问合并如何避免LazyInitException？

EN

package biz.tugay.learningjpa.persistence;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

public class PersistenceUtilImpl implements PersistenceUtil {

    private EntityManagerFactory entityManagerFactory;

    public PersistenceUtilImpl(final String persistenceUnit) {
        entityManagerFactory = Persistence.createEntityManagerFactory(persistenceUnit);
    }

    public EntityManager getEntityManager() {
        final EntityManager entityManager = entityManagerFactory.createEntityManager();
        return entityManager;
    }

    public void closeFactory() {
        entityManagerFactory.close();
    }
}

package biz.tugay.learningjpa.model;

import javax.persistence.*;
import java.util.List;

@Entity
public class Actor {

    private Integer id;
    private String firstname;
    private String lastname;

    private List<Film> films;

    @Id
    @Column(name = "actor_id")
    @GeneratedValue(strategy = GenerationType.AUTO)
    public Integer getId() {
        return id;
    }

    public void setId(Integer actor_id) {
        this.id = actor_id;
    }

    @Basic
    @Column(name = "first_name")
    public String getFirstname() {
        return firstname;
    }

    public void setFirstname(String first_name) {
        this.firstname = first_name;
    }

    @Basic
    @Column(name = "last_name")
    public String getLastname() {
        return lastname;
    }

    public void setLastname(String last_name) {
        this.lastname = last_name;
    }

    @ManyToMany
    @JoinTable(name = "film_actor",
            joinColumns = @JoinColumn(name = "actor_id"),
            inverseJoinColumns = @JoinColumn(name = "film_id")
    )
    public List<Film> getFilms() {
        return films;
    }

    public void setFilms(List<Film> films) {
        this.films = films;
    }
}

package biz.tugay.learningjpa.model;

import javax.persistence.*;
import java.util.List;

@Entity
public class Film {

    private Integer id;
    private String title;

    private List<Actor> actors;

    @Id
    @Column(name = "film_id")
    @GeneratedValue(strategy = GenerationType.AUTO)
    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    @Basic
    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    @ManyToMany
    @JoinTable(name = "film_actor",
            joinColumns = @JoinColumn(name = "film_id"),
            inverseJoinColumns = @JoinColumn(name = "actor_id"))
    public List<Actor> getActors() {
        return actors;
    }

    public void setActors(List<Actor> actors) {
        this.actors = actors;
    }
}

public final class ActorServiceImpl implements ActorService {

    private final PersistenceUtil persistenceUtil;

    public ActorServiceImpl(PersistenceUtil persistenceUtil) {
        this.persistenceUtil = persistenceUtil;
    }

    @Override
    public List<Actor> getActorsOfFilm(Film film) {
        final EntityManager entityManager = persistenceUtil.getEntityManager();
        final Film mergedFilm = entityManager.merge(film);
        final ArrayList<Actor> actors = new ArrayList<Actor>();
        actors.addAll(mergedFilm.getActors());
        entityManager.close();
        return actors;
    }
}

public final class FilmServiceImpl implements FilmService {

    private final PersistenceUtil persistenceUtil;

    public FilmServiceImpl(final PersistenceUtil persistenceUtil) {
        this.persistenceUtil = persistenceUtil;
    }

    @Override
    public Film filmById(int filmId) {
        final EntityManager entityManager = persistenceUtil.getEntityManager();
        final Film film = entityManager.find(Film.class, filmId);
        entityManager.close();
        return film;
    }
}

public class ActorServiceImplTest {

    private PersistenceUtilImpl persistenceUtil;
    private ActorServiceImpl actorService;

    @Before
    public void init() {
        persistenceUtil = new PersistenceUtilImpl("sakiladb");
        actorService = new ActorServiceImpl(persistenceUtil);
    }

    @After
    public void tearDown() {
        persistenceUtil.closeFactory();
    }

    @Test(expected = LazyInitializationException.class)
    public void testGetActorsOfFilmLazyInit() {
        final FilmServiceImpl filmService = new FilmServiceImpl(persistenceUtil);
        final Film film = filmService.filmById(1);
        for (Actor actor : film.getActors()) {
            System.out.println(actor.getFirstname());
        }
    }

    @Test
    public void testGetActorsOfFilm() {
        final FilmServiceImpl filmService = new FilmServiceImpl(persistenceUtil);
        final Film film = filmService.filmById(1);
        final List<Actor> actorsOfFilm = actorService.getActorsOfFilm(film);
        for (Actor actor : actorsOfFilm) {
            System.out.println(actor.getFirstname());
        }
    }
}

@Test
public void testGetActorsOfFilm() {
    final Film film = new Film();
    film.setId(1);
    final List<Actor> actorsOfFilm = actorService.getActorsOfFilm(film);
    for (Actor actor : actorsOfFilm) {
        System.out.println(actor.getFirstname());
    }
}

java.lang.NullPointerException
    at java.util.ArrayList.addAll(ArrayList.java:559)
    at biz.tugay.learningjpa.service.ActorServiceImpl.getActorsOfFilm(ActorServiceImpl.java:57)