MessageBusConnection connection = myProject.getMessageBus().connect(myProject);
connection.subscribe(MavenImportListener.TOPIC, new MavenImportListener() {
  @Override
  public void importFinished(@NotNull Collection<MavenProject> importedProjects, @NotNull List<Module> newModules) {
    // ...
  }
});

See documentation of Messaging API at http://www.jetbrains.org/intellij/sdk/docs/reference_guide/messaging_infrastructure.html 

Usually, project is available as a method parameter and if I used the above mentioned method, then each time that method is invoked, a new Listener will be subscribed. Is there a sort of a central place (like plugin.xml) in code base where I can subscribe through a listener only once.

You can use project service or component.

http://www.jetbrains.org/intellij/sdk/docs/basics/plugin_structure/plugin_services.html#how-to-declare-a-service