@Entity
@NamedQuery(name="User.findAll", query="SELECT u FROM User u")
public class User implements Serializable {
    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private int idUser;

    private String email;

    private String firstname;

    //bi-directional many-to-many association to Item
    @ManyToMany
    @JoinTable(
        name="UserHasRent"
        , joinColumns={
            @JoinColumn(name="User_idUser")
            }
        , inverseJoinColumns={
            @JoinColumn(name="Item_idItem")
            }
        )


    private List<Item> items;


    public List<Item> getItems() {
        return this.items;
    }

    public void setItems(List<Item> items) {
        this.items = items;
     }
    }

@Entity
@NamedQuery(name="Item.findAll", query="SELECT a FROM Item a")
public class Item implements Serializable {
    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private int idItem;

    private String description;

    private String image;

    private String name;

    //bi-directional many-to-many association to User
    @ManyToMany(mappedBy="items")
    private List<User> users;


    public List<Usuario> getUser() {
        return this.users;
    }

    public void setUsuarios(List<User> users) {
        this.usuarios = users;
    }

}

@RequestMapping(value ="/rent", method = RequestMethod.POST)
public UsersResponse rent(@RequestParam("idUser") int idUser,
              @RequestParam("idItem") int idItem){

    UsersResponse us = new UsersResponse();
    Boolean state = false;

    if(idUser >= -1 && idItem >= -1){

    state = usersService.rentItem(idUser, idItem);

    }

    if(state){
        us.setCode(200);
        us.setCodeMessage("item rented succesfully");
    }
    return us;
}

@Override
@Transactional
public Boolean rentItem(int idUser, int idItem) {

    User user = usersRepository.findOne(idUser);
    Item item = rentService.getItemById(idItem);

    List <Item> rents = new ArrayList<Item>();
    rents.add(item);

    user.setItems(rents);

    Usuario nuser = usersRepository.save(user);

    return (nuser == null) ? false : true;
}

java.lang.NullPointerException: null at com.app.services.UsersService.rentItem(UsersService.java:96)