@Entity
public class Parent {
    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "seq")
    private int id;

    @ManyToMany(fetch = FetchType.LAZY, targetEntity = Child.class, cascade = CascadeType.MERGE)
    @JoinTable(name = "parent_child", joinColumns = @JoinColumn(name = "parent_id"), inverseJoinColumns = @JoinColumn(name = "child_id"))
    private List<Child> children;

    public int getId() {
        return id;
    }

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

    public List<Child> getChildren() {
        return children;
    }

    public void setChildren(List<Child> children) {
        this.children = children;
    }
}

@Entity
public class Child {
    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "seq")
    private int id;

    @ManyToMany(fetch = FetchType.LAZY, targetEntity = Parent.class, cascade = CascadeType.MERGE)
    @JoinTable(name = "parent_child", joinColumns = @JoinColumn(name = "child_id"), inverseJoinColumns = @JoinColumn(name = "parent_id"))
    private List<Parent> parents;

    public int getId() {
        return id;
    }

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

    public List<Parent> getParents() {
        return parents;
    }

    public void setParents(List<Parent> parents) {
        this.parents = parents;
    }
}

    @Test
    public void hhh123Test() throws Exception {
        // persist parent entity in a transaction

        EntityManager em = entityManagerFactory.createEntityManager();
        em.getTransaction().begin();

        Parent parent = new Parent();
        em.persist(parent);
        //em.flush();
        int id = parent.getId();

        em.getTransaction().commit();
        em.close();

        // relate and persist child entity in a new transaction

        em = entityManagerFactory.createEntityManager();
        em.getTransaction().begin();

        parent = em.find(Parent.class, id);
        // *: parent.getChildren().size();
        assertEquals(0, parent.getChildren().size());
        Child child = new Child();
        child.setParents(Arrays.asList(parent));
        parent.setChildren(Arrays.asList(child));
        em.persist(child);
        em.persist(parent);
        //em.flush();

        assertEquals(1, parent.getChildren().size()); // -> [Child@36bf7916, Child@36bf7916]

        em.getTransaction().commit();
        em.close();

        em = entityManagerFactory.createEntityManager();
        em.getTransaction().begin();

        Parent p = em.find(Parent.class, id);
        // FAILURE: 1 expected, but 2 childs in List
        Assert.assertEquals(1, p.getChildren().size());

        em.getTransaction().commit();
        em.close();
    }